Introduction to VLSI Design

Page 1

Lecture 0

VLSI Design Introduction to the subject Rajesh Ghongade VIIT


• Teaching Scheme – Lectures: 3 Hrs/week – Practical: 2 Hrs/Week

• Examination Scheme – Paper: 100 Marks – Practical: 50 Marks – Oral: 25 Marks


Unit-I VHDL Modeling and Design Flow Introduction to VLSI: complete VLSI design flow (with reference to an EDA tool). Sequential, Data flow, and Structural Modeling. Functions. Procedures, attributes. Test benches, Synthesizable, and non synthesizable statements; packages and configurations Modeling in VHDL with examples of circuits such as counters, shift registers, bidirectional bus, etc.


Unit 2 FSM And Sequential Logic Principles Sequential Circuits, Meta-stability Synchronization, Design of Finite State Machines, and State minimization, FSM CASE STUDIES - Traffic Light control. Lift Control and UART STA and DTA


Unit 3 Programmable Logic Devices Introduction to the CPLDs, Study of architecture of CPLD. and Study of the Architecture of FPGA


Unit 4 System On Chip One, two phase clock, Clock distribution. Power distribution. Power optimization, SRC and DRC, Design validation, Global routing, Switch box routing. Off chip connections, I/O Architectures, Wire parasitics, EMI immune design. Study of memory-Basics of memory includes types of memory cells and memory architectures. Types of memory, based on architecture specific and application specific viz. SRAM, DRAM, SDRAM, FLASH, FIFO.


Unit 5 CMOS VLSI CMOS parasitics, equivalent circuit, body effect, Technology Scaling, A. parameter. Detail study of Inverter Characteristics, power dissipation, power delay product, CMOS combinational logic design and W/L calculations. Transmission gates, Introduction to CMOS layout.


Unit 6 Testability Need of Design for testability, Introduction to Fault Coverage, Testability. Design- forTestability, Controllability and Observability, Stuck-at Fault Model. Stuck-Open and Stuck-Short faults. Boundary Scan check. JTAG technology; TAP Controller and TAP Controller State Diagram. Scan path. Full and Partial scan. BIST


Text Books 1. John F. Wakerly, "Digital Design, Principles and Practices", Prentice Hall Publication 2. Neil H. E Weste and Kamran Eshraghian, "Principles of CMOS VLSI Design". 3. Wayne Wolf, "Modern VLSI Design" 4. Sudhkar Yalamachalli, "Introductory VHDL from simulation to Synthesis"


Reference Books 1. 2. 3. 4. 5. 6. 7. 8.

Perry "VHDL" Charles Roth, "Digital System Design using VHDL". McGraw hill. Xilinx Data Manual "The Programmable Logic Data Book". Stephen Brown and Zvonko Vranesic, "Fundamentals of Digital Logic with VHDL Design", Second Edition, McGraw-Hill, 2005. Michael John Sebastian Smith, "Application-Specific Integrated Circuits", Addison Wesley. Wayne Wolf, "FPGA-Based System Design", Prentice Hall, Miron Abramovici, "Digital Systems Testing and Testable Design", Jaico Publishing. Sung-Mo (Steve) kang, Yusuf Leblebici, " CMOS Digital Integrated Circuit", Tata McGraw-Hill Publication.


LIST OF EXPERIMENTS Any 8 assignments out of the following: Simulation, Synthesis, and Implementation of: 1. 8: 1 Multiplexer, 2:4 Decoder, Comparator and Adder. 2. Flip Flop, Shift Register and Counter 3. Lift Controller /Traffic Light Controller/ UART. Anyone of the three. 4. Parity generator and Checker. 5. Implementation of RAM/FIFO. 6. Ramp waveform generator using DAC 7. Bi-directional buffer 8. Temperature sensing using ADC, Displaying on 7-Segment display and threshold setting using keyboard 9. Implementation of 4-bit RISC processor


EDA Tools • Active-HDL 6.3 sp1 – (VHDL compiling and simulation)

• Synpilfy Pro 8.2 – Synthesis

• Xilinx Webpack 8.X – Implementation


Alternate EDA Tools • Libero Gold 6.2 – (VHDL compiling & implementation)

• Modelsim 6.0 – Simulation

• Synpilfy 8.2 – Synthesis


Hardware available • • • •

Actel ProASIC PLUS evaluation board with APA300-PQ208 device Xilinx CPLD Dedicated Trainer XC 9572 PC 84 Xilinx FPGA Dedicated Trainer XC 2S50 – TQ 144(Spartan II) Universal Trainer kit (BASE UNIT) (ADD on Modules) – Xilinx CPLD Module • XC 95108 – PLCC 84

– Xilinx FPGA Module • XC V100E – PQ 240 XC (Virtex)

– Altera Flex 10K Series FPGA – Atmel CPLD Device ATF 150 4AS / ATF 1508 AS PC 84 – Cypress CPLD Module Device CY37064V PC84


Lecture 1

Introduction to VLSI R.B.Ghongade


Microelectronics Is the art, science and technology of designing and fabricating integrated circuits with small-dimension electronic devices Areas of Microelectronics are : • • • • • • •

VLSI Design VLSI CAD Tools Technology & Fabrication Physics Modeling and Simulation Characterization Testing

Nearly all the advances in the modern day electronic systems and devices are a direct outcome of VLSI technology


Some keywords! • Very-large-scale-integration (VLSI) is defined as a technology that allows the construction and interconnection of large numbers (millions) of transistors on a single integrated circuit. • Integrated circuit is a collection of one or more gates fabricated on a single silicon chip. • Wafer is a thin slice of semiconductor material on which semiconductor devices are made. Also called a slice or substrate. • Chip is a small piece of semiconductor material upon which miniaturized electronic circuits can be built. • Die is an individual circuit or subsystem that is one of several identical chips that are produced after dicing up a wafer. If you use these key-words often, people will think that you are an expert VLSI engineer!!!


The origin of this terminology can be traced as the logical extension to the integration techniques namely the Small Scale Integration, SSI (the ICs which functioned as logic gates, flip-flops), the Medium Scale Integration, MSI (multiplexers, decoders)., the Large Scale Integration LSI (early microprocessors, small memories, PAL, GAL

Technology

Number of gates/transistors* per chip

Examples

Year

SSI

1 to 20

74XX series, 4xxx series

60’s

MSI

100 to 1000

74XXX series, 45XX series

70’s

LSI

1000 to 10,000/100 to 100,000*

8085,

80’s

CPLD, FPGA, advanced μC, SoC

90’s

VLSI

10,000 to 100,000/1,000,000 *

Dividing line between LSI and VLSI is somewhat fuzzy hence number of transistors provides a good criterion


The advances in the integration techniques can be attributed directly to :

•Advances in photolithography techniques •New designs of semiconductor devices •Newer methods of metallization

The development of integration technology has followed the famous Moore’s Law. It was stated by Gordon Moore, co-founder of Intel, in the year 1965, that “the number of transistors per chip would grow exponentially (double every 18 months)”. In fact the doubling period has shortened to a mere 12 months!


Increasing transistor density


The number of transistors/gates that can fit in to the semiconductor die dictates the complexity of the functionality that the device can perform. The important factors that fuel the research in VLSI technology can be summarized as below: • • • • • • • •

Increased functionality Higher reliability Small footprint Very low power consumption Increased speed of operation Re-programmability( except ASIC devices) Mass production Low cost


VLSI is thus a technology that can be harnessed for various applications covering analog, digital and mixed signal electronics. The current trend is to reduce the entire system design to a single chip solution called as system on chip (SoC)


Building blocks of VLSI system on chip Digital MCU/MPU Memory

MEMS CCD sensors microtransformers microresonators

Mixed-Signal DSP Audio ,Video circuits MPEG engine

RF/Analog Frequency generation mixers filters VCO LNA RF power amplifiers Opamps

Power Management converter regulator on-chip power supply

Applications multimedia computing communications biomedical ...


VLSI Design Process VLSI technology thus provides a platform for developing systems for various applications The integrated circuits so developed can be further classified as :


ASIC • An Application Specific Integrated Circuit (ASIC) is a semiconductor device designed especially for a particular customer (versus a Standard Product, which is designed for general use by any customer) • The three major categories of ASIC Technology are : – Gate Array-Based – Standard Cell-Based – Full custom


Gate Arrays • There are two types of gate arrays: – a channeled gate array – channel-less gate array • A channeled gate-array is manufactured with single or double rows of basic cells across the silicon • A basic cell consists of a number of transistors • The channels between the rows of cells are used for interconnecting the basic cells during the final customization process • A channel-less gate array is manufactured with a “sea” of basic cells across the silicon and there are no dedicated channels for interconnection • Gate arrays contain from a few thousand equivalent gates to hundreds of thousand of equivalent gates • Due to the limited routing space on channeled gate arrays, typically only 70% to 90% of the total number of available gates can be used


• The library of cells provided by a gate array vendor will contain: – – – –

primitive logic gates registers, hard-macros soft-macros

Hard-macros and soft-macros are usually of MSI and LSI complexity, such as multiplexers, comparators and counters. Hard macros are defined by the manufacturer in terms of cell primitives Soft-macros are characterized by the designer, for example, specifying the width of a particular counter


Standard Cell • Standard cell devices do not have the concept of a basic cell and no components are prefabricated on the silicon chip • The manufacturer creates custom masks for every stage of the device’s process which leads to a more efficient utilization of silicon as compared to gate arrays • Manufacturers supply hard-macro and soft-macro libraries containing elements of LSI and VLSI complexity, such as controllers, ALUs and microprocessors. • Additionally, soft-macro libraries contain RAM functions that cannot be implemented efficiently in gate array devices; ROM functions are more efficiently implemented in cell primitives


The Characteristics of ASICs The remarks that follow further discuss some trade-offs of ASICs with respect to the following categories: • • • • • • •

Complexity Silicon Efficiency Design Risks Prototype Turnaround NRE CAD / CAE Support Performance


Complexity • • •

Complexity here means the number of transistors (or the amount of logic and/or memory) per given amount of area, plus the associated interconnect capability Current Array-Based and Cell-Based chips accommodate as many as 20,000,000 usable logic gates on a single die Array-Based designs -especially in a Channel-Free Array technology - are capable of realizing functions that represent actual system building blocks and incorporate system memory functions on the same die The Array-Based memories do tend to be about 5 times less dense than Cell-Based memories because they are constructed out of the gates on the master slice. And full custom memories would provide much higher densities than do Array-Based memories But in fact many designers who are using the Array-Based technologies to get fast turn around tend to be using very small “scratch pad” or “cache” types of memories which fit very well into the ASIC concept


Silicon Efficiency • Array-Based technologies focus on fast implementation of logic integration onto a single chip, rather than on absolute highest density. • Cell-Based designs allow you to get more logic onto a chip in a given area. • Cell-Based designs feature transistors and routing tracks whose gradations of size are finer than those in Array-Based products. Thus CellBased designs use silicon more efficiently than Array-Based designs


NRE • NRE (“Non-Recurring Engineering”) charges are the costs associated with developing an ASIC • NRE is based on a number of factors like: – the complexity of the design, – the technology chosen (# of masks required) – the amount of work to be done by the customer and by the silicon vendor – the need for special cells or procedures – the type of package required – the schedule the number of layers of metal –…


• The more work the silicon vendor does and the more special the requirements, the higher will be the NRE . The more work the customer does, the lower the NRE ! • Array-Based designs require the fewest number of design-specific masks and therefore offer the lowest NRE to prototypes. • Cell- Based designs require all masks to be generated for the chosen process and therefore the NRE charge will be higher for a Cell-Based design than for an Array-Based design


Design Risks • The penalty for discovering a design error is higher for a Cell-Based ASIC than for an Array-Based ASIC • Mistakes after prototype fabrication in Array-Based designs usually only require that the metal mask layers be redone. On the other hand, design changes for a CellBased design may require that all masks be redone !


Prototype Turnaround Time (TAT) • Designs that require a complete mask set (CellBased) will always require more time to manufacture than designs which use a basic set of diffusion masks and only require customization at the metal layers (Array-Based) • This difference in time could be anywhere from one week to 4 weeks depending on how fast the silicon vendor can get masks from the mask shop and depending on how long the FAB cycle is for a given process


CAD / CAE Support The use of EDA tools ensure: • Clean documentation • Reusable data • Functional verification • Easy modification • Automated rule check • Back-annotation (synchronization between schematic and layout) • Bill of material


Performance • The two most critical parameters that have been used to measure the worth of new technologies have been speed and power • High power circuits are normally fast, but the increased power requires larger power supplies and tends to heat up the junctions on silicon chips which slows the devices. • In today's most dominant ASIC technology - CMOS high power can cause accelerated junction temperatures which can slow down speed • One way to reduce the power and still maintain speed is to develop circuits such as differential pairs that do not switch from voltage rail to voltage rail


SYSTEM REQUIREMENTS

ASIC Design Flow ARCHITECTURE DEFINITION AND LOGIC DESIGN LOGIC DIAGRAM/DESCRIPTION

VLSI DESIGN AND LAYOUT

DESIGN VERIFICATION FAIL

TECHNOLOGY DESIGN RULES DEVICE MODELS

DESIGN RULE CHECK SIMULATION (SPICE)

PASS MASK GENERATION

SILICON PROCESSING

WAFER TESTING, PACKAGING, RELIABILITY QUALIFICATION


INITIAL DESIGN REVIEW

ASIC Design Flow (detailed)

DESIGN LANDMARKS FRONT-END TOOLS

DESIGN ENTRY

SIMULATION & POWER ANALYSIS

FLOORPLANNING

TEST& VERIFICATION TOOLS LAYOUT & PHYSICAL VERIFICATION TOOLS

LOGIC & TEST SYNTHESIS

CLOCK PLANNING/ CLOCK TREE SYNTHESIS

STATIC TIMING ANALYSIS

TIMING ASSERTIO NS

GATE-LEVEL SIMULATION

FORMAL VERIFICATION

RELEASE TO LAYOUT

TIMING DRIVEN LAYOUT/OPTIMIZATION

STATIC TIMING ANALYSIS

POST LAYOUT TECHNOLOGY CHECKS

AUTOMATIC TEST-PATTERN GENERATION

RELEASE TO MANUFACTURING

TEST STRUCTURE VERIFICATION

POWER ESTIMATION

PRE-LAYOUT TECHNOLOGY CHECKS


Programmable logic device (PLD) • It is an integrated circuit able to implement combinational and/or sequential digital functions defined by the designer and programmed into this circuit • There are a wide variety of ICs that can have their logic function “programmed” into them after they are manufactured. Most of these devices use technology that also allows the function to be reprogrammed


• Historically, programmable logic arrays (PLAs) were the first programmable logic devices • PLAs contained a two-level structure of AND and OR gates with user-programmable connections • Using this structure, a designer could accommodate any logic function up to a certain level of complexity using the well-known theory of logic synthesis and minimization • PLA structure was enhanced and PLA costs were reduced with the introduction of programmable array logic (PAL) devices • Today, such devices are generically called programmable logic devices (PLDs), and are the “MSI” of the programmable logic industry • The ever-increasing capacity of integrated circuits created an opportunity for IC manufacturers to design larger PLDs for larger digital-design applications • However, the basic two-level AND-OR structure of PLDs could not be scaled to larger sizes. Instead, IC manufacturers devised complex PLD (CPLD) architectures to achieve the required scale


• A typical CPLD is merely a collection of multiple PLDs and an interconnection structure, all on the same chip. In addition to the individual PLDs, the on-chip interconnection structure is also programmable, providing a rich variety of design possibilities • CPLDs can be scaled to larger sizes by increasing the number of individual PLDs and the richness of the interconnection structure on the CPLD chip • At about the same time that CPLDs were being invented, other IC manufacturers took a different approach to scaling the size of programmable logic chips. • Compared to a CPLD, a field-programmable gate arrays (FPGA) contains a much larger number of smaller individual logic blocks, and provides a large, distributed interconnection structure that dominates the entire chip


CPLD and FPGA


Top-Down design methodology • Means describing a complete system at an abstract level using hardware description language(HDL) and the use of EDA tools like partitioners and synthesizers • More time is spent on designing HDL models, considering different architectures and considering system test & testability issues. Practically no time is spent on designing at gate level ABSTRACT

To consider a concept without thinking of a specific example; consider abstractly or theoretically.


SYSTEM CONCEPT ALGORITHM

RTL / DATAFLOW GATE TRANSISTOR / SWITCH

INCREASING DETAILED REALIZATION & COMPLEXITY

INCREASING BEHAVIOURAL ABSTRACTION

Levels of behavioural abstraction The process of formulating general concepts by abstracting common properties of instances


System Level: All the specifications (input and output) are described at this level. This level completely ignores the hardware structure. However HDLs are not useful at this stage. It simply treats the design like a black box. Algorithmic (also called behavioural) level: This is the highest level of abstraction provided by most HDLs. A module can be implemented in terms of the desired deign algorithm without the concern of the hardware implementation details. Design at this level is very similar to a conventional high level language programming like C. RTL (Register Transfer Level) (also called dataflow): At this level the module is designed by specifying the data flow between the registers. The designer is aware of how data flows between hardware registers and how the data is processed in the design. Gate Level: The module is implemented in terms of logic gates and interconnections between these gates. Design at this level is similar to describing a design in terms of gate-level logic diagram. Transistor (also called Switch) Level: This is the lowest level of abstraction. A module can be implemented in terms of switches, storage nodes, and the interconnections between them. Design at this level requires knowledge of switch-level implementation details.


Lecture 2

Introduction to VHDL R.B.Ghongade


PLD based design flow •A decision has to be arrived at regarding the selection of the type of a PLD since we have two options the CPLD and the FPGA •The selected device is then called the target device Steps involved: • • • • • • • • •

Specifications Design Entry Compilation Functional Simulation/Verification Synthesis Post-synthesis simulation Implementation Timing Simulation Hardware Implementation


SPECIFICATIONS

HDL based design flow

(STEP 1) CREATE A DIGITAL DESIGN BY VHDL CODE SCHEMATIC ENTRY STATE DIAGRAM

(RTL Level) (STEP 2) COMPILATI ON

Netlist (Gate Level)

Optimized Netli st (Gate Level)

(STEP 3) FUNCTIONAL SIMULATION

Active-HDL Xilinx ISE Libero IDE FPGA Advantage Lattice ISP LEVER

Active-HDL Modelsim Xilinx XST

(STEP 4) (SPECIFY TARGET DEVICE) SYNTHESIS

Synplify Leonardo Spectrum

(STEP 5) SI MULATION (POST SYNTHESIS)

Active-HDL Modelsim

(STEP 6) IMPLEMENTATION PLACE & ROUTE

Palace

(STEP 7) SI MULATION (TIMING ASPECTS)

Active-HDL Modelsim

(STEP 8) HARDWARE IMPLEMENTATION

Xilinx IMPACT Actel Flash Pro


Detailed HDL based design flow

NETLIST Placement & Routing (Device Vendor's Tool)

Idea

Algorithmic Simulation (VHDL)

Design Entry (Schematic)

Design Entry (Text Editor)

VHDL Template Model Generator

Functional Simulation (VHDL Simulator)

Target Device Library

Test Vectors (Input Stimuli & Output Expected)

Back-annotation

Program Data (Fuse Map: JED, HEX...) Structural VHDL (VITAL primitives)

Synthesis (Synthesizer Tool)

SDF (Delay Information) Post- Simulation (VHDL Simulator)

Netlist (EDIF,XNF,DSL...)

Timing Analysis TO PLACE & ROUTE

Device Programming JTAG-ISP VITAL Primitive Library

Chip


Specifications • • • •

It may again include the input, output and ambient specifications Target device may be finalized Choice of target device as CPLD or FPGA depends on various factors Specific type of device may be selected by comparing the specifications provided by the manufacturer in the datasheet and the actual design requirements

• XILINX • LATTICE • LUCENT • ALTERA • ACTEL • CYPRESS • AT&T • AMD

5

5

Xilinx 31

Actel Cypress

15

Altera Lattice 11 6 3 24

AMD AT&T Others


Design Entry • This is essentially the design entry point in an EDA tool • It can be done by the following means: • Block Diagram/Schematic capture • State Diagram entry • HDL code entry


Block Diagram/Schematic capture •

A schematic circuit is literally “drawn” in an appropriate graphical editor • The EDA tool associated with this task is called Schematic Capture Tool • An electrical rule check (ERC) is usually run • The main job of the ERC tool is to check for incorrect electrical connections for example if a VCC pin of an IC is accidentally shorted to ground, then the ERC tool will point out such a discrepancy • For this tool to be effective the IC pins have to be earlier declared as power, ground, input, output, bidirectional etc. • After removing the ERC errors a netlist is generated by the editor • A netlist is a text file showing the nets i.e. a set of components connected together • It is also possible to generate VHDL netlist


Block Diagram/Schematic capture


State Diagram entry • Many designs are most effectively described /designed by state diagram approach. • Effective for sequential designs • The EDA tools provide a graphical interface so that the designer can directly make an entry of the state diagram and generate the netlist. • This method is preferred since it is a fast way of creating the design


State Diagram entry


HDL code entry • • • • •

A designer can enter his /her design using a hardware description language (HDL) The HDLs prominent in the industry are “VHDL” and “Verilog” There is another language that is recently making ground called as “System C” Being similar to C language gives it an advantage to be more user friendly and comfortable to designers familiar with C Using the code entry method is the most preferred one since it offers: – – – –

Design flexibility Code re-use Easy modification Tighter control over resources

A netlist is again created by compiling the HDL code


HDL code entry


Compilation • At this stage the design is said to be at the Register Transfer Level (RTL) • All the data manipulation is done here at the register of the host CPU or we can say that the design is not in terms of the logic gates but “internal” to the environment • After successful compilation of the design using any one of the three methods a netlist is generated which is called the gate-level netlist • The design now is translated in terms of the logic gates and modular entities like multiplexers, decoders. Thus we now have the design at Gate-level


Functional Simulation/Verification • There are two very different tasks in digital system development – Verification is the process of determining whether a design meets the specification and performance goals – It concerns the correctness of the initial design as well as the refinement processes – Testing is the process of detecting the physical defects of a die or a package that occurred during manufacturing


Functional Simulation/Verification •

A functional test is done by simulating the gate-level design using logic simulators that may be available as a built-in feature of the EDA tool. There are two ways of functional verification: – Interactive mode • In the interactive mode the designer can change the input signals and observe the corresponding output behaviour of the design. This method is becomes cumbersome for designs involving large number of inputs and outputs.

– Batch mode • Batch mode uses the concept of test-benches (also a piece of VHDL code) that generates test patterns and checks the validity of the output. This mode is attractive for larger designs.

• •

If any undesirable behaviour is observed, the designer can correct the flaw by going back to the design entry level It is important here to note that none of the timing aspects have been considered during this simulation. Functional verification can thus be compared to the algorithm testing in conventional programming languages


Synthesis • Synthesis means to properly put together so as to make the whole complex design • At this stage it is necessary to specify the target device, since the synthesis tool (again dedicated software) requires knowing the resources available with the target device • Synthesis optimally maps the design functionality (at the gate-level) in to the actual devices available with the target device • For example if the design uses a four- input AND gate but since this is not available with the target device, the synthesis tool can break down the four- input AND gate into two two-input AND gates and map correspondingly • Optimization is very important otherwise the design may get “blown-up” and the target device may prove too small for the design • Synthesis tools have built-in proprietary algorithms for optimization


Post-synthesis simulation • After synthesis the design needs to be rechecked to confirm its functionality • Simulation at this level ensures that the mapped design behaves correctly as desired • A possibility may exist wherein, the synthesis tool may incorrectly recompose the design while mapping • Again timing parameters are ignored here


Implementation • This is the process of physical placing of the design into the target device. • Though it is a physical placement, it still takes place in the virtual environment provided by the EDA tool • A physical map of the target device is loaded into the environment and the components are virtually fitted into the target device. • Again this process may have two phases: – Physical synthesis • Physical synthesis means optimal relocation of the design into the target device. Proprietary software tools are available for this task and may be quite costly. This phase is however optional.

– Place and route • Place and route is the phase where the tool completes the task of virtually placing the components of the design in to the target device and then wiring the individual modules to complete the design.


Timing Simulation • One most important change the design undergoes is after the implementation. • The modules in the design now may be physically placed apart from each other. • This factor introduces the delay aspect in the signal propagation. • Many synchronous circuits will fail if the timing aspects are ignored, even though they appear to be functionally perfect! • Hence a simulation is necessary again to test the timing behaviour of the design. • This provides the designer with a better view of the design functionality. • In fact the real-world behaviour of the device can be very accurately studied by the simulation with timing aspects


Hardware Implementation • The final step in design is to “download” the functionality into the actual hardware i.e. the target device • The synthesis tool generates the output in terms of “bit-stream” that is used to configure the target device • Vendor specific tools can be used for downloading the bit-stream into the physical device


HDL • •

A hardware description language (HDL) is a software coding language used to model the intended operation of a piece of hardware There are two aspects to the description of hardware that an HDL facilitates: – true abstract behaviour modeling – hardware structure modeling

Abstract behaviour modeling – A hardware description language is declarative in order to facilitate the abstract description of hardware behaviour for specification purpose. This behaviour is not influenced by structural or design aspects of the hardware intent.

Hardware structure modeling – Hardware structure is capable of being modeled in a hardware description language irrespective of the design’s behaviour.


VHDL • What is VHDL? – Digital system design using Hardware Description Language is an established methodology in EDA – VHDL stands for “VERY HIGH SPEED INTEGRATED CIRCUITS HARDWARE DESCRIPTION LANUAGE”

EDA stands for “ELECTRONIC DESIGN AUTOMATION”


FEATURES • VHDL is an amalgamation of following languages – Concurrent language – Sequential Language – Timing Specification – Simulation Language – Test Language


• VHDL has got powerful language constructs – {if…else}, {with…select} etc

• Design hierarchies to create modular designs • Supports Design Libraries • Facilitates device independent design and portability


Concurrent Language • Concurrent statements execute at the same time in parallel as in hardware Z <= C + X ; X <= A + B ;

A

+

X

B + C

Z


Sequential Language • Sequential statements execute one at a time in sequence • As the case with any conventional programming language the sequence of statements is important Z <= C + X ; X <= A + B;

X <= A + B; Z <= C + X ;

Sequential statements are required to design sequential circuits


Timing Specification • Providing timing attributes in a sequential digital design is of prime importance since the operations are synchronized to a common clock • Example: process begin clk <= ‘0’ ; wait for 20 ns ; clk <= ‘1’ ; wait for 12 ns ; end process ;

0

20

32

Timing can be specified in a process only

52

64

84

ns


Simulation language • For analyzing a digital design it is important the design be simulated • Simulation has different flavours – Functional simulation – Post-synthesis simulation – Post- layout simulation

• Any HDL should thus be equipped with simulation capability for verification and troubleshooting purposes


Test Language • Testbench – It is a part of a VHDL module that generates a set of test vectors (test inputs) and sends them to the module being tested – It collects the responses generated by the module under test and compares them against a specification of correct results – Thus testbench is required to ensure that the design is correct and that the module is operating as desired Equivalent to checking of logical errors in any conventional programming language


Testbench use

Test tst_a tst_b tst_c ABC_testbench.vhd

MODULE UNDER TEST

ABC.vhd

Equivalent to mechanical test jigs used for testing functionality of mass produced pcbs as in TV sets or motherboards


Design Hierarchy • Hierarchy can be represented using VHDL • Example – A full adder which is the top level module being composed of three lower level modules that are; half adder and OR gate

A B Cin

HALF ADDER

SUM

HALF ADDER

OR

CARRY

Design hierarchy simplifies the design procedure and manageability in case of complex designs


Design Libraries • Design Unit – It is any block of VHDL code or collection of VHDL codes that may be independently analyzed and inserted into a design library

• Design Library – It is a storage facility in which analysed VHDL descriptions are stored for repeated uses DESIGN UNIT

1

Design Library

2 Analyze

3

4

5 Simulator


Logic systems • Need for multi-valued logic system – Conventional logic systems have only three values i.e. ‘0’, ‘1’, ‘Z’

• Example – Consider the truth-table for AND gate

A 0 0 1 1 0

B 0 1 0 1 Z

Y 0 0 0 1 ?

HOW TO RESOLVE THIS CONDITION ?


• For this problem a 9-valued logic system or package was developed that is called “STD_LOGIC_1164” and it is accepted as IEEE STD 1164-1993 • Multi-valued logic – Unknown: value was known but not anymore – Un-initialized: value was never known – High impedance: net has no driver – Drive strengths: handle different output drivers – Don’t care: value is immaterial


Levels of abstraction • Different styles are adopted for writing VHDL code • Abstraction defines how much detail about the design is specified in a particular description • Four levels are: – – – –

Layout level Logic level Register Transfer level Behavioral level

BEHAVIORAL RTL LOGIC LAYOUT


Layout Level • This is the lowest level and describes the CMOS layout level design on silicon


Logic Level • Design has information about – Function – Architecture – Technology – Detailed timings

• Layout information and analog effects are ignored


Register Transfer Level • Using HDL every register in the design and the logic in between is defined • Design contains: – Architecture information – No details of technology – No specification of absolute timing delays


Behavioral Level • Describing function of a design using HDL without specifying the architecture of registers • Contains timing information required to represent a function


Basic building blocks of VHDL code • A VHDL design is composed of following blocks: – Library declarations – Entity – Architecture – Configuration


Basic VHDL code LIBRARY DECLARATIONS

ENTITY BASIC VHDL CODE

ARCHITECTURE

CONFIGURATION


Detailed anatomy of VHDL code

Library Package

Functions Procedures Types Constants Components

Entity

Generics

Architecture (Style I) DATAFLOW

Concurrent Statements

Ports

Architecture (Style II) BEHAVIOURAL

Concurrent Statements

Architecture (Style III) STRUCTURAL

Process

Sequential Statements


Lecture 3

Elements of VHDL R.B.Ghongade


Basic building blocks LIBRARY DECLARATIONS

ENTITY BASIC VHDL CODE

ARCHITECTURE

CONFIGURATION


Overview


Library • It is a collection of compiled VHDL units • It enables sharing of compiled designs and hides the source code from the users • Commonly used functions, procedures and user data types can be compiled into a user defined library for use with all designs • Library should be declared before each entity declaration even if it is in the same VHDL file


Library • To declare a library (i.e. to make it visible to the design) two lines of code are needed , one containing name of the library, the other a use clause • A library structure can be as follows: LIBRARY PACKAGE

FUNCTIONS PROCEDURES TYPES CONSTANTS COMPONENTS


Library syntax LIBRARY library_name ; USE library_name.package_name.package_parts ;

Example LIBRARY IEEE ; -- semicolon indicates USE IEEE.std_logic_1164.all ; -- end of statement or -- declaration LIBRARY work ; -- double dash (--) -- indicates a comment USE work.all ;


Library details IEEE.MATH_COMPLEX.all

This package defines a standard for designers to use in describing VHDL models that make use of common COMPLEX constants and common COMPLEX mathematical functions and operators.

IEEE.MATH_REAL.all

This package defines a standard for designers to use in describing VHDL models that make use of common REAL constants and common REAL elementary mathematical functions.

IEEE.NUMERIC_BIT.all

This package defines numeric types and arithmetic functions for use with synthesis tools. Two numeric types are defined: -- UNSIGNED: represents an UNSIGNED number in vector form -- SIGNED: represents a SIGNED number in vector form The base element type is type BIT.


Library details IEEE.NUMERIC_STD.alll

This package defines numeric types and arithmetic functions for use with synthesis tools. Two numeric types are defined: -- UNSIGNED: represents UNSIGNED number in vector form -- SIGNED: represents a SIGNED number in vector form -- The base element type is type STD_LOGIC.

IEEE.STD_LOGIC_1164.all

This package defines a standard for designers to use in describing the interconnection data types used in VHDL modeling. Defines multi-value logic types and related functions.

IEEE.STD_LOGIC_ARITH.all

This package defines a set of arithmetic, conversion, and comparison functions for SIGNED, UNSIGNED, SMALL_INT, INTEGER, STD_ULOGIC, STD_LOGIC, and STD_LOGIC_VECTOR.


Library details IEEE.STD_LOGIC_MISC.alll

This package defines supplemental types, subtypes, constants, and functions for the Std_logic_1164 Package.

IEEE.STD_LOGIC_SIGNED.all

This package defines a set of signed arithmetic, conversion, and comparison functions for STD_LOGIC_VECTOR.

IEEE.STD_LOGIC_TEXTIO.all

This package overloads the standard TEXTIO procedures READ and WRITE.

IEEE.STD_LOGIC_UNSIGNED.all This package defines a set of unsigned arithmetic, conversion and comparison functions for STD_LOGIC_VECTOR.


Entity – It is the design’s interface to the external circuitry – Equivalent to pinout /package of an IC – VHDL design must include one and only one entity per module – It can be used as a component in other entities after being compiled into a library


Entity declaration • Defines the input and output ports of the design • Name of the entity can be anything other than the reserved VHDL word • Each port in the port list must be allotted: – a name ( should be self-explanatory that provides information about its function – data flow direction or mode – a type

• Ports should be well documented with comments at the end of line providing additional information about the signal


Entity syntax

entity entity_name is port ( port_name : signal_mode signal_type ; port_name : signal_mode signal_type ; port_name : signal_mode signal_type ) ; end entity_name ;


Modes • Ports in the portlist have modes which indicate the driver direction • Mode also indicates whether or not the port can be read from within the entity • Four modes are available: – Mode IN – Mode OUT – Mode INOUT – Mode BUFFER


• Mode IN Value can be read from but not assigned to (by the entity) Port signal A

port ( A : in std_logic ) ;

Drivers reside outside the entity

ENTITY


• Mode OUT Value can be assigned to but not read from (by the entity) port ( B : out std_logic ) ;

Port signal B

Drivers reside inside the entity

ENTITY


• Mode INOUT Bi-directional , value can be assigned to as well as read from (by the entity) Port signal C

port ( C : inout std_logic ) ;

Drivers reside both inside and outside the entity

ENTITY


• Mode BUFFER Output port with internal read capability Drivers reside inside the entity Port signal D

port ( D : buffer std_logic ) ;

ENTITY DO NOT USE UNLESS REQUIRED

Signal can be read inside the entity


Entity example entity and_gate is port ( 1A , 2A , 3A, 4A : in std_logic ; 1B , 2B , 3B, 4B : in std_logic ; 1Y , 2Y , 3Y, 4Y : out std_logic ) ; end and_gate ; 1 2 3 4 5 6 7

1A

VCC

1B

4B

1Y

4A

2A

4Y

2B

3B

2Y

3A

GND

3Y

14 13 12 11 10 9 8


Entity example entity ALU is port ( In1 : in std_logic_vector ( 3 downto 0) ; -- 1st operand In2 ; in std_logic_vector ( 3 downto 0) ; -- 2nd operand Opsel : in std_logic_vector ( 3 downto 0) ; -- opn select Cin : in std_logic ; Mode : in std_logic ; Result : out std_logic_vector ( 3 downto 0 ) ; Cout : out std_logic ; Equal : out std_logic ) ; end ALU ; In1

In2

Op sel

Mode

Cout

Cin

ALU Result

Equal


Architecture It specifies • Behaviour • Function • Relationship between inputs and outputs of an entity

Syntax architecture achitecture_name of entity_name is [declarations] -- optional begin code -- concurrent statements only end achitecture_name ;


• Architecture can contain only concurrent statements • A design can be described in an architecture using various levels of abstraction • An entity can have more than one architectures since a function can be implemented in a number of ways • There can be no architecture without an entity


Architectural bodies • Behavioural – It is the high-level description – It contains a set of assignment statements to represent behaviour – No need to focus on the gate-level implementation of a design Example: architecture behave of and_gate is begin process ( a, b ) if a=‘1’ and b=‘1’ then c <= ‘1’ ; else c <=‘0’ ; end if ; end process ; end behave ;


• Dataflow – It uses concurrent signal assignment statements Example: architecture dataflow of and_gate is begin c<= a and b ; end dataflow ;


• Structural – Components from libraries are connected together – Designs are hierarchical – each component can be individually simulated – it makes use of component instantiation Top level design Functional units A B Cin

HALF ADDER

SUM

HALF ADDER

OR

CARRY


Configuration • Since a number of architectures can exist for an entity , using configuration statement we can bind a particular architecture to the entity Syntax configuration CONFIGURATION_NAME of ENTITY_NAME is for ARCHITECTURE_NAME end for; end CONFIGURATION_NAME;


Next class

Language elements


Language Elements I R.B.Ghongade Lecture 4


• VHDL is a strongly typed language – Designers have to declare the type before using it

• VHDL is not case sensitive ( but avoid mixed cases as a good programming practice) • VHDL supports a wide variety of data types and operators – OBJECTS – OPERATORS – AGGREGATES


Objects • They are used to represent and store the data in the design being described • Object contains a value of specific type Class

SIGNAL

Object

COUNT

Data type

: INTEGER

This results in an object called COUNT that holds INTEGER value that belongs to class SIGNAL

• The name given to the object is called as identifier • Do not use reserved words as identifiers


• Each object has a data type and class • Class indicates how the object is used in the module and what can be done with that object • Type indicates what type of data the object contains • Each object belongs to one of the following class: – CONSTANT – SIGNAL – VARIABLE CLASS – FILES CONSTANT

SIGNAL

VARIABLE

FILES


Data Types • In order to write VHDL code efficiently it is necessary to study the specification and use of data types • Following are the categories of data types: – – – – – – –

Pre-defined User defined Subtypes Arrays Port arrays Records Signed and unsigned


Pre-defined data types • Specified by IEEE 1076 and IEEE 1164 Package

Library Type/Functions

standard

std

BIT, BOOLEAN, INTEGER, REAL

std_logic_1164

ieee

STD_LOGIC, STD_ULOGIC

std_logic_arith

ieee

std_logic_signed std_logic_unsigned

ieee ieee

SIGNED, UNSIGNED / data conversion functions Functions that allow operations with STD_LOGIC_VECTOR


BIT (and BIT_VECTOR): 2 level logic (‘0’, ‘1’)

Examples: SIGNAL X : BIT ;

SIGNAL Y : BIT_VECTOR (3 downto 0);

SIGNAL W : BIT_VECTOR (0 to 7);

X is declared as a onedigit SIGNAL of type BIT Y is 4-bit vector, leftmost bit is MSB W is 8-bit vector, rightmost bit is MSB


• To assign a value to the signal use the operator “ < = ” • Assignment examples: X <= ‘1’ ;

X is a single bit SIGNAL whose value is ‘1’ Note that single quotes are used for a single bit

Y <= “0111” ; W <= “01110001” ;

Y is a 4- bit SIGNAL whose value is “0111” . Note that double quotes are used for vectors W is an 8- bit SIGNAL whose value is “0111001” . MSB is ‘1’


STD_LOGIC (and STD_LOGIC_VECTOR): 8 valued logic (introduced in IEEE 1164 standard) Symbol ‘X’ ‘0’ ‘1’ ‘Z’ ‘W’ ‘L’ ‘H’ ‘-’

Description Remark Forcing unknown Synthesizable unknown Synthesizable logic ‘0’ Forcing low Synthesizable logic ‘1’ Forcing high High impedance Synthesizable tri-state buffer Weak unknown Weak low Weak high Don’t care


Examples: SIGNAL X : STD_LOGIC ;

X is declared as a one-digit (scalar) SIGNAL of type STD_LOGIC

SIGNAL Y : STD_LOGIC_VECTOR (3 downto 0);

Y is 4-bit vector, leftmost bit is MSB

for (optional) SIGNAL Y : STD_LOGIC_VECTOR (3 downto 0) : = “0001” initial value use “ := ”

Most of std_logic levels are intended for simulation only. However ‘0’, ‘1’ and ‘Z’ are synthesizable with no restrictions


• With respect to “weak” values, they are resolved in favour of the “forcing” values in multiple-driven nodes. If any two std_logic signals are connected to the same node, then conflicting logic levels are resolved by using the shown table

X 0 1 Z WL H X X X X X X X X X 0 X 0 X 0 0 0 0 X 1 X X 1 1 1 1 1 X Z X 0 1 Z WL H Z WX 0 1 W WWWX L X 0 1 L WL WX H X 0 1 H WWH X -

X X X X X X X X

•The STD_ULOGIC has 9 valued logic levels : additional value is ‘U’ for “Un-resolved or ”Un-initialized”


• Other types BOOLEAN

TRUE, FALSE

INTEGER

32-bit integers (from - 2,147,483,647 to + 2,147,483,647

NATURAL

Non-negative numbers (from 0 to 2,147,483,647

REAL

Real numbers (from -1.0E-38 to +1.0E38)

Physical literals

Used to inform physical quantities like , time, voltage etc. Useful for simulation but not synthesizable

Character literals SIGNED, UNSIGNED

Single ASCII character or a string of such characters. Not synthesizable They have appearance of STD_LOGIC_VECTOR, but accept arithmetic operations which are typical of INTEGER data type


User defined data types • VHDL allows user defined data types • Two categories of this data type are: – Integer – Enumerated

• User defined integer type TYPE my_integer IS RANGE -32 to +32 ; TYPE student_grade IS RANGE 0 to 100 ;


• User defined enumerated type TYPE my_logic IS (‘0’, ‘1’, ‘Z’ ); TYPE my_state IS ( idle, forward, backward, stop) ; An enumerated type, typically used in state machines

• The encoding of enumerated types is done sequentially and automatically • Since here there are 4 states only two bits are required hence “00” is assigned to first state ( idle), “01” to second state (forward) and so on.


Subtypes • A SUBTYPE is a TYPE with a constraint • Though operations between data of different types are not allowed, they are allowed between the subtype and its corresponding base type SUBTYPE sub_state IS my_state RANGE idle to backward ;

This means that the subtype sub_state =(idle, forward, backward)


Arrays • Arrays are collections of objects of same type • Can be 1-dimensional, 2-dimensional or 1D X 1D • Higher dimensional arrays are possible but not synthesizable

0 Scalar

0 1 0 0

1D

1D x 1D

1 1 0 0

0

1

0

1

0

1 0 1 0

1

0

1

1

0

1 1 0 1

0

1

0

1

0

0 1 0 0 2D data array


Array syntax To specify an array : TYPE type_name IS ARRAY (specification) OF data_type ;

To use an array : SIGNAL signal_name : type_name [:= initial_value]


Example : 1D x 1D array – We want to build an array containing 4 vectors, each of size 8 bits – we will call each vector as row and the complete array as matrix TYPE row IS ARRAY (7 downto 0 ) OF STD_LOGIC ; TYPE matrix IS ARRAY (3 downto 0 ) OF row ; SIGNAL X : matrix ;

1D x 1D SIGNAL


Example : 2D array – This array will be created with scalars only TYPE matrix2D IS ARRAY (0 TO 3, 7 DOWNTO 0 ) OF STD_LOGIC ;

L M

L

ROWS

M

COLUMNS


Port Arrays • In the specification of the input or output pins (PORTS) of a circuit (which is made in the ENTITY), we might need to specify the ports as arrays of vectors. • Since TYPE declarations are not allowed in an ENTITY, the solution is to declare user-defined data types in a PACKAGE, which will then be visible to the whole design (thus including the ENTITY)


------- Package: -------------------------LIBRARY ieee; USE ieee.std_logic_1164.all; ---------------------------PACKAGE my_data_types IS TYPE vector_array IS ARRAY (NATURAL RANGE <>) OF STD_LOGIC_VECTOR(7 DOWNTO 0); END my_data_types; -------------------------------------------------- Main code: ------------------------LIBRARY ieee; USE ieee.std_logic_1164.all; USE work.my_data_types.all; -- user-defined package --------------------------ENTITY mux IS PORT (inp: IN VECTOR_ARRAY (0 TO 3); ... ); END mux; ... ; --------------------------------------------


• As can be seen in the example above, a userdefined data type, called vector_array,was created, which can contain an indefinite number of vectors of size eight bits each (NATURAL RANGE <> signifies that the range is not fixed, with the only restriction that it must fall within the NATURAL range, which goes from 0 to +2,147,483,647) • The data type was saved in a PACKAGE called my_data_types, and later used in an ENTITY to specify a PORT called inp • Notice in the main code the inclusion of an additional USE clause to make the user-defined package my_data_types visible to the design.


Records • Records are similar to arrays, with the only difference that they contain objects of different types. TYPE birthday IS RECORD day: INTEGER RANGE 1 TO 31; month: month_name; END RECORD;


Signed and Unsigned data types • These types are defined in the std_logic_arith package of the ieee library

Examples: SIGNAL x: SIGNED (7 DOWNTO 0); SIGNAL y: UNSIGNED (0 TO 3);


• An UNSIGNED value is a number never lower than zero. For example, ‘‘0101’’ represents the decimal 5, while ‘‘1101’’ signifies 13. • If type SIGNED is used instead, the value can be positive or negative (in two’s complement format). Therefore,‘‘0101’’ would represent the decimal 5, while ‘‘1101’’ would mean 3. • To use SIGNED or UNSIGNED data types, the std_logic_arith package, of the ieee library, must be declared


Next class Language Elements II


Language Elements II R.B.Ghongade Lecture 5


Operators • VHDL provides several kinds of predefined operators – – – – – –

Assignment operators Logical operators Arithmetic operators Relational operators Shift operators Concatenation operators


Assignment operators • Are used to assign values to signals, variables, and constants.

<= := =>

Used to assign a value to a SIGNAL Used to assign a value to a VARIABLE, CONSTANT, or GENERIC Used also for establishing initial values Used to assign values to individual vector elements or with OTHERS


SIGNAL x : STD_LOGIC; VARIABLE y : STD_LOGIC_VECTOR(3 DOWNTO 0); SIGNAL w: STD_LOGIC_VECTOR(0 TO 7);

Then the following assignments are legal:

x <= '1'; -- '1' is assigned to SIGNAL x using "<=" y := "0000"; -- "0000" is assigned to VARIABLE y using --":=" w <= "10000000"; -- LSB is '1', the others are '0' w <= (0 =>'1', OTHERS =>'0'); -- LSB is '1', the others -- are '0'


Logical operators • Used to perform logical operations. • The data must be of type: – – – – –

BIT, STD_LOGIC STD_ULOGIC BIT_VECTOR STD_LOGIC_VECTOR STD_ULOGIC_VECTOR


• The logical operators are: – NOT – – – – – –

AND OR NAND NOR XOR XNOR

The NOT operator has precedence over the others

Examples: y <= NOT a AND b; -- (a'.b) y <= NOT (a AND b); -- (a.b)' y <= a NAND b; -- (a.b)'


Arithmetic operators • Used to perform arithmetic operations. The data can be of type INTEGER, SIGNED, UNSIGNED, or REAL (the last cannot be synthesized directly). • Also, if the std_logic_signed or the std_logic_unsigned package of the ieee library is used, then STD_LOGIC_VECTOR can also be employed directly in addition and subtraction operations


¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾

+ * / ** MOD REM ABS

( Addition) (Subtraction) (Multiplication) (Division) (Exponentiation) ( Modulus) ( Remainder) ( Absolute value)

For mod, rem, abs , there generally is little or no synthesis support

•There are no synthesis restrictions regarding addition and subtraction, and the same is generally true for multiplication •For division, only power of two dividers (shift operation) are allowed •For exponentiation, only static values of base and exponent are accepted •Regarding the mod and rem operators, y mod x returns the remainder of y/x with the signal of x, while y rem x returns the remainder of y/x with the signal of y •Finally, abs returns the absolute value


Comparison operators ¾ ¾ ¾ ¾ ¾ ¾

= /= < > <= >=

Equal to Not equal to Less than Greater than Less than or equal to Greater than or equal to

Also called RELATIONAL operators


Shift operators ¾ ¾ ¾ ¾ ¾ ¾

sll srl sla sra ror rol

shift left logical shift right logical shift left arithmetic shift right arithmetic rotate left logical rotate right logical

• LOGICAL • ARITHMETIC • ROTATE

LOGICAL SHIFTING 0

ARITHMETIC SHIFTING (retains sign bit)

0

ROTATE


Concatenation operator &

Concatenation

• Operands can be one-dimensional array type or element type • “ &” works on vectors only Example: SIGNAL SIGNAL BEGIN b <= a <= . . .

a : STD_LOGIC_VECTOR ( 5 DOWNTO 0 ) ; b,c,d : STD_LOGIC_VECTOR ( 2 DOWNTO 0 ) ; ‘0’ & c(1) c & d ;

& d(2) ;


Operator summary Operator type

Operators

Data types

NOT, AND, AND OR, NOR, XOR, XNOR

BIT, BIT_VECTOR, STD_LOGIC, STD_LOGIC_VECTOR, STD_ULOGIC, STD_ULOGIC_VECTOR

Arithmetic

+, -,*,/,** (mod, rem , abs)

INTEGER, SIGNED, UNSIGNED

Comparison

=, /=, <, >, <=, >=

All above

Shift

sll, srl, sla, sra, rol, ror

BIT_VECTOR

Concatenation

&, ( , , , )

Same as for logical operators, plus SIGNED and UNSIGNED

Logical


Operator overloading • Operators can be user-defined • Let us consider the pre-defined arithmetic operators seen earlier (+,- , *, /, etc.). They specify arithmetic operations between data of certain types (INTEGER, for example) • For instance, the pre-defined ‘‘+’’ operator does not allow addition between data of type BIT. • We can define our own operators, using the same name as the pre-defined ones


• For example, we could use ‘‘+’’ to indicate a new kind of addition, this time between values of type BIT_VECTOR. This technique is called operator overloading • Example: Consider that we want to add an integer to a binary 1-bit number. Then the following FUNCTION could be used FUNCTION "+" (a: INTEGER, b: BIT) RETURN INTEGER IS BEGIN IF (b='1') THEN RETURN a+1; ELSE RETURN a; END IF; END "+";


A call to the function above could thus be the following: SIGNAL inp1, outp: INTEGER RANGE 0 TO 15; SIGNAL inp2: BIT; (...) outp <= 3 + inp1 + inp2; (...) • In ‘‘outp<=3+inp1+inp2;’’, the first ‘‘+’’ is the predefined addition operator (adds two integers), while the second is the overloaded user-defined addition operator (adds an integer and a bit).


Aggregates • It assigns values to elements of an array a <= (OTHERS => ‘0’ ) ;

a <= “0000” ;

• We can assign values to some bits in a vector and use “OTHERS” clause to assign default values to remaining bits a <= (0 => ‘1’, 2 => ‘1’, OTHERS => ‘0’ ) ; is equivalent to a <= “00101” ;

Useful when we are dealing with large vectors


Classes re-visited • Each object has a data type and class • Class indicates how the object is used in the module and what can be done with that object • Type indicates what type of data the object contains • Each object belongs to one of the following class: – CONSTANT CLASS – SIGNAL – VARIABLE

CONSTANT

SIGNAL

VARIABLE


Constants • These are identifiers with fixed values • The value is assigned only once when declared • Values cannot be changed during simulation CONSTANT bus_width : INTEGER :=16 ; CONSTANT CLK_PERIOD : TIME :=15 ns ;

• Constants make the design description more readable • Design changed at later time becomes easy


Signals Example: architecture and_gate of myand is signal TEMP : STD_LOGIC ; begin U1 : AND2 portmap ( a, b, TEMP ) ; U2 : AND2 portmap (TEMP, c , d ) ;

Equivalent to wires within a circuit

end and_gate ; TEMP

a b AND2 d c AND2


• Thus signals are used : – to connect design entities together and communicate changes in values within a design – instead of INOUT mode

• Each signal has a history of values i.e. they hold a list of values which include current value of the signal and a set of possible future values that can appear on the signal • Computed value is assigned to signal after specified delay called DELTA DELAY


Variables • These are objects with single current value • They are used to store the intermediate values between the sequential statements • Variable assignment occurs immediately • Variables can be declared and used inside the process statement only. But they retain their value throughout the entire simulation


Example : process ( a ) variable count : INTEGER : = 1 ; begin count : = count+ 1 ; end process ;

count contains the total number of events that occurred on signal a


Next class

Language elements III


Language Elements III R.B.Ghongade Lecture 6


Attributes • An attribute is data that are attached to VHDL objects or predefined data about VHDL objects • Examples are the current drive capability of a buffer or the maximum operating temperature of the device • Types are – Data Attributes – Signal Attributes – User-defined Attributes


Data Attributes The pre-defined, synthesizable data attributes are the following: • d’LOW : Returns lower array index • d’HIGH : Returns upper array index • d’LEFT : Returns leftmost array index • d’RIGHT : Returns rightmost array index • d’LENGTH : Returns vector size • d’RANGE : Returns vector range • d’REVERSE_RANGE: Returns vector range in reverse order


Example Consider the following signal: SIGNAL d : STD_LOGIC_VECTOR (7 DOWNTO 0);

Then: d'LOW=0, d'HIGH=7, d'LEFT=7, d'RIGHT=0, d'LENGTH=8, d'RANGE=(7 downto 0), d'REVERSE_RANGE=(0 to 7)


If the signal is of enumerated type, then: • d’VAL(pos) : Returns value in the position specified • d’POS(value) : Returns position of the value specified • d’LEFTOF(value) : Returns value in the position to the left of the value specified • d’VAL(row, column) : Returns value in the position specified; etc There is little or no synthesis support for enumerated data type attributes


Signal Attributes Let us consider a signal s Then: • s’EVENT : Returns true when an event occurs on s • s’STABLE : Returns true if no event has occurred on s • s’ACTIVE : Returns true if s = ‘1’ • s’QUIET <time> : Returns true if no event has occurred during the time specified • s’LAST_EVENT : Returns the time elapsed since last event • s’LAST_ACTIVE: Returns the time elapsed since last s=‘1’ • s’LAST_VALUE : Returns the value of s before the last event; etc.


Example All four assignments shown below are synthesizable and equivalent. They return TRUE when an event (a change) occurs on clk, AND if such event is upward (in other words, when a rising edge occurs on clk) IF (clk'EVENT AND clk='1')... -- EVENT attribute -- used with IF IF (NOT clk'STABLE AND clk='1')... -- STABLE --attribute used -- with IF WAIT UNTIL (clk'EVENT AND clk='1'); -- EVENT --attribute used -- with WAIT IF RISING_EDGE(clk)... -- call to a function


User-defined Attributes • VHDL also allows the construction of user-defined attributes • To employ a user-defined attribute, it must be declared and specified Attribute Declaration: ATTRIBUTE attribute_name: attribute_type

;

Attribute Specification: ATTRIBUTE attribute_name OF target_name: class IS value; where: attribute_type: any data type (BIT, INTEGER, STD_LOGIC_VECTOR, etc.) class: TYPE, SIGNAL, FUNCTION, etc. value: ‘0’, 27, ‘‘00 11 10 01’’, etc.


Example ATTRIBUTE number_of_inputs: INTEGER; ATTRIBUTE number_of_inputs OF nand3: SIGNAL IS 3; ... inputs <= nand3'number_of_inputs; -- attribute call, -- returns 3


Generics • As the name suggests, GENERIC is a way of specifying a generic parameter • a static parameter that can be easily modified and adapted to different applications • The purpose is to make the code more flexible and reusable • must be declared in the ENTITY • More than one GENERIC parameter can be specified in an ENTITY


Syntax GENERIC (parameter_name : parameter_type := parameter_value);

Example The GENERIC statement below specifies a parameter called n, of type INTEGER, whose default value is 8. Therefore, whenever n is found in the ENTITY itself or in the ARCHITECTURE (one or more) that follows, its value will be assumed to be 8 ENTITY my_entity IS GENERIC (n : INTEGER := 8; vector: BIT_VECTOR := "00001111");

PORT (...); END my_entity; ARCHITECTURE my_architecture OF my_entity IS ... END my_architecture;


Example ARCHITECTURE generic_decoder OF decoder IS BEGIN PROCESS (ena, sel) VARIABLE temp1 : STD_LOGIC_VECTOR (x'HIGH DOWNTO 0); VARIABLE temp2 : INTEGER RANGE 0 TO x'HIGH; ....


Delays in VHDL • In VHDL, there are three types of delay that are encountered – Inertial delay – Transport delay – Delta delay


Inertial Delay • Inertial delay is the default in VHDL • Behaves similarly to the actual device • Output signal of the device has inertia, which must be overcome for the signal to change value • The inertial delay model is by far the most commonly used in all currently available simulators


LIBRARY IEEE; USE IEEE.std_logic_1164.ALL; ENTITY buf IS PORT ( a : IN std_logic; PORT ( b : OUT std_logic); END buf; ARCHITECTURE buf OF buf IS BEGIN b <= a AFTER 20 ns; END buf;

Due to inertial delay pulse is swallowed up

Inertial delay prevents prolific propagation of spikes throughout the circuit


Transport Delay • It represents a wire delay in which any pulse, no matter how small, is propagated to the output signal delayed by the delay value specified • Especially useful for modeling delay line devices, wire delays on a PCB, and path delays on an ASIC


LIBRARY IEEE; USE IEEE.std_logic_1164.ALL; ENTITY delay_line IS PORT ( a : IN std_logic; PORT ( b : OUT std_logic); END delay_line; ARCHITECTURE delay_line OF delay_line IS BEGIN b <= TRANSPORT a AFTER 20 ns; END delay_line;

Pulse is simply delayed


Delta delay • These are used since the PC that processes and simulates a concurrent phenomenon is basically a sequential machine • The simulation program mimics concurrency by scheduling events in some order • Simulation deltas are used to order some types of events during a simulation • Specifically, zero delay events must be ordered to produce consistent results


Circuit that shows the difference! A

Assumptions Zero delay components CLK=‘1’ A=‘1’ E

CLK B

DFF F D

Q

CLK

Q'

D C


Problem when no delta delay concept is used 1) A becomes 0

A A

2) Evaluate inverter 3) B <= 1 4) Evaluate AND with C=1

E E

CLK CLK B B

DFF DFF

5) D<=1 F

C C

D D

Q Q

CLK CLK

Q' Q'

6) Evaluate NAND 7) C<=0 8) Evaluate AND 9) D<=0

D

Unwanted D assertion


Problem when no delta delay concept is used A

1) A becomes 0 2) Evaluate inverter 3) B <= 1 E

CLK B

DFF

4) Evaluate NAND FF

C

D

Q

CLK

Q'

5) C<=0 6) Evaluate AND 7) D<=0

D


Delta delay use 10 ns

Delta 1

A<=0 Evaluate inverter

Delta 2

B<=0 Evaluate AND Evaluate NAND

Delta 3

D<= 1 C <=0 Evaluate AND

Delta 4

D<= 0

11 ns

To summarize, simulation deltas are an infinitesimal amount of time used as a synchronization mechanism when 0 delay events are present.


Concurrent Statements and Constructs


Combinational vs. Sequential Logic The output of the circuit depends solely on the current inputs

Output does depend on previous inputs hence storage elements are required input

output

Combinational Logic

input

Combinational Logic

output

Present State

Next State Storage Elements


Concurrent Code • Consider the following statement: X=X+Y; In conventional software X and Y are register locations hence contents of X and Y are added and stored in X

Register X

Register Y

+


Difference in VHDL • In VHDL the same statement will mean a feedback in a purely combinational logic which is invalid

Y

X

+


• VHDL code is inherently concurrent (parallel) • Only statements placed inside a PROCESS, FUNCTION, or PROCEDURE are sequential • Concurrent code is also called dataflow code • Order does not matter • We can only build combinational logic circuits with concurrent code • Concurrent assignment produces one driver for each assignment statement a

z <= a;

z


Multiple driver assignment architecture ABC of XYZ is signal z,a,b,c,d : std_logic ; begin z <= a and b; z <= c and d; Care has to be taken to handle such conditions . . . with a hi-impedance state

a b ? c d

z


Next Class

Concurrent constructs


Concurrent Constructs R.B.Ghongade Lecture 7


Types of concurrent constructs

• when … else • with … select

These constructs need not be in the process


when…else • A concurrent statement which assigns one of several expressions to a signal, depending on the values of Boolean conditions which are tested in sequence • Equivalent to a process containing an if statement Syntax [Label:] Target <= [Options] Expression [after TimeExpression] when Condition else Expression [after TimeExpression] when Condition else ... Expression [after TimeExpression] [when Condition];


Where to use ? architecture – begin – HERE - end block – begin – HERE - end generate – begin – HERE - end Rules: • The reserved word guarded may only appear in a signal assignment within a guarded block. A guarded assignment only executes when the guard expression on the surrounding block is true • An Expression on the right hand side may be replaced by the reserved word “unaffected”


Synthesis • Conditional signal assignments are synthesized to combinational logic • The Expressions on the right hand side are multiplexed onto the Target signal • The resulting logic will be priority encoded, because the conditions are tested in sequence

Remarks: • Conditional and selected signal assignments are a concise way to describe combinational logic in Register Transfer Level descriptions, although processes can be easier to read and maintain in some cases • A conditional assignment is a neat way to convert from a Boolean condition to the type Std_logic


Example z <= a when s1=‘1’ else b when s2=‘1’ else c ;

c MUX21

b

s2

MUX21

a s1

z


Example (Tri-state Buffer) architecture tri_buff of tri_buff_part is begin out1 <= in1 when control=‘1’ else ‘z’; end tri_buff ;

control

in1

out1


with…select • A concurrent statement which assigns one of several expressions to a signal, depending on the value of the expression at the top. • Equivalent to a process containing a case statement Syntax [Label:] with Expression select Target <= [Options] Expression [after TimeExpression] when Choices, Expression [after TimeExpression] when Choices, Expression when others;


Where to use ? architecture – begin – HERE – end block – begin – HERE – end generate – begin – HERE – end Rules: • Every case of the Expression at the top must be covered once and only once by the choices • An Expression on the right hand side may be replaced by the reserved word “unaffected” • All possible choices must be enumerated • “others” clause is important since we have 9valued logic


Synthesis • Selected signal assignments are synthesized to combinational logic • The Expressions on the right hand side are multiplexed onto the Target signal Remarks: • Conditional and selected signal assignments are a good way to describe combinational logic in Register Transfer Level descriptions


Example (Multiplexer) architecture mux41 of mux is -- Assumptions begin -- a,b,c,d,z are with control select -- std_logic z <= a when “00” , -- control is b when “01” ,-- std_logic_vector(1 downto 0) c when “10” , d when “11” , ‘Z’ when others ; end mux41 ;

a b c d

z MUX41

control


Block

• There are two types of blocks – Simple – Guarded


Simple block • The BLOCK statement, in its simple form, represents only a way of locally partitioning the code • It allows a set of concurrent statements to be clustered into a BLOCK, with the purpose of turning the overall code more readable and more manageable (which might be helpful when dealing with long codes)


Syntax label: BLOCK [declarative part] BEGIN (concurrent statements) END BLOCK label;

General form of architecture using block for partitioning

ARCHITECTURE example ... BEGIN ... block1: BLOCK BEGIN ... END BLOCK block1 ; ... block2: BLOCK BEGIN ... END BLOCK block2 ; ... END example ;


• Block can be nested inside another block Syntax

label1: BLOCK [declarative part of top block] BEGIN [concurrent statements of top block] label2: BLOCK [declarative part nested block] BEGIN (concurrent statements of nested block) END BLOCK label2; [more concurrent statements of top block] END BLOCK label1;


Guarded block • A guarded BLOCK is a special kind of BLOCK, which includes an additional expression, called guard expression • A guarded statement in a guarded BLOCK is executed only when the guard expression is TRUE Syntax label: BLOCK (guard expression) [declarative part] BEGIN (concurrent guarded and unguarded statements) END BLOCK label;


• Even though only concurrent statements can be written within a BLOCK, with a guarded BLOCK even sequential circuits can be constructed LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY latch IS PORT (d, clk: IN STD_LOGIC; q: OUT STD_LOGIC); END latch; ARCHITECTURE latch OF latch IS BEGIN b1: BLOCK (clk='1') BEGIN q <= GUARDED d; END BLOCK b1; END latch;

Latch


LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY DFF IS PORT (d, clk, rst: IN STD_LOGIC; q: OUT STD_LOGIC); END DFF;

DFF

ARCHITECTURE DFF OF DFF IS BEGIN b1: BLOCK (clk’EVENT AND clk='1') BEGIN q <= GUARDED ‘0’ WHEN rst=‘1’ ELSE d; END BLOCK b1; END DFF; Here, a positive-edge sensitive D-type flip-flop, with synchronous reset, is designed. In it, clk'EVENT AND clk='1' is the guard expression, while q <= GUARDED '0‘ WHEN rst='1' is a guarded statement. Therefore, q<='0' will occur when the guard expression is true and rst is ‘1’


Homework Problems 1)Generic encoder

2) 8- bit ALU


For ALU in problem 2 sel

Operation

Function

0000 y <= a

Transfer a

0001 y <= a+1

Increment a

0010 y <= a-1

Decrement a

0011 y <= b

Transfer b

0100 y <= b+1

Increment b

0101 y <= b-1

Decrement b

0110 y <= a + b

Add a and b

0111 y <= a + b + cin

Add a and b with carry

1000 y <= NOT a

Complement a

1001 y <= NOT b

Complement b

1010 y <= a AND b

AND

1011 y <= a OR b

OR

1100 y <= a NAND b

NAND

1101 y <= a NOR b

NOR

1110 y <= a XOR b

XOR

1111 y <= a XNOR b

XNOR

Unit

Arithmetic

Logic


3) Priority Encoder

The circuit must encode the address of the input bit of highest order that is active. ‘‘000’’ should indicate that there is no request at the input (no bit active)


Expected waveform for Problem 3


Next Class

Component Instantiation DO NOT MISS IN ANY CASE !


Component Instantiation R.B.Ghongade Lecture 8,9,10


Component • A component is analogous to a chip socket; it gives an indirect way to use one hierarchical block within another • A component is instantiated within an architecture, and is associated with a (lower level) entity and architecture during elaboration using information from a configuration. • A component declaration is similar in form to an entity declaration, in that it includes the required ports and parameters of the component • The difference is that it refers to a design described in a separate VHDL file • The ports and parameters in the component declaration may be a subset of those in the component file, but they must have the same names


Component can be declared in the main code itself


Component can be declared in a package


Syntax : COMPONENT component_name GENERIC ( parameter_name : string := default_value ; parameter_name : integer := default_value); PORT (input_name, input_name : IN STD_LOGIC; bidir_name, bidir_name : INOUT STD_LOGIC; output_name, output_name : OUT STD_LOGIC); END COMPONENT;

Where : package - <HERE> - end architecture - is - <HERE> - begin - end block - <HERE> - begin - end generate - <HERE> - begin - end


Rules: • For default configuration, the component name must match the name of the corresponding entity to be used in its place, and generics and ports must also match in name, mode and type Synthesis: • A component without a corresponding design entity is synthesized as a black box In VHDL'93, components are not necessary. It is possible instead to directly instantiate an entity within an architecture.


Example component Counter generic (N: INTEGER); port (Clock, Reset, Enable: in Std_logic; Q: buffer Std_logic_vector (N-1 downto 0)); end component ;


Instantiation • A concurrent statement used to define the design hierarchy by making a copy of a lower level design entity within an architecture • In VHDL'93, a direct instantiation of an entity bypasses the component and configuration


Syntax: InstanceLabel: [component] ComponentName [GenericMap] [PortMap]; InstanceLabel: entity EntityName[(ArchitectureName)] [GenericMap] [PortMap]; InstanceLabel: configuration ConfigurationName [GenericMap] [PortMap];

Where: architecture – begin - <HERE> - end block – begin - <HERE> - end generate – begin - <HERE> - end


Rules: • An entity, architecture or configuration must be compiled into a library before the corresponding instance can be compiled • However, an instance of a component can be compiled before the corresponding design entity has even been written Example : G1: NAND2 generic map (1.2 ns) port map (N1, N2, N3); G2: entity WORK.Counter(RTL) port map (Clk, Rst, Count);


Generic Map • Used to define the values of generics • Usually given in an Instance, but may also appear in a configuration Syntax generic map ([Formal =>] Actual, ...)

Formal = {either} Name FunctionCall Actual = Expression


Where : Label : ComponentName <HERE> port map (…); for - use - <HERE> port map (…) block – generic (…); <HERE> ; port – begin - end

Rules : The two forms of syntax (ordered list or explicitly named choices) can be mixed, but the ordered list must come before the named choices

A generic map does not end with a semicolon!


Example: architecture Structure of Ent is component NAND2 generic (TPLH, TPHL: TIME := 0 NS); port (A, B: in STD_LOGIC; F : out STD_LOGIC); end component; begin G1: NAND2 generic map (1.9 NS, 2.8 NS) port map (N1, N2, N3); G2: NAND2 generic map (TPLH => 2 NS, TPHL => 3 NS) port map (N4, N5, N6); end Structure;


Port Map • A port map is typically used to define the interconnection between instances in a structural description (or netlist) • A port map maps signals in an architecture to ports on an instance within that architecture • Port maps can also appear in a configuration or a block


Syntax: port map ([Formal =>] Actual, ...);

Formal = {either} Name FunctionCall Actual = {either} Name FunctionCall open Where: Label : ComponentName generic map (…) <HERE>; for - use - generic map (…) <HERE>; block - port (…) ; <HERE>; - begin - end


Rules: • The two forms of syntax (ordered list or explicitly named ports) can be mixed, but the ordered list must come before the named ports • Within an instance, the formals are ports on the component or entity being instanced, the actuals are signals visible in the architecture containing the instance • Within a configuration, the formals are ports on the entity, the actuals are ports on the component • If the actual is a conversion function, this is called implicitly as values are passed in • If the formal is a conversion function, this is called implicitly as values are passed out Use the port names rather than order to improve readability and reduce the risk of making connection errors


Example: component COUNTER port (CLK, RESET: in Std_logic; UpDown: in Std_logic := '0';-- default value Q: out Std_logic_vector(3 downto 0)); end component; ... -- Positional association... G1: COUNTER port map (Clk32MHz, RST, open, Count); -- Named association (order doesn't matter)... G2: COUNTER port map ( RESET => RST, CLK => Clk32MHz, Q(3) => Q2MHz, Q(2) => open, -- unconnected Q(1 downto 0) => Cnt2, UpDown => open);


Top Level Entity and Lower Level Entity

RESET

RST

Clk32MHz

Q2MHz

Cnt2 TOP LEVEL ENTITY

COUNT

CLK

Q

Updown COUNTER (LOWER LEVEL ENTITY)


RESET => RST CLK => Clk32MHz Q(3) => Q2MHz

RST

TOP LEVEL ENTITY

Clk32MHz Q2MHz

RESET

RESET

CLK

CLK

Cnt2

Q Updown

Q(3)

G1 Updown

G2

Q(0)

COUNT

UpDown => open


A still simpler example entity ND4 is port (in1,in2,in3,in4 : in std_logic ; z : out std_logic); end ND4; architecture ND4_CI of ND4 is component ND2 port (a , b : in std_logic; c : out std_logic); end component ; signal temp1, temp2 : std_logic; begin U1 : ND2 port map (a => in1 , b => in2 , c => temp1); U2 : ND2 port map (a => in3 , b => in4 , c => temp2); U3 : ND2 port map (a => temp1 , b => temp2 , c => z); end ND4_CI ;


infers‌ ND4 IN1

a

c => temp1 for U1 a => temp1 for U3

c b

U1

IN2

a c b

U3

IN3

a c b IN4

U2

c => temp2 for U2 b => temp2 for U3

Z


Generate statement • A concurrent statement used to create regular structures or conditional structures during elaboration • Used to create multiple copies of components , processes or blocks • It provides a compact description of regular structures such as memories , registers and counters


• Two flavours of generate statement are: – for … generate • Number of copies is determined by a discrete range

– if … generate • Zero or one copy is made conditionally

• Range must be a computable integer in any of the following forms: – integer_expression to integer_expression – integer_expression downto integer_expression – Each integer_expression evaluates to an integer


Syntax : Label: for ParameterName in Range generate [Declarations... begin] ConcurrentStatements... end generate [Label]; Label: if Condition generate [Declarations... begin] ConcurrentStatements... end generate [Label];


Where: architecture – begin - <HERE> - end block – begin - <HERE> - end generate – begin - <HERE> - end Rules : • The Range and Condition must both be static, i.e. they cannot include signals • The Label at the beginning of the generate statement cannot be omitted Synthesis: • Synthesis is straightforward, but not all synthesis tools support generate!


Example: architecture ABC of full_add4 is component full_add port (PA , PB , PC : in std_logic ; PCOUT , PSUM : out std_logic) ; end component ; signal c: std_logic_vector(4 downto 0); begin c(0) <= cin ; -- cin is declared in entity GK : for k in 3 downto 0 generate FA :full_add port map(A(k),B(k),C(k),C(k+1),SUM(k); end generate GK ; cout <= c(4) ; -- cout is declared in entity end ABC ;


infers… A(3)

Cout

B(3)

A(2)

FA3

B(1)

C(2)

SUM(2)

A(0)

FA1

FA2 C(3)

SUM(3)

A(1)

B(2)

B(0)

FA0 C(1)

SUM(1)

SUM(0)

Cin


Another example architecture SHIFTER_ARCH of SHIFTER is component DFF port (D , CLK : in std_logic ; Q : out std_logic) ; end component ; begin GK : for k in 0 to 3 generate GK0 : IF k=0 generate DFILPFLOP : DFF port map (count , clock , Q(k)); end generate GK0 ; GK1_3 : if k > 0 generate DFILPFLOP : DFF port map (Q(k-1), clock , Q(k)); end generate GK1_3 ; end generate GK ; end SHIFTER_ARCH ;


infers…

CLOCK

COUNT

DF0

DF2

DF1

Q(0)

Q(1)

DF3

Q(2)

Q(3)


Ways to describe a circuit! • Three types of descriptions possible with VHDL – Structural – Dataflow – Behavioral


Structural Method • At the structural level, which is the lowest level, you have to first manually design the circuit. • Use VHDL to specify the components and gates that are needed by the circuit and how they are connected together by following your circuit exactly • Synthesizing a structural VHDL description of a circuit will produce a netlist that is exactly like your original circuit • The advantage of working at the structural level is that you have full control as to what components are used and how they are connected. • But you need to first come up with the circuit and so the full capabilities of the synthesizer are not utilized


Dataflow Method • At the dataflow level, you use the built-in logical functions of VHDL in signal assignment statements to describe a circuit, which again you have to first design manually • Boolean functions that describe a circuit can be easily converted to signal assignment statements using the built-in logical functions • The only drawback is that the built-in logical functions such as the AND and OR function only take two operands. This is like having only 2-input gates to work with ! All the statements use in the structural and dataflow level are executed concurrently


Behavioral Method • Describing a circuit at the behavioral level is most similar to writing a computer program • You have all the standard high-level programming constructs such as the FOR LOOP, WHILE LOOP, IF THEN ELSE, CASE, and variable assignments • The statements are enclosed in a process block and are executed sequentially


Example BCD to 7- segment display decoder a I3 I2 I1 I0

BCD to 7-segment display decoder

segs(6) {seg 'a'} segs(5) {seg 'b'} segs(4) {seg 'c'} segs(3) {seg 'd'} segs(2) {seg 'e'} segs(1) {seg 'f'} segs(0) {seg 'g'}

f

b g

e

c d


Truth-table


Logic Equations a = I 3 + I1 + ( I 2 : I 0 ) b = I 2 + ( I1 : I 0 ) '

c = I 2 + I1 + I 0 '

d = I1I 0 + I 2 I 0 + I 2 I1 + I 2 I1 I 0 '

'

e = I1I 0 + I 2 I 0 '

'

'

'

'

'

f = I 3 + I 2 I1 + I 2 I 0 + I1 I 0 '

'

g = I 3 + ( I 2 ⊕ I1 ) + I1I 0

'

'

'


Logic gates


Structural VHDL description ENTITY myxnor2 IS PORT(i1, i2: IN BIT;o: OUT BIT); END myxnor2; ARCHITECTURE Dataflow OF myxnor2 IS BEGIN o <= not(i1 XOR i2); END Dataflow; ENTITY myxor2 IS PORT(i1, i2: IN BIT;o: OUT BIT); END myxor2; ARCHITECTURE Dataflow OF myxor2 IS BEGIN o <= i1 XOR i2; END Dataflow; ENTITY myand2 IS PORT(i1, i2: IN BIT;o: OUT BIT); END myand2; ARCHITECTURE Dataflow OF myand2 IS BEGIN o <= i1 AND i2; END Dataflow;


ENTITY myand3 IS PORT(i1, i2, i3: IN BIT; o: OUT BIT); END myand3; ARCHITECTURE Dataflow OF myand3 IS BEGIN o <= (i1 AND i2 AND i3); END Dataflow; ENTITY myor2 IS PORT(i1, i2: IN BIT;o: OUT BIT); END myor2; ARCHITECTURE Dataflow OF myor2 IS BEGIN o <= i1 OR i2; END Dataflow; ENTITY myor3 IS PORT(i1, i2, i3: IN BIT;o: OUT BIT); END myor3; ARCHITECTURE Dataflow OF myor3 IS BEGIN o <= i1 OR i2 OR i3; END Dataflow;


ENTITY myor4 IS PORT(i1, i2, i3, i4: IN BIT; o: OUT BIT); END myor4; ARCHITECTURE Dataflow OF myor4 IS BEGIN o <= i1 OR i2 OR i3 OR i4; END Dataflow; ENTITY inv IS PORT (i: IN BIT; o: OUT BIT); END inv; ARCHITECTURE Dataflow OF inv IS BEGIN o <= not i; END Dataflow;


LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY bcd IS PORT(i0, i1, i2, i3: IN BIT; a, b, c, d, e, f, g: OUT BIT); END bcd; ARCHITECTURE Structural OF bcd IS COMPONENT inv PORT (i: IN BIT ;o: OUT BIT); END COMPONENT; COMPONENT myand2 PORT(i1, i2: IN BIT;o: OUT BIT); END COMPONENT; COMPONENT myand3 PORT(i1, i2, i3: IN BIT;o: OUT BIT); END COMPONENT; COMPONENT myor2 PORT(i1, i2: IN BIT;o: OUT BIT); END COMPONENT; COMPONENT myor3 PORT(i1, i2, i3: IN BIT;o: OUT BIT); END COMPONENT; COMPONENT myor4 PORT(i1, i2, i3, i4: IN BIT;o: OUT BIT); END COMPONENT; COMPONENT myxnor2 PORT(i1, i2: IN BIT;o: OUT BIT); END COMPONENT; COMPONENT myxor2 PORT(i1, i2: IN BIT;o: OUT BIT); END COMPONENT;


SIGNAL j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z: BIT; BEGIN U1: INV port map(i2,j); U2: INV port map(i1,k); U3: INV port map(i0,l); U4: myXNOR2 port map(i2, i0, z); U5: myOR3 port map(i3, i1, z, a); U6: myXNOR2 port map(i1, i0, y); U7: myOR2 port map(j, y, b); U8: myOR3 port map(i2, k, i0, c); U9: myAND2 port map(i1, l, x); U10: myAND2 port map(j, l, w); U11: myAND2 port map(j, i1, v); U12: myAND3 port map(i2, k, i0, t); U13: myOR4 port map(x, w, v, t, d); U14: myAND2 port map(i1, l, s); U15: myAND2 port map(j, l, r); U16: myOR2 port map(s, r, e); U17: myAND2 port map(i2, k, q); U18: myAND2 port map(i2, l, p); U19: myAND2 port map(k, l, o); U20: myOR4 port map(i3, q, p, o, f); U21: myXOR2 port map(i2, i1, n); U22: myAND2 port map(i1, l, m); U23: myOR3 port map(i3, n, m, g); END Structural;


Dataflow VHDL description LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY bcd IS PORT ( I: IN STD_LOGIC_VECTOR (3 DOWNTO 0); Segs: OUT std_logic_vector (1 TO 7)); END bcd; ARCHITECTURE Dataflow OF bcd IS BEGIN Segs(1) <= I(3) OR I(1) OR NOT (I(2) XOR I(0)); -- seg a Segs(2) <= (NOT I(2)) OR NOT (I(1) XOR I(0)); -- seg b Segs(3) <= I(2) OR (NOT I(1)) OR I(0); -- seg c Segs(4) <= (I(1) AND NOT I(0)) OR (NOT I(2) AND NOT I(0)) -- seg d OR (NOT I(2) AND I(1)) OR (I(2) AND NOT I(1) AND I(0)); Segs(5) <= (I(1) AND NOT I(0)) OR (NOT I(2) AND NOT I(0)); -- seg e Segs(6) <= I(3) OR (I(2) AND NOT I(1)) -- seg f OR (I(2) AND NOT I(0)) OR (NOT I(1) AND NOT I(0)); Segs(7) <= I(3) OR (I(2) XOR I(1)) OR (I(1) AND NOT I(0)); -- seg g END Dataflow;


Behavioral VHDL description library IEEE; use IEEE.STD_LOGIC_1164.all; entity BCD is port( I : in STD_LOGIC_VECTOR(3 downto 0); segs : out STD_LOGIC_VECTOR(6 downto 0) ); end BCD; architecture Behavioral of BCD is begin with I select Segs <= "1111110" when "0000", "0110000" when "0001", "1101001" when "0010", "1111001" when "0011", "0110011" when "0100", "1011011" when "0101", "1011111" when "0110", "1110000" when "0111", "1111111" when "1000", "1110011" when "1001", "0000000" when others; end

Behavioral;


Output


Assignment No 3

Equations for carry_generate(G) and carry_propagate(P) for ALU 74181

xi

hsi

yi

si

xi-1 x0 yi-1

ci Carry Lookahead Logic

g i= x i . y i pi=xi + yi

y0 c0

ci+1= gi + pi . ci


c1= g0 + p0 . c0

Additional Information

c2= g1 + p1 . g0 + p1.p0.c0

c3= g2 + p2 . g1 + p2.p1.g0+p2.p1.p0.c0

c4= g3 + p3 . g2 + p3.p2.g1+p3.p2.p1.g0+p3.p2.p1.p0.c0


Equations for implementation of G_L , P_L outputs

G_L= (g3+p3.g2+p3.p2.g1+p3.p2.p1.g0)’ P_L=(p3.p2.p1.p0)’

Implement the Carry_Generate and Carry_Propagate outputs also to complete the ALU assignment


Next Class

Sequential Statements


‘generate’d doubt !

A[3:0] B[3:0] Cin

[3:0] [3:0]

[0] [0]

[1] [1] [1]

fulladder PA PB PC

PCOUT PSUM

[1] [0]

[2] [2] [2]

fulladder PA PB PC

PCOUT PSUM

[2] [1]

[3] [3] [3]

fulladder PA PB PC

PCOUT PSUM

[3] [2]

fulladder PA PB PC

PCOUT PSUM

[4] [3]

[4] [3:0]

COUT SUM[3:0]

[4] [3:0]

COUT SUM[3:0]

GK.3.FA

GK.2.FA

GK.1.FA

GK.0.FA

A[3:0] B[3:0] Cin

[3:0] [3:0]

[0] [0]

[1] [1] [1]

fulladder PA PB PC

PCOUT PSUM

[1] [0]

[2] [2] [2]

fulladder PA PB PC

PCOUT PSUM

[2] [1]

[3] [3] [3]

fulladder PA PB PC

PCOUT PSUM

[3] [2]

fulladder PA PB PC

PCOUT PSUM

[4] [3]

GK.3.FA

GK.2.FA

GK.1.FA

GK.0.FA

GK : for k in 3 downto 0 generate

=

GK : for k in 0 to 3 generate


Sequential Statements R.B.Ghongade Lecture 12


Sequential Statements • VHDL code is inherently concurrent • Sections of code that are executed sequentially are : – PROCESS – FUNCTION – PROCEDURE

• One important aspect of sequential code is that it is not limited to sequential logic • We can build sequential circuits as well as combinational circuits • Sequential code is also called behavioral code • Thus a PROCESS is a concurrent statement which describes behaviour


• Sequential statements are allowed only inside PROCESSES, FUNCTIONS, or PROCEDURES • Sequential statements are: – IF – WAIT – CASE – LOOP

• VARIABLES are also restricted to be used in sequential code only VARIABLE can never be global, so its value can not be passed out directly


SIGNALS and VARIABLES revisited ! • VHDL has two ways of passing non-static values around: by means of a SIGNAL or by means of a VARIABLE • A SIGNAL can be declared in a PACKAGE, ENTITY or ARCHITECTURE (in its declarative part), while a VARIABLE can only be declared inside a piece of sequential code • SIGNAL is global while VARIABLE is local • The value of a VARIABLE can never be passed out of the PROCESS directly; if necessary, then it must be assigned to a SIGNAL • Update of VARIABLE is immediate whereas new value for SIGNAL is generally only guaranteed to be available after the conclusion of the present run of the PROCESS • Assignment operator for SIGNAL is “<= “ while that for VARIABLE is “ : = “


Process • A PROCESS is a sequential section of VHDL code • It is characterized by the presence of IF, WAIT, CASE, or LOOP, and by a sensitivity list (except when WAIT is used) • A PROCESS must be installed in the main code, and is executed every time a signal in the sensitivity list changes (or the condition related to WAIT is fulfilled)


Syntax [label:] [postponed] PROCESS (sensitivity list) [VARIABLE name type [range] [:= initial_value;]] BEGIN (sequential code) END [postponed] PROCESS [label];

Where entity - begin - <HERE> - end architecture - begin - <HERE> - end block - begin - <HERE> - end generate - begin - <HERE> - end

“POSTPONED” is a reserved VHDL word


Rules • A process must contain either a sensitivity list or wait statements, but not both • Every process executes once during initialization, before simulation starts • A postponed process is not executed until the final simulation cycle of a particular simulation time, and thus sees the stable values of signals and variables A process with neither a sensitivity list nor a wait will loop forever !


Using EVENT attribute • To construct a synchronous circuit, monitoring a signal (clock, for example) is necessary • A common way of detecting a signal change is by means of the EVENT attribute • For instance, if clk is a signal to be monitored, then clk ’ EVENT returns TRUE when a change on clk occurs (rising or falling edge)


IF construct • A sequential statement which executes one branch from a set of branches dependent upon the conditions, which are tested in sequence Syntax [Label:] if Condition then SequentialStatements... [elsif Condition then SequentialStatements...] ... {any number of elsif parts} [else SequentialStatements...] end if [Label]; Be careful about the spelling of elsif and end if


Synthesis • Assignments within if statements generally synthesize to multiplexers • Incomplete assignments, where outputs remain unchanged for certain input conditions, synthesize to transparent latches in unclocked processes, and to flip-flops in clocked processes • In some circumstances, nested if statements synthesize to multiple logic levels. This can be avoided by using a case statement instead


• A set of elsif branches can be used to impart priority to the conditions tested first • To decode a value without giving priority to certain conditions, use a case statement instead Example IF (x<y) THEN temp:="11111111"; ELSIF (x=y AND w='0') THEN temp := "11110000"; ELSE temp:=(OTHERS =>'0');


D Flip-Flop with asynchronous reset • A D-type flip-flop is the most basic building block in sequential logic circuits. In it, the output must copy the input at either the positive or negative transition of the clock signal (rising or falling edge) If rst = ‘1’, then the output must be q = ‘0’ ,regardless of the status of clk. Otherwise, the output must copy the input (that is, q = d) at the positive edge of clk


LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY dff IS PORT (d, clk, rst: IN STD_LOGIC; q: OUT STD_LOGIC); END dff; ARCHITECTURE behavior OF dff IS BEGIN PROCESS (clk, rst) BEGIN IF (rst='1') THEN q <= '0'; ELSIF (clk'EVENT AND clk='1') THEN q <= d; END IF; END PROCESS; END behavior;


Output

clk d

D[0]

Q[0] R

rst

q

q


Changing the statement ELSIF (clk'EVENT AND clk=‘0') THEN

clk d

D[0]

Q[0] R

rst

q

q


One Digit counter example Progressive 1-digit decimal counter (0 -> 9 ->0) Single bit input (clk) and a 4-bit output (digit).


LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY counter IS PORT (clk : IN STD_LOGIC; digit : OUT INTEGER RANGE 0 TO 9); END counter; ARCHITECTURE counter OF counter IS BEGIN count: PROCESS (clk) VARIABLE temp : INTEGER RANGE 0 TO 10; BEGIN IF (clk'EVENT AND clk='1') THEN temp := temp + 1; IF (temp=10) THEN temp := 0; END IF; END IF; digit <= temp; END PROCESS count; END counter;


Output

In a counter like circuits always use comparison statements with constant values This ensures simple comparator inference as against full comparator inference for comparison with unknown values


[3]

digit[3:0]

[1] [32]

[0] [31]

[31]

0

[29]

0

1

count.un9_temp

[1]

[3]

temp_3[1]

[1] [32]

D[3:0]

Q[3:0]

count.temp[3:0]

[3:0] [3:0]

+

1

[29]

un2_temp[29:32]

[31:32]

clk

[29]

0

0

1

[3]

temp_3[3]

Changing the statement IF (temp>=10) THEN temp := 0; [3] [30] [1] [32] [29:32] 1010

<

count.un1_temp

0

0

[31]

1

[1]

temp_3[1] clk

Extra Hardware

0

0

[29]

1

[3]

temp_3[3]

[3] [30] [1] [32]

D[3:0]

Q[3:0]

count.temp[3:0]

[3:0] [3:0] 1

+

[29:32]

un2_temp[29:32]

digit[3:0]


Next Class

Sequential Statements cont..


Sequential Statements II R.B.Ghongade Lecture 13


Wait statement • The operation of WAIT is sometimes similar to that of IF • PROCESS cannot have a sensitivity list when WAIT is employed • Three flavours of WAIT statements are: – WAIT UNTIL – WAIT ON – WAIT FOR


Syntax

WAIT UNTIL signal_condition; • The WAIT UNTIL statement accepts only one signal, thus being more appropriate for synchronous code than asynchronous • Since the PROCESS has no sensitivity list in this case, WAIT UNTIL must be the first statement in the PROCESS • The PROCESS will be executed every time the condition is met


Example ( 8-bit register ) PROCESS -- no sensitivity list BEGIN WAIT UNTIL (clk'EVENT AND clk='1'); IF (rst='1') THEN op <= "00000000"; ELSIF (clk'EVENT AND clk='1') THEN op <= inp; END IF; END PROCESS;


Output and Inference

Output changes only with clk

clk inp[7:0] rst

[7:0] [7:0]

D[7:0] R

Q[7:0]

op[7:0]

[7:0] [7:0]

op[7:0]


Syntax

WAIT ON signal1 [, signal2, ... ];

• The WAIT ON statement accepts multiple signals • The PROCESS is put on hold until any of the signals listed changes


Example ( 8-bit register ) PROCESS BEGIN WAIT ON clk, rst; IF (rst='1') THEN op <= "00000000"; ELSIF (clk'EVENT AND clk='1') THEN op <= inp; END IF; END PROCESS;


Output and Inference

Output changes with clk and rst

clk inp[7:0]

[7:0] [7:0]

D[7:0]

Q[7:0] R

rst

op[7:0]

[7:0] [7:0]

op[7:0]


DFF revisited with WAIT! LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY dff IS PORT (d, clk, rst: IN STD_LOGIC; q: OUT STD_LOGIC); END dff; ARCHITECTURE dff OF dff IS BEGIN PROCESS BEGIN WAIT ON rst, clk; IF (rst='1') THEN q <= '0'; ELSIF (clk'EVENT AND clk='1') THEN q <= d; END IF; END PROCESS; END dff;


Output and Inference

clk d

D[0]

Q[0]

q

R rst

q

Infers exactly the same hardware as the earlier design As a homework problem repeat the one digit counter with WAIT statement


Syntax

WAIT FOR time; • WAIT FOR is intended for simulation only (waveform generation for test-benches)


Case statement • CASE is another statement intended exclusively for sequential code • The CASE statement (sequential) is very similar to WHEN (combinational) • All permutations must be tested, so the keyword OTHERS is often helpful • Another important keyword is NULL (the counterpart of UNAFFECTED), which should be used when no action is to take place • CASE allows multiple assignments for each test condition while WHEN allows only one


Syntax [Label:] case Expression is when Choices => SequentialStatements... when Choices => SequentialStatements... ... {any number of when parts} end case [Label];

Choices = Choice | Choice | ... Choice = {either} ConstantExpression Range others {the last branch}


Where process – begin - <HERE> - end function – begin - <HERE> - end procedure – begin - <HERE> - end if – then - <HERE> - elsif – then - <HERE>else - <HERE> - end case - => - <HERE> - when - => - <HERE>end loop-<HERE>-end


Rules • The Expression must not be enclosed in parenthesis • The type of the Expression must be enumeration, integer, physical, or a one dimensional array • Every case of the Expression must be covered once and only once by the Choices


Synthesis • Assignments within case statements generally synthesize to multiplexers • Incomplete assignments (i.e. where outputs remain unassigned for certain input conditions) in unclocked processes synthesize to transparent latches • Incomplete assignments in clocked processes synthesize to recirculation around registers


Example case ADDRESS when 0 => -A <= '1'; when 1 => A <= '1'; --B <= '1'; when 2 to 15

is Select a single value

More than one statement in a branch => -- Select a range of ADDRESS -- values

C <= '1'; when 16 | 20 | 24 => -- Pick out several -- ADDRESS values B <= '1'; C <= '1'; D <= '1'; when others => -- Mop up the rest null; end case;


2 - digit counter with SSD output a

a f

b

f

b

g e

g

c d

e

c d

CLK COUNTER 7 BITSDIGIT 2 DIGIT 1

RST

Progressive 2-digit decimal counter (0-> 99-> 0) , with external asynchronous reset plus binary-coded decimal (BCD) to seven-segment display (SSD) conversion


LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY counter IS PORT (clk, rst : IN STD_LOGIC; digit1, digit2 : OUT STD_LOGIC_VECTOR (6 DOWNTO 0)); END counter; ARCHITECTURE counter OF counter IS BEGIN PROCESS (clk, rst) VARIABLE temp1: INTEGER RANGE 0 TO 10; VARIABLE temp2: INTEGER RANGE 0 TO 10; BEGIN IF (rst='1') THEN temp1 := 0; temp2 := 0; ELSIF (clk'EVENT AND clk='1') THEN temp1 := temp1 + 1; IF (temp1=10) THEN temp1 := 0; temp2 := temp2 + 1; IF (temp2=10) THEN temp2 := 0; END IF; END IF; END IF;


CASE temp1 IS WHEN 0 => digit1 <= "1111110"; --7E WHEN 1 => digit1 <= "0110000"; --30 WHEN 2 => digit1 <= "1101101"; --6D WHEN 3 => digit1 <= "1111001"; --79 WHEN 4 => digit1 <= "0110011"; --33 WHEN 5 => digit1 <= "1011011"; --5B WHEN 6 => digit1 <= "1011111"; --5F WHEN 7 => digit1 <= "1110000"; --70 WHEN 8 => digit1 <= "1111111"; --7F WHEN 9 => digit1 <= "1111011"; --7B WHEN OTHERS => NULL; END CASE; CASE temp2 IS WHEN 0 => digit2 <= "1111110"; --7E WHEN 1 => digit2 <= "0110000"; --30 WHEN 2 => digit2 <= "1101101"; --6D WHEN 3 => digit2 <= "1111001"; --79 WHEN 4 => digit2 <= "0110011"; --33 WHEN 5 => digit2 <= "1011011"; --5B WHEN 6 => digit2 <= "1011111"; --5F WHEN 7 => digit2 <= "1110000"; --70 WHEN 8 => digit2 <= "1111111"; --7F WHEN 9 => digit2 <= "1111011"; --7B WHEN OTHERS => NULL; END CASE; END PROCESS; END counter;


Output


Inference

DO NOT TRY TO WORK OUT HOW THIS CIRCUIT WORKS !!!


Next Class

Test !!!


Sequential Statements III R.B.Ghongade Lecture 14


Loop statement • LOOP is useful when a piece of code must be instantiated several times • Like IF, WAIT, and CASE, LOOP is intended exclusively for sequential code, so it too can only be used inside a PROCESS, FUNCTION, or PROCEDURE. • There are several ways of using LOOP • A loop is an infinite loop (and thus an error) unless it contains an exit or wait statement


Syntax [label:] LOOP (SequentialStatements) END LOOP [label]; FOR / LOOP: The loop is repeated a fixed number of times [label:] FOR identifier IN range LOOP (sequential statements) END LOOP [label]; WHILE / LOOP: The loop is repeated until a condition no longer holds [label:] WHILE condition LOOP (sequential statements) END LOOP [label];


WAIT : Continue looping until..

EXIT: Used for ending the loop [label:] EXIT [label] [WHEN condition];

NEXT: Used for skipping loop steps [label:] NEXT [loop_label] [WHEN condition];


Example of FOR / LOOP:

The loop will be repeated unconditionally until i reaches 5 (that is, six times)

FOR i IN 0 TO 5 LOOP x(i) <= enable AND w(i+2); y(0, i) <= w(i); END LOOP;

• One important remark regarding FOR / LOOP is that both limits of the range must be static • Thus a declaration of the type "FOR i IN 0 TO choice LOOP", where choice is an input (non-static) parameter, is generally not synthesizable


Example of WHILE / LOOP: WHILE (i < 10) LOOP WAIT UNTIL clk'EVENT AND clk='1'; (other statements) END LOOP;

Example of WAIT in LOOP : LOOP WAIT UNTIL Clock = '1'; EXIT WHEN Reset = '1'; Div2 <= NOT Div2; END LOOP;

In this example, LOOP will keep repeating while i < 10


Example of EXIT : FOR i IN data'RANGE LOOP CASE data(i) IS WHEN '0' => count:=count+1; WHEN OTHERS => EXIT; END CASE; END LOOP;

EXIT implies not an escape from the current iteration of the loop, but rather a definite exit (that is, even if i is still within the data range, the LOOP statement will be considered as concluded). In this case, the loop will end as soon as a value different from ‘0’ is found in the data vector


Example with NEXT: FOR i IN 0 TO 15 LOOP NEXT WHEN i=skip; -- jumps to next iteration (...) END LOOP;

NEXT causes LOOP to skip one iteration when i = skip


Synthesis • Not generally synthesizable. Some tools do allow loops containing wait statements to describe implicit finite state machines, but this is not a recommended practice


Example : 8-bit unsigned Carry Ripple Adder LOWER LEVEL

a

b TOP LEVEL

cin

+

cout a

b

a0 b0

s

cin

c0

+

s0

a1 b1 c1

a7 b7 c2

+

c7

s1 s

sj = aj XOR bj XOR cj cj+1 = (aj AND bj) OR (aj AND cj) OR (bj AND cj)

+

s7

c8

cout


LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY adder IS GENERIC (length : INTEGER := 8); PORT ( a, b: IN STD_LOGIC_VECTOR (length-1 DOWNTO 0); cin: IN STD_LOGIC; s: OUT STD_LOGIC_VECTOR (length-1 DOWNTO 0); cout: OUT STD_LOGIC); END adder; ARCHITECTURE adder OF adder IS BEGIN PROCESS (a, b, cin) VARIABLE carry : STD_LOGIC_VECTOR (length DOWNTO 0); BEGIN carry(0) := cin; FOR i IN 0 TO length-1 LOOP s(i) <= a(i) XOR b(i) XOR carry(i); carry(i+1) := (a(i) AND b(i)) OR (a(i) AND carry(i)) OR (b(i) AND carry(i)); END LOOP; cout <= carry(length); END PROCESS; END adder;


Output


Example: Leading Zeros The circuit should count the number of leading zeros in a binary vector, starting from the left end

0 0 0 1 1 0 0 0

3 2 1 0


LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY LeadingZeros IS PORT ( data: IN STD_LOGIC_VECTOR (7 DOWNTO 0); zeros: OUT INTEGER RANGE 0 TO 8); END LeadingZeros; ARCHITECTURE behavior OF LeadingZeros IS BEGIN PROCESS (data) VARIABLE count: INTEGER RANGE 0 TO 8; BEGIN count := 0; FOR i IN data'RANGE LOOP CASE data(i) IS WHEN '0' => count := count + 1; WHEN OTHERS => EXIT; END CASE; END LOOP; zeros <= count; END PROCESS; END behavior;


Output

Modify the design for trailing zeros !


Comparison between Concurrent and Sequential Constructs R.B.Ghongade Lecture 15


CASE versus IF • Though in principle the presence of ELSE in the IF/ELSE statement might infer the implementation of a priority decoder (which would never occur with CASE), this will generally not happen • When IF (a sequential statement) is used to implement a fully combinational circuit, a multiplexer might be inferred instead • Therefore, after optimization, the general tendency is for a circuit synthesized from a VHDL code based on IF not to differ from that based on CASE


Same inference ! ---- With IF: -------------IF (sel="00") THEN x<=a; ELSIF (sel="01") THEN x<=b; ELSIF (sel="10") THEN x<=c; ELSE x<=d; ---- With CASE: -----------CASE sel IS WHEN "00" => x<=a; WHEN "01" => x<=b; WHEN "10" => x<=c; WHEN OTHERS => x<=d; END CASE;


CASE versus WHEN • CASE and WHEN are very similar. However, while one is concurrent (WHEN), the other is sequential (CASE) WHEN

CASE

Statement type

Concurrent

Sequential

Usage

Only outside PROCESSES, FUNCTIONS, or PROCEDURES

Only inside PROCESSES, FUNCTIONS, or PROCEDURES

All permutations must be tested

Yes for WITH/SELECT/WHEN

Yes

Max. # of assignments per test No-action keyword

One

Any

UNAFFECTED

NULL


Same functionality ! ---- With WHEN: ---------------WITH sel SELECT x <= aWHEN "000", b WHEN "001", c WHEN "010", UNAFFECTED WHEN OTHERS; ---- With CASE: ---------------CASE sel IS WHEN "000" => x<=a; WHEN "001" => x<=b; WHEN "010" => x<=c; WHEN OTHERS => NULL; END CASE;


Using Sequential Code to Design Combinational Circuits • Sequential code can be used to implement either sequential or combinational circuits • Registers are for sequential circuits necessary, so will be inferred by the compiler • This should not happen for combinational circuits • Also for a combinational circuit, the complete truth-table should be clearly specified in the code


Rules to be followed while using sequential code for designing combinational circuits • Rule 1: Make sure that all input signals used (read) in the PROCESS appear in its sensitivity list • Rule 2: Make sure that all combinations of the input/output signals are included in the code; that is, make sure that, by looking at the code, the circuit’s complete truth-table can be obtained (indeed, this is true for both sequential as well as concurrent code)


• Failing to comply with rule 1 will generally cause the compiler to simply issue a warning saying that a given input signal was not included in the sensitivity list, and then proceed as if the signal were included. Even though no damage is caused to the design in this case, it is a good design practice to always take rule 1 into consideration • With respect to rule 2, however, the consequences can be more serious because incomplete specifications of the output signals might cause the synthesizer to infer latches in order to hold their previous values


Bad Combinational Design

x should behave as a multiplexer; that is, should be equal to the input selected by sel; y, on the other hand, should be equal to ‘0’ when sel = ‘‘00’’, or ‘1’ if sel = ‘‘01’’


LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY example IS PORT (a, b, c, d: IN STD_LOGIC; sel: IN INTEGER RANGE 0 TO 3; x,y: OUT STD_LOGIC); END example; ARCHITECTURE example OF example IS BEGIN PROCESS (a, b, c, d, sel) BEGIN IF (sel=0) THEN x<=a; y<=‘0’; ELSIF (sel=1) THEN x<=b; y<=‘1’; ELSIF (sel=2) THEN x<=c; ELSE x<=d; END IF; END PROCESS; END example;


Output

Value of y remains ‘1’ which is not expected

Cure for the problem: y<='X'; must be included for other conditions of sel i.e. 2 & 3


Some templates for use! process (Inputs) -- All inputs in sensitivity list begin ... -- Outputs assigned for all input conditions ... -- No feedback end process; -- Gives pure combinational logic

process (Inputs) -- All inputs in sensitivity list begin if Enable = '1' then ... -- Latched actions end if; end process; -- Gives transparent latches + logic


process (Clock) -- Clock only in sensitivity list begin if Rising_edge(Clock) then -- Test clock edge only ... -- Synchronous actions end if; end process; -- Gives flipflops + logic

process (Clock, Reset) -- Clock and reset only in -- sensitivity list begin if Reset = '0' then -- Test active level of -- asynchronous reset ... -- Asynchronous actions elsif Rising_edge(Clock) then -- Test clock edge -- only ... -- Synchronous actions end if; end process; -- Gives flipflops + logic


process -- No sensitivity list begin wait until Rising_edge(Clock); ... -- Synchronous actions end process; -- Gives flipflops + logic


Implementing RAM

The circuit has a data input bus (data_in), a data output bus (data_out), an address bus (addr), plus clock (clk) and write enable (wr_ena) pins When wr_ena is asserted, at the next rising edge of clk the vector present at data_in must be stored in the position specified by addr The output, data_out, on the other hand, must constantly display the data selected by addr


LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY ram IS GENERIC ( bits: INTEGER := 8; -- # of bits per word words: INTEGER := 16); -- # of words in the memory PORT ( wr_ena, clk: IN STD_LOGIC; addr: IN INTEGER RANGE 0 TO words-1; data_in: IN STD_LOGIC_VECTOR (bits-1 DOWNTO 0); data_out: OUT STD_LOGIC_VECTOR (bits-1 DOWNTO 0)); END ram; ARCHITECTURE ram OF ram IS TYPE vector_array IS ARRAY (0 TO words-1) OF STD_LOGIC_VECTOR (bits-1 DOWNTO 0); SIGNAL memory: vector_array; BEGIN PROCESS (clk, wr_ena) BEGIN IF (wr_ena='1') THEN IF (clk'EVENT AND clk='1') THEN memory(addr) <= data_in; END IF; END IF; END PROCESS; data_out <= memory(addr); END ram;


Output

data_in[7:0] addr[3:0] wr_ena clk

RADDR[3:0]

[7:0]

DATA[7:0]

[7:0] [3:0] [3:0]

ram1

[3:0]

WADDR[3:0] WE[0] CLK

DOUT[7:0]

memory[7:0]

[7:0] [7:0]

data_out[7:0]


Packages, Functions and Procedures R.B.Ghongade Lecture 16,17


Fundamental units of VHDL code

COVERED SO FAR


Packages • Frequently used pieces of VHDL code are usually written in the form of COMPONENTS, FUNCTIONS, or PROCEDURES • Such codes are then placed inside a PACKAGE and compiled into the destination LIBRARY • Packages allow code partitioning, code sharing, and code reuse • Besides COMPONENTS, FUNCTIONS, and PROCEDURES, it can also contain TYPE and CONSTANT definitions • A package is split into a declaration and a body • The package declaration defines the external interface to the package, the package body typically contains the bodies of any functions or procedures defined in the package declaration


Syntax {declaration} package PackageName is Declarations... end [package] [PackageName]; {body} package body PackageName is Declarations... end [package body] [PackageName];


REMEMBER THIS !

• Where a function or procedure is placed in a package, the declaration and body must conform, i.e. the parameters must be identical between the two • Only definitions placed in the package declaration are visible outside the package Common, shared declarations of types, subtypes, constants, procedures, functions and components are best put in a package


A simple package LIBRARY ieee; USE ieee.std_logic_1164.all; PACKAGE my_package IS TYPE state IS (st1, st2, st3, st4); TYPE color IS (red, green, blue); CONSTANT vec: STD_LOGIC_VECTOR(7 DOWNTO 0) := "11111111"; END my_package;

The example above shows a PACKAGE called my_package. It contains only TYPE and CONSTANT declarations, so a PACKAGE BODY is not necessary


Example LIBRARY ieee; USE ieee.std_logic_1164.all; PACKAGE my_package IS TYPE state IS (st1, st2, st3, st4); TYPE color IS (red, green, blue); CONSTANT vec: STD_LOGIC_VECTOR(7 DOWNTO 0) := "11111111"; FUNCTION positive_edge(SIGNAL s: STD_LOGIC) RETURN BOOLEAN; END my_package; PACKAGE BODY my_package IS FUNCTION positive_edge(SIGNAL s: STD_LOGIC) RETURN BOOLEAN IS BEGIN RETURN (s'EVENT AND s='1'); END positive_edge; END my_package;

• • •

This example contains, besides TYPE and CONSTANT declarations, a FUNCTION. Therefore, a PACKAGE BODY is now needed This function returns TRUE when a positive edge occurs on clk


• Any of the PACKAGES in the previous examples can now be compiled, becoming then part of our work LIBRARY (or any other) • To make use of it in a VHDL code, we have to add a new USE clause to the main code (USE work.my_package.all), as shown below: LIBRARY ieee; USE ieee.std_logic_1164.all; USE work.my_package.all; ENTITY ... ... ARCHITECTURE ... ...


Functions & Procedures • FUNCTIONS and PROCEDURES are collectively called subprograms • They are very similar to a PROCESS , for they are the only pieces of sequential VHDL code, and thus employ the same sequential statements (IF, CASE, and LOOP; WAIT is not allowed) • PROCESS is intended for immediate use in the main code, FUNCTIONS & PROCEDURES are intended mainly for LIBRARY allocation, that is, their purpose is to store commonly used pieces of code, so they can be reused or shared by other projects. • A FUNCTION or PROCEDURE can also be installed in the main code itself


Functions • Used to group together executable, sequential statements to define new mathematical or logical functions • Also used to define bus resolution functions, operators, and conversion functions between data types • When defined in a package, the function must be split into a declaration and a body


Syntax: Function Body FUNCTION function_name [<parameter list>] RETURN data_type IS [declarations] BEGIN (sequential statements) END function_name;

<parameter list> = [CONSTANT] constant_name : constant_type ; or <parameter list> = SIGNAL signal_name : signal_type ;


Where :(FUNCTION DECLARATION) package - <HERE> - end package body - <HERE> - end entity – is - <HERE> - begin - end architecture –is - <HERE> - begin- end block - <HERE> - begin - end generate - <HERE> - begin - end process - <HERE> - begin - end function – is - <HERE> - begin - end procedure – is - <HERE> - begin - end Function Body is NOT allowed inside a Package


Rules : • The function_name may be an identifier or an operator • Functions cannot assign signals or variables defined outside themselves, nor can then contain wait statements • A function must execute a return statement • Pure functions cannot have side effects they must do nothing but return a value


• There can be any number of such parameters (even zero), can only be CONSTANT (default) or SIGNAL (VARIABLES are not allowed). • Their types can be any of the synthesizable data types (BOOLEAN,STD_LOGIC, INTEGER, etc.) • No range specification should be included (for example, do not enter RANGE when using INTEGER, or TO/DOWNTO when using STD_LOGIC_VECTOR) • On the other hand, there is only one return value, whose type is specified by data_type


REMEMBER THIS !

• The return type must be a name; it cannot include a constraint • Variables defined inside a function are initialized each time the function is called • The declaration and body must conform, i.e. the parameters and return type must be identical between the two • The function declaration ends with a ";", whereas the function body has is at the corresponding point in the syntax


Synthesis : • Each call to a function is synthesized as a separate block of combinational logic Example : FUNCTION f1 (a, b: INTEGER; SIGNAL c: STD_LOGIC_VECTOR) RETURN BOOLEAN IS BEGIN (sequential statements) END f1;

• The function, named f1, receives three parameters (a, b, and c) • a and b are CONSTANTS (notice that the word CONSTANT can be omitted, for it is the default object), while c is a SIGNAL. • a and b are of type INTEGER, while c is of type STD_LOGIC_VECTOR • Notice that neither RANGE nor DOWNTO was specified. • The output parameter (there can be only one) is of type BOOLEAN


Function Call • A function is called as part of an expression. The expression can obviously appear by itself or associated to a statement (either concurrent or sequential) Examples of function calls: x <= conv_integer(a); -- converts a to an integer -- (expression appears by -- itself) y <= maximum(a, b); -- returns the largest of a -- and b -- (expression appears by itself) IF x > maximum(a, b) ... -- compares x to the -- largest of a, b -- (expression associated to a -- statement)


Function positive_edge( ) • The FUNCTION below detects a positive (rising) clock edge. • It is similar to the IF (clk’EVENT and clk = ‘1’) statement ------ Function body: ----------------------------FUNCTION positive_edge(SIGNAL s: STD_LOGIC) RETURN BOOLEAN IS BEGIN RETURN (s'EVENT AND s='1'); END positive_edge; ------ Function call: ----------------------------... IF positive_edge(clk) THEN... ...


Function locations PACKAGE + (PACKAGE BODY)

LIBRARY

FUNCTION/PROCEDURE LOCATION ENTITY MAIN CODE

ARCHITECTURE DECLARATIVE PART


Though a FUNCTION is usually placed in a PACKAGE (for code partitioning, code reuse, and code sharing purposes), it can also be located in the main code (either inside the ARCHITECTURE or inside the ENTITY)

When placed in a PACKAGE, then a PACKAGE BODY is necessary, which must contain the body of each FUNCTION (or PROCEDURE) declared in the declarative part of the PACKAGE


FUNCTION Located in the Main Code LIBRARY ieee; USE ieee.std_logic_1164.all; ENTITY dff IS PORT ( d, clk, rst: IN STD_LOGIC; q: OUT STD_LOGIC); END dff; ARCHITECTURE my_arch OF dff IS FUNCTION positive_edge(SIGNAL s: STD_LOGIC) RETURN BOOLEAN IS BEGIN RETURN s'EVENT AND s='1'; END positive_edge; BEGIN PROCESS (clk, rst) BEGIN IF (rst='1') THEN q <= '0'; ELSIF positive_edge(clk) THEN q <= d; END IF; END PROCESS; END my_arch;


FUNCTION Located in a PACKAGE LIBRARY ieee; USE ieee.std_logic_1164.all; PACKAGE my_package IS FUNCTION positive_edge(SIGNAL s: STD_LOGIC) RETURN BOOLEAN; END my_package; PACKAGE BODY my_package IS FUNCTION positive_edge(SIGNAL s: STD_LOGIC) RETURN BOOLEAN IS BEGIN RETURN s'EVENT AND s='1'; END positive_edge; END my_package;


LIBRARY ieee; USE ieee.std_logic_1164.all; USE work.my_package.all;

WORK DESIGN SPACE SHOULD BE VISIBLE TO THE TOP-LEVEL ENTITY

ENTITY dff IS PORT ( d, clk, rst: IN STD_LOGIC; q: OUT STD_LOGIC); END dff; ARCHITECTURE my_arch OF dff IS BEGIN PROCESS (clk, rst) BEGIN IF (rst='1') THEN q <= '0'; ELSIF positive_edge(clk) THEN q <= d; END IF; END PROCESS; END my_arch;


Function conv_integer( )

conv_integer( ) function converts a STD_LOGIC_VECTOR value into an INTEGER value

LIBRARY ieee; USE ieee.std_logic_1164.all; PACKAGE my_package IS FUNCTION conv_integer (SIGNAL vector: STD_LOGIC_VECTOR) RETURN INTEGER; END my_package; PACKAGE BODY my_package IS FUNCTION conv_integer (SIGNAL vector: STD_LOGIC_VECTOR) RETURN INTEGER IS VARIABLE result: INTEGER RANGE 0 TO 2**vector'LENGTH-1; BEGIN IF (vector ( vector'HIGH )='1') THEN result:=1; ELSE result:=0; END IF; FOR i IN (vector'HIGH-1) DOWNTO (vector'LOW) LOOP result:=result*2; IF(vector(i)='1') THEN result:=result+1; END IF; END LOOP; RETURN result; END conv_integer; END my_package;


Algorithm 3

2

1

0

0

1

0

1

vector’LENGTH=4 vector’HIGH=3 vector ( vector'HIGH )= 0

vector Since ‘ vector ( vector'HIGH )= 0 ’

result=0 Iteration 1:

3 downto 0 ( i = 2 to start with ) result= 0 x 2 result= 0 + 1

since vector(2)= 1 ( i = 1)

Iteration 2: result= 1 x 2 result= 2 + 0

since vector(1)= 0 ( i = 0)

Iteration 3: result= 2 x 2 result= 4 + 1 RETURNED : result = 5

since vector(0)= 1


LIBRARY ieee; USE ieee.std_logic_1164.all; USE work.my_package.all; ENTITY conv_int2 IS PORT ( a: IN STD_LOGIC_VECTOR(0 TO 3); y: OUT INTEGER RANGE 0 TO 15); END conv_int2; ARCHITECTURE my_arch OF conv_int2 IS BEGIN y <= conv_integer(a); END my_arch;


PROCEDURE • A PROCEDURE is very similar to a FUNCTION and has the same basic purposes • A procedure can return more than one value • Like a FUNCTION, two parts are necessary to construct and use a PROCEDURE: – the procedure itself (procedure body) – procedure call


Procedure Body PROCEDURE procedure_name [<parameter list>] IS [declarations] BEGIN (sequential statements) END procedure_name;

<parameter list> = [CONSTANT] constant_name: mode type; <parameter list> = SIGNAL signal_name: mode type; <parameter list> = VARIABLE variable_name: mode type;


A PROCEDURE can have any number of IN, OUT, or INOUT parameters, which can be SIGNALS, VARIABLES, or CONSTANTS. For input signals (mode IN), the default is CONSTANT, whereas for output signals (mode OUT or INOUT) the default is VARIABLE WAIT, SIGNAL declarations, and COMPONENTS are not synthesizable when used in a FUNCTION. The same is true for a PROCEDURE, with the exception that a SIGNAL can be declared, but then the PROCEDURE must be declared in a PROCESS. Moreover, besides WAIT, any other edge detection is also not synthesizable with a PROCEDURE (that is, contrary to a function, a synthesizable procedure should not infer registers)


Example PROCEDURE my_procedure ( a: IN BIT; SIGNAL b, c: IN BIT; SIGNAL x: OUT BIT_VECTOR(7 DOWNTO 0); SIGNAL y: INOUT INTEGER RANGE 0 TO 99) IS BEGIN ... END my_procedure; • •

The PROCEDURE has three inputs, a, b, and c (mode IN) a is a CONSTANT of type BIT, while b and c are SIGNALS, also of type BIT. Notice that the word CONSTANT can be omitted for input parameters, for it is the default object (recall, however, that for outputs the default object is VARIABLE) There are also two return signals, x (mode OUT, type BIT_VECTOR) and y (mode INOUT, type INTEGER)


Procedure Call • Contrary to a FUNCTION, which is called as part of an expression, a PROCEDURE call is a statement on its own • It can appear by itself or associated to a statement (either concurrent or sequential) Examples of procedure calls: compute_min_max(in1, in2, 1n3, out1, out2); -- statement by itself divide (dividend, divisor, quotient, remainder); -- statement by itself IF (a>b) THEN compute_min_max(in1, in2, 1n3, out1, out2); -- procedure call -- associated to another -- statement


SIMULATION ISSUES R.B.Ghongade Lecture 20,21,22


SIMULATION ISSUES • • • •

SIMULATION SIMULATION PROCESS DELAY MODELING TYPES OF SIMULATION


Simulation • Simulation is a functional emulation of a circuit design through software programs, that use models to replicate how a device will perform in terms of timing and results


Simulation • Simulation eliminates the time-consuming need for constant physical prototyping • Simulation can be performed during ALL stages of verification • Motivation of simulation comes from the need to verify that the HDL code is correctly implementing the design • Simply verify that the design meets its required specification


Flavours of Simulation • Functional Simulation: Is functional verification of the design without any delays • Pre- Layout simulation: Is functional verification of the design including logic cell delays • Post- Layout simulation: Is performed after physical place and route( interconnect delays are taken into account)


Simulation at different Levels

0 a1 Vcc1 b1 b2 a2 b3 a3 a4 GND b4 0

1 2 3 4

0 1 2 3 4

a1

Vcc1

b1

a2

b2

a3

b3

a4

GND

b4

5 6 7

1 2 3 4

0 a1 Vcc1 b1 a2 b2 a3 b3 a4 GND b4 0

5 6 7 8

1 2 3 4

0 a1 Vcc1 b1 a2 b2 a3 b3 a4 GND b4 0

5 6 7 8

Register

5 6 7 8

A

Q1

D

Q4

ENB

8

Long card

0

Module

Subsystem

Chip

System


Comparison for simulation Module Difficult Higher

Subsystem

Chip

Stimuli Development

Easier

Simulation Efficiency

Low

Nature of detected problems Low level Low Early

System

Bug correction time Time in project

All kinds Higher Late


Steps in simulation ELABORATION

INITIALISATION

EXECUTION


Steps in Simulation • COMPILATION * Checks VHDL source code to check syntax and semantic rules of VHDL * If a syntax or semantic error occurs, then the compiler flags off an error message * Else the compiler generates an intermediate MECHANISM code SEMANTICS

SCIENCE OF MEANING

SYNTAX

OF LANGUAGE


Steps in Simulation

cont...

• ELABORATION * Ports are created for each instance of a component * Memory storage is allocated for the required signal * Interconnections among the port signals are specified * Mechanism is established for executing the VHDL process in proper sequence


Steps in Simulation

cont...

• INITIALIZATION * Initial values preset in the declarations statements are assigned to signals/variables


Steps in Simulation

cont...

• EXECUTION * Every process is executed until it suspends. Signal values are updated only after this. * Simulator accepts simulation commands like: RUN, ASSIGN,WATCH , which control the simulation of the system * Simulation ends when all signals have been updated and new values have been assigned to the signals


SIMULATION PROCESS BASIC DEFINITIONS •EVENT A CHANGE ON COMPONENT OUTPUT THAT WILL BE PROPAGATED •TRANSACTION A CHANGE ON COMPONENT INPUT THAT DOES NOT PROPAGATE


SIMULATION PROCESS BASIC DEFINITIONS

cont.

•SCHEDULING FUTURE EVENTS GENERATED BY EVALUATION OF A PROCESS •EVALUATION CALCULATING THE OUTPUTS OF COMPONENTS BASED ON INPUTS AND CURRENT STATE


SIMULATION PROCESS Time Jargon

Real Time (As it happens‌)

Simulation Time (Relative to some arbitrary t=0)


SIMULATION PROCESS Simulation executes as follows: At t=0 , all gate outputs are set to an unknown value Two queues are set up

SIGNALS TO BE UPDATED

PROCESSES TO BE EXECUTED


SIMULATION PROCESS • When Simulation time is incremented : • Signal is updated • All processes sensitive to that signal are placed on the process execution queue One loop is called DELTA CYCLE

• Each resumed process is executed until it suspends • Effects of the logic changes that have occurred as a result of process execution are evaluated • Simulation time is set to the next event in queue or halted if simulation time gets exhausted


SIMULATION DELTA • What is simulation delta? •Several logic changes occur simultaneously in a circuit (concurrent operations) •But simulator being run by a sequential machine, hence it cannot process events concurrently. •To take care this, time is frozen within the simulator. •The real time the simulator takes to complete all concurrent operations in the queue is called SIMULATION DELTA


SIMULATION DELTA Zero simulation time

t=0 ns

t=1 ns

t=2 ns

t=3 ns

t=4 ns

Many delta cycles can occur

Real Time

The minimum time interval possible for the simulator is assumed to be 1 ns here


SIMULATION DELTA • SUMMARY Simulation deltas allow ordering of events that occur at the same simulation time during simulation Simulation deltas are infinitesimal amount of time used as synchronism mechanism when zero delay events are present


DELAY MODELING Delays are timing parameters given by the user for modeling physical characteristics of hardware Types of delays

INERTIAL DELAY

TRANSPORT DELAY

Delays are specified in signal assignment statements only Delays should NOT be used in variable assignments


INERTIAL DELAY • It is used to model the inherent inertia of physical devices • Example: – The input value must be stable for a specified minimum pulse duration before the value is allowed to propagate to the output – If the input is not stable for the specified limit, no output change occurs


TRANSPORT DELAY • It represents pure propagation delay i.e., wires and interconnect delays • Signal value is assigned with a specified delay independent of the width of the input waveform


DELAYS

10 ns X

3 ns

6 ns

10 ns 2 ns

Z1 Z2 Z3


TYPES OF SIMULATION • • • • • •

FUNCTIONAL SIMULATION BEHAVIORAL SIMULATION STATIC TIMING SIMULATION GATE-LEVEL SIMULATION SWITCH-LEVEL SIMULATION TRANSISTOR-LEVEL OR CIRCUITLEVEL SIMULATION


TYPES OF SIMULATION • FUNCTIONAL SIMULATION It ignores timing aspects Verifies only the functionality of the design

• BEHAVIORAL SIMULATION A given functionality is modeled using HDL Timing aspects are considered


TYPES OF SIMULATION • STATIC TIMING SIMULATION A built in tool that computes delay for each timing path Does not require input stimuli

• GATE-LEVEL SIMULATION Is used to check the timing performance of design Delay parameters of logic cells are used to verify things


TYPES OF SIMULATION • SWITCH-LEVEL SIMULATION Is one level below the gate level simulation It models transistors as switches It provides more accurate timing predictions than gate-level simulation


TYPES OF SIMULATION • TRANSISTOR-LEVEL SIMULATION Requires transistor models. Circuit is described in terms of resistances, capacitances and voltage and current sources A set of mathematical equations relating current and voltages is setup and solved numerically Gives analog results and is most accurate Requires large amount of computing resources


And finally!! Simulation time depends on : • Simulation levels of logic • Physical Memory of PC • Speed of PC


THANK YOU !!!


Test benches in VHDL

R.B.Ghongade Lecture 18,19

Copyright, R.B.Ghongade


Introduction • A design is always incomplete without verification • There are several ways to verify VHDL designs • Test benches are one of them • Test benches are also called Test cases


• A testbench is an environment, where a design ( called design or unit under test UUT) is checked – applying signals (stimuli) – monitoring its responses by signal probes and monitors

• A testbench substitutes the design’s environment in such a way that the behaviour of the design can be observed and analyzed.


A testbench always consists of following elements: – a socket for the unit under test (UUT) – a stimuli generator (a subsystem that applies stimuli to UUT, either generating them internally or reading from an external source) – tools for observing UUT responses to the stimuli


Concept of Testbench


Elements of a VHDL Test Bench • A VHDL test bench is just another specification with its own • entity • architecture • In addition, it has special structure with some elements that are characteristic to this type of specification: • Test bench entity has no ports, • UUT component instantiation - the relationship between the test bench and UUT is specified through component instantiation and structural-type specification, • Stimuli - it is a set of signals that are declared internally in the test bench architecture and assigned to UUT's ports in its instantiation. The stimuli are defined as waveforms in one or more behavioral processes.



Using Test Benches • The design must be verifiable. • It is much more convenient to use a test bench for design verification. • Writing a test bench -> very complex task • Therefore some guidelines for future stimuli development should be written as you progress with the design. • It is the test bench which is simulated, not the unit under test. • The UUT is only one of the components instantiated in a test bench. • There is no limitation on the test bench size. • The only limitation is the VHDL simulator used capability.


Writing stimuli can be performed Simulation of the test bench is the last DESIGN phaseconcurrently with of a design process. Here youwriting specifications fortoeach new design will receive an answer the question; When stimuliThe are specified, aset test should block. stimuli Design Specification "does the system behave as bench specification can be written contain such set of input (and state) expected?" Remember that the answer It will contain the stimuli and an as much signal values that covers received from simulating test bench is instantiation of the designed real life situations as possible. The design process consists of design and VERIFICATION reliable only to the extent determined under test). It isfirst the one is verification system phases (unit The objective of the bytest thebench test bench accuracy and and not design that will the to create a new VHDL specification that meets coverage area. during The better a test bench be simulated the verification Stimuli Definitions system requirements. is,phase. the more confident you can be that your system is properly designed.

Test bench Specifications Test bench Simulation


Example : Multiplexer


Example : J-K FlipFlop


Closer look at Testbench UUT: 2- bit Multiplexer


Testbench


Waveforms


Testbench : Using assert‌report


Waveforms


Error Report


SUMMARY

• A Test bench thus is an effective builtin tool for verifying VHDL designs •Troubleshooting becomes easier and faster because of the ASSERT…REPORT clause •Automation of verification is possible because of the seamless integration of language elements.


Synthesis Issues R.B.Ghongade Lecture 19


Agenda • • • • • •

What is synthesis ? Synthesis Tools : Expectations Synthesis Tools : Features Hardware modeling examples Good coding Practices Synthesis guidelines


What is synthesis ? • Synthesis is an automatic process that converts user’s hardware description into structural logic description • When we use VHDL as textual file, the process is called VHDL Synthesis • Synthesis is a means of converting HDL into real world hardware • Synthesis tools generate a gate-level netlist for the target technology


What is synthesis ? SYNTHESIS = TRANSLATION + OPTIMIZATION x <= ( a and b ) or ( c and d ); a a

b

a

b

x x

c d

c d

b

LUT

c d

• Synthesis is target device technology specific • Synthesizer will try to use the best architectural resource available with the target

x


Several steps work out behind the scene! Translation Optimization Hardware Description - written with hardware in mind

Mix of Boolean , other operations and memory elements

Mapping Gate Level – Technology Specific

• Translation ( language synthesis ) : Design at higher level of description is compiled into known language elements • Optimization : Algorithms are applied to make the design compact and fast • Design is mapped using architecture specific techniques


Synthesis process – design flow HDL behavioral description

Netlist

RTL Synthesis RTL Optimization

Logic extraction

High-level description with Boolean equations

Structured Boolean equations

Logic Optimization

Structured Boolean equations

Technology mapping Gate-level optimization

Optimized Netlist


Synthesis process •

Translation : Inputs are transformed into a description based Boolean equations – If the input data consists of gate level netlist then it is necessary to extract Boolean equations to determine functionality of all used gates Technology independent logic optimization: This process aims to improve structure of Boolean equations by applying rules of Boolean algebra . This removes the redundant logic and reduces the space requirement Technology Mapping: This is the process of transforming technology independent netlist into technology dependent one. During mapping , timing and area information of all usable gates of the target technology must be available. It is split into two phases – Flattening – Structuring


Flattening • The aim is to generate Boolean equations for each output of module in such a way that the output value is a direct function of inputs. These equations reflect two level logic in SOP form. • Resulting equations do not imply any structure • Good optimization results are obtained • Caution: In case of structured logic this process would destroy the characteristic structure and its associated properties . e.g. carry look ahead adder • Flattening cannot be applied to every logic circuit because the number of product terms may become very large


Structuring • New intermediate variables are inserted during the structuring process • E.g. If (A’.B’.C’) occurs 10 times then the tool may assign X= (A’.B’.C’) and use X everywhere • Finally , the sub-functions are substituted into original equations • Compared to the logic before structuring, the resulting area is reduced


Synthesis process- review CONSTRAINT(area , speed) LIBRARIES

DESIGN

SYNTHESIS

NETLIST

•Translation process converts RTL to a Boolean form •Optimization is done on the converted Boolean equations •Optimized logic is mapped to technology library

REPORT

•Flattening is a process where all the design hierarchy is removed, the entire design is transformed into a flat , generic , SOP form •Structuring is the opposite of flattening, its adds structure to the generic design by extracting common logic factors and representing them as intermediate nodes to produce compact logic


Synthesis tool : expectations A good synthesis tool should – perform technology specific optimizations i.e. vendor specific FPGAs and CPLds – have best optimization techniques – allow designer control – have broad language coverage – provide user friendly debugging environment – have fast compile times – provide clean and seamless link with backend tools


Synthesis Tools - features Tool cost depends on features provided by it Desirable features are: – Replicating the logic – Duplicate flip-flops, remove unused logic – optimization across design hierarchy – resource sharing of adders , incrementors, multipliers – automatic RAM inference (RAM logic is automatically mapped to technology specific RAM cells)


Replicate logic – Replicate logic to meet fan-out demands – E.g. WR’ may be connected to 100 points hence add buffers to split internally

BUF BUF BUF BUF BUF BUF


Duplicating logic • We can duplicate the logic which generates the signal , for minimizing fan-out • Trade-off : Loading effect of signals is reduced hence lowering propagation delay but at the cost of logic and interconnect complexity D

Q

D

Q

D

Q


Resource sharing • Some synthesis tools automatically perform a limited amount of resource sharing ( for arithmetic expressions that are mutually exclusive) • Consider the code: ADDSEL: process( sel, a ,b,c,d) begin if (sel=‘1’ ) then y<= a + b ; else y<= c + d ; end if ; end process ADDSEL ;


Resource sharing a a c

+ +

b y

c

b

+

d

d sel

sel

Before resource sharing

After resource sharing

An adder requires more floor space than a multiplexer

y


Simulation vs. Synthesis • Some simulation constructs are not supported by synthesis tools – e.g. wait statements

• Synthesis tools ignore initial values • Care should be taken so that simulationsynthesis mismatch does not occur


Using Signals or Variables • Variables are used only for convenience of describing behaviour • Variables are used and declared in a process however it cannot be used to communicate between processes • Variable assignments are done immediately and are executed sequentially • Variables may or may not represent physical wires • Signal assignments are done at the end of process • Signals represent physical wires in the circuit


Using Signals or Variables • Use variables in combinational processes because there is less simulation overhead • Order dependency – Signal assignments are order independent – Variable assignments are order dependent – Signal assignments under a clocked process are translated into registers – Variable assignments under a clocked process may or may not be translated into registers


Signals or Variables process (clk, a, b, c, d) variable y, x, w : std_logic ; begin if (clk=‘1’ and clk’event) then z1<= y ; variables are read y : = x ; before being written x : = a and b ; to , this infers a w : = c and d ; memory element z2<= w ; end if; end process ;


Hardware inferred


Same process with order of statements changed process (clk, a, b, c, d) variable y, x, w : std_logic ; begin if (clk='1' and clk'event) then x := a and b ; y := x ; variables are read z1<= y ; before being written z2<= w ; to , this infers a w := c and d ; memory element end if; end process ;


Hardware inferred


Hardware modeling examples “For loop� process(word) variable result : std_logic; begin result:='0'; for i in 0 to 7 loop result := result xor word(i); end loop; op<=result; end process;


Multiplexer optimization The hardware inferred depends on the condition given in the “when others” clause case sel is when “000” => y<= data(0); when “001” => y<= data(1); when “010” => y<= data(2); when “011” => y<= data(3); 1) when others => y<= ‘0’; 2) when others => y<= ‘Z’; 3) when others => y<= ‘X’; 4) when others => NULL; end case;


Case 1 when others => y<= ‘0’;

Case 2 when others => y<= ‘Z’;


Case 3 when others => y<= ‘X’;

Case 4 when others => NULL;


Good coding practices • Good coding style means that the synthesis tool can identify constructs within your code that it can easily map to technology features • All programmable devices may have their unique architectural resources e.g. Xilinx Virtex series has built-in RAM • Coding for performance : – common mistake is to ignore hardware and start coding as if programming. To achieve best performance the designer must think about hardware


Good coding practices • Improve performance by – avoiding unnecessary priority structures in logic – optimizing logic for late arriving signals – structuring arithmetic for performance – avoiding area inefficient code – buffering high fan-out signals


Good coding practices • Use “constants” to enhance readability and code maintenance • Comparison with a constant is preferred since it is much “cheaper” to implement • To avoid accidental latches – specify all clauses of “if” and “case” statements – specify all outputs

• Use “case” rather than “if-then-else” whenever possible • Use parentheses for better operation • Never use mode “buffer”


Design constraints • Constraining designs: – constraints are means of communicating our requirements to the synthesis and back-end tools

• Categories of constraints are : – Timing constraints • • • •

maximum frequency duty cycle input delays output delays

– Layout constraints


Design constraints • Avoid over-constraining the design • Consequences of over-constraining are: – Design performance suffers: • critical timing paths get the best placement and fastest routing options • as the number of critical paths increase , the ability to obtain the design performance objectives decrease

– Run times increase


Synthesis guidelines • Simulate your design before synthesis • Avoid combinational loops in processes • If a port is declared to be an integer data type, the range should be specified, else the synthesis tool will infer a 32-bit port • Avoid mixed clock edges – if a large number of both positive and negative edge flip-flops are required they should be placed in different modules


Synthesis guidelines • For best results: – use technology primitives (macros) from the target technology libraries wherever possible – try small designs on target technology to find its limitations and strengths – Partition the design correctly • eliminate glue logic at the top level • partition block size based on the logic function, CPU resources and memory • separate random logic from structured logic • separate timing-sensitive modules from area -sensitive ones


Next class

CPLDs and FPGAs


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