Logic devices

Logic devices • • • • • • • • • • •

Or gate And gate Nor gate Nand gate Buffers Inverters Exclusive-OR Exclusive-Nor Set reset flip flop Jk flip flop Toggle flip flop

History of logic • The first programmed logic devices were invented in the 1970's where they were only Or gate and And gate just using these two type of devices they were able to implement a simple logic function.

• A PLA device can be define by three function which are as follows; 1/ Number of inputs 2/ Number of and gates (in terms) 3/ Number of Or gates (= number of output) • How the idea of logic devices were introduced was down to, the inteligent of John Birkner and H.t Chua of monolithic memories with them working with Andy Chan to introduce a more streamlined architeture they called it programmed array logic. After introducing to these PLA device they later developed the ideas of using theses devices for simple logic function and complex function.

• At this current time we are, all aware of all the type of logic devices, that are available for a use.

The type of logic devics available for use • • • • •

Using a ROM as a PLD Early programmed logic Programmed logic Array Generic Array logics Combinational programmed logic devices • Other variants

• PLD programmed language • PLD programmed device • These are the devices that are available to be used.

Logic gate functions • The function of logic gate is to perform basic logic function and are used to build a blocksof digital intergrated circuit. Nearly all of logic gates can take an inputs of two binary values to give you an output of a single value . I know you can use logic gates with more then 2 inputs to give you an ouput of more then one value but the basic function og logic gate would be used for a simple circuit with 2 input to give you an output of one value.

The use of a basic logic gate • A basic logic gates are used to construct a simple logic circuit or if you need to make a complex logic circuit you would use more inputs with combinations of different types of logic gate ( i.e and gate, or gate, nand gate and nor gate) These logic gates are mainly used in electronic engineering particularly in digital signal processing and a simple control system circuits. A drill motor is activated by limit switch which is controlled by logic gate and this another example of how useful logic gate is in engineering environment.

Using ROM as a PLD • The idea of using ROM as a PLD is to reconfigure a electronic circuits.

• Theses logic device can be used for more then 1 inputs.

• They are usually much slower then dedicated logic circuits.

• they cannot necessarily provide safe "covers" for asynchronous logic transitions so the PROM's outputs may glitch as the inputs switch.

• they consume more power • This device is more expensive then programme logic particularly if there is high speed required.

Early Programme Logic • Early programme logic was used in Motorola, which they offered the XC157 in 1969 and this model can programme gate array which consist of 12 gates and 30 uncommitted input/output pins. • In 1974 GE entered into an agreement with Monolithic Memories to develop a maskprogrammable logic device incorporating the GE innovations. The device was named the 'Programmable Associative Logic Array' or PALA. The MMI 5760 was completed in 1976 and could implement multilevel or sequential circuits of over 100 gates. The device was supported by a GE design environment where Boolean equations would be converted to mask patterns for configuring the device.

Programmed Logic Array •

PLA is programme logic device used to implement combination logic circuits.The desired outputs for each combination of inputs could be programmed into a read-only memory, with the inputs being loaded onto the address bus and the outputs being read out as data. However, that would require a separate memory location for every possible combination of inputs, including combinations that are never supposed to occur, and also duplicating data for "don't care" conditions (for example, logic like "if input A is 1, then, as far as output X is concerned, we don't care what input B is": in a ROM this would have to be written out twice, once for each possible value of B, and as more "don't care" inputs are added, the duplication grows exponentially); therefore, a programmable logic array can often implement a piece of logic using fewer transistors than the equivalent in read-only memory.

Generic Array logics •

Generic array logic was invented as a an improvement of PAL.This device was intended to make easier for people who are using it. The main benifit of using GAL was it can be eraseable and can be re-programmed making prototyping and design changes easier for the engineers However there is a similiar device called PEEL (programmable electrically eraseable logic) was introduced by the international CMOS technology corporation.

CMOS VS TTL • • • • • • • • • •

Characteristics of CMOS logic: 1. Dissipates low power: The power dissipation is dependent on the power supply voltage, frequency, output load, and input rise time. At 1 MHz and 50 pF load, the power dissipation is typically 10 nW per gate.

2. Short propagation delays: Depending on the power supply, the propagation delays are usually around 25 nS to 50 nS. 3. Rise and fall times are controlled: The rise and falls are usually ramps instead of step functions, and they are 20 - 40% longer than the propagation delays.

4. Noise immunity approaches 50% or 45% of the full logic swing.

5. Levels of the logic signal will be essentially equal to the power supplied since the input impedance is so high.

Characteristics of TTL logic: 1. Power dissipation is usually 10 mW per gate.

2. Propagation delays are 10 nS when driving a 15 pF/400 ohm load.

3. Voltage levels range from 0 to Vcc where Vcc is typically 4.75V - 5.25V. Voltage range 0V - 0.8V creates logic level 0. Voltage range 2V - Vcc creates logic level 1.

References • Bolton, WB, 2010. Programmable logic controllers. 5th ed. Oxford: Newnes. • http://en.wikipedia.org/wiki/Logic_gate