Boolean Algebra and Digital Logic Based on slides from Jones & Bartlett
3.2 Boolean Algebra • Boolean algebra is a mathematical system for the manipulation of variables that can have one of two values. – In formal logic, these values are “true” and “false.” – In digital systems, these values are “on” and “off,” 1 and 0, or “high” and “low” depending on the usage context.
• Boolean expressions are created by performing operations on Boolean variables. – Common Boolean operators include AND, OR, and NOT. COMP 228, Fall 2009
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3.2 Boolean Algebra • A Boolean operator can be completely described using a truth table. • The truth table for the Boolean operators AND and OR are shown at the right. • The AND operator is also known as a Boolean product. The OR operator is the Boolean sum. COMP 228, Fall 2009
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3.2 Boolean Algebra • The truth table for the Boolean NOT operator is shown at the right. • The NOT operation is most often designated by an overbar. It is also sometimes written as X’ or ~X.
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3.2 Boolean Algebra • A Boolean function has: • • •
At least one Boolean variable, At least one Boolean operator, and At least one input from the set {0,1}.
• It produces an output that is also a member of the set {0,1}. Now you know why the binary numbering system is so handy in digital systems.
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3.2 Boolean Algebra • The truth table for the Boolean function:
is shown at the right. • To make evaluation of the Boolean function easier, the truth table contains extra (shaded) columns to hold evaluations of subparts of the function. COMP 228, Fall 2009
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3.2 Boolean Algebra • As with common arithmetic, Boolean operations have rules of precedence. • The NOT operator has highest priority, followed by AND and then OR. • This is how we chose the (shaded) function subparts in our table. COMP 228, Fall 2009
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3.2 Boolean Algebra • Digital computers contain circuits that implement Boolean functions. • The simpler that we can make a Boolean function, the smaller the circuit that will likely result. – Simpler circuits are cheaper to build, consume less power, and run faster than complex circuits.
• With this in mind, we always want to reduce our Boolean functions to their simplest form. • There are a number of Boolean identities that help us to do this. COMP 228, Fall 2009
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3.3 Logic Gates • The three simplest gates are the AND, OR, and NOT gates.
• They correspond directly to their respective Boolean operations, as you can see by their truth tables. COMP 228, Fall 2009
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3.3 Logic Gates • NAND (NOT-AND) and NOR (NOT-OR) are often used in practice because they are easily implemented in an IC. • DeMorgan’s Theorem – (XY)’ = X’ + Y’ – (X+Y)’ = X’Y’
• (XX)’ = X’ • Hence every Boolean function can be implemented using NAND gates (or NOR gates) only. • This property is known as functional completeness. COMP 228, Fall 2009
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3.3 Logic Gates • We often ‘minimize/simplify’ a Boolean function so that it may have a cheaper implementation using fewer logic gates • Karnaugh map is a well-developed technique to facilitate this x 0 1
x 0 1
yz00
01
11 10
1 1 1 1 0 1 0 0
yz00
01
x’+y’z
11 10
1 1 0 1 0 1 0 0
x’z’+y’z
Assuming inverted input is available, this can be implemented using 3 NAND gates.
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Clicker 3.1 How many distinct Boolean functions are there involving 3 or fewer Boolean variables? A: B: C: D: E:
512 256 128 64 None of the above
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Clicker 3.2 • Consider the Karnaugh map bc
a 0 1
00
01
11
0 1
1 1
1 1
10
0 1
It corresponds to the following minimized logic function: A: B: C: D:
a + a’c a+c a + bc + b’c None of the above
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Clicker 3.3 • Consider the Karnaugh map yz wx 00 01 11 10
00
01
11
0 1 0 1
1 1 1 1
1 1 1 1
10
0 1 0 1
It corresponds to the following minimized logic function: A: w’ + x’ + y’z + yz’ B: w’x’ + y C: wx’ + w’x + z D: None of the above COMP 228, Fall 2009
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3.3 Logic Gates • Another very useful gate is the exclusive OR (XOR) gate. • The output of the XOR operation is true only when the values of the inputs differ.
Note the special symbol ⊕ for the XOR operation. COMP 228, Fall 2009
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3.5 Combinational Circuits • Combinational logic circuits give us many useful devices. • One of the simplest is the half adder, which finds the sum of two bits. • We can gain some insight as to the construction of a half adder by looking at its truth table, shown at the right.
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3.5 Combinational Circuits • As we see, the sum can be found using the XOR operation and the carry using the AND operation.
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3.5 Combinational Circuits • We can change our half adder into to a full adder by including gates for processing the carry bit. • The truth table for a full adder is shown at the right.
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3.5 Combinational Circuits • How can we change the half adder shown below to make it a full adder?
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3.5 Combinational Circuits • Here’s our completed full adder.
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3.5 Combinational Circuits • Just as we combined half adders to make a full adder, full adders can connected in series. • The carry bit “ripples” from one adder to the next; hence, this configuration is called a ripple-carry adder.
Today’s systems employ more efficient adders. COMP 228, Fall 2009
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Clicker 3.4 • Assuming each logic gate incurs a 10 psec delay. What is the time delay needed in order that the 16-bit ripple adder has enough time to generate a stable output? [Refer to the full-adder implementation on p. 18] A: B: C: D:
160 psec 320 psec 480 psec None of the above
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3.5 Combinational Circuits • Decoders are another important type of combinational circuit. • Among other things, they are useful in selecting a memory location according a binary value placed on the address lines of a memory bus. • Address decoders with n inputs can select any of 2n locations. This is a block diagram for a decoder. COMP 228, Fall 2009
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3.5 Combinational Circuits • This is what a 2-to-4 decoder looks like on the inside.
If x = 0 and y = 1, which output line is enabled? COMP 228, Fall 2009
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Clicker 3.5 How many input bits are there in a decoder with 16 output bits? A: B: C: D:
16 8 4 None of the above
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3.5 Combinational Circuits • A multiplexer does just the opposite of a decoder. • It selects a single output from several inputs. • The particular input chosen for output is determined by the value of the multiplexer’s control lines. • To be able to select among n inputs, log2n control lines are needed. COMP 228, Fall 2009
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This is a block diagram for a multiplexer. 25
3.5 Combinational Circuits • This is what a 4-to-1 multiplexer looks like on the inside.
If S0 = 1 and S1 = 0, which input is transferred to the output? COMP 228, Fall 2009
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3.6 Sequential Circuits • Combinational logic circuits are perfect for situations when we require the immediate application of a Boolean function to a set of inputs. • There are other times, however, when we need a circuit to change its value with consideration to its current state as well as its inputs. – These circuits have to “remember” their current state.
• Sequential logic circuits provide this functionality for us. COMP 228, Fall 2009
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3.6 Sequential Circuits • As the name implies, sequential logic circuits require a means by which events can be sequenced. • Synchronous sequential circuits operate with a clock. State changes are controlled by clocks. • Asynchronous sequential circuits operate without a clock.
• A “clock” is a special circuit that sends periodic electrical pulses through a circuit. • Clocks produce electrical waveforms such as the one shown below.
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3.6 Sequential Circuits • To retain their state values, sequential circuits rely on feedback. • Feedback in digital circuits occurs when an output is looped back to the input. • A simple example of this concept is shown below. – If Q is 0 it will always be 0; if it is 1, it will always be 1. Why?
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3.6 Sequential Circuits • The D flip-flop is the fundamental circuit of computer memory. – D flip-flops are usually illustrated using the block diagram shown below.
• The characteristic table for the D flip-flop is shown at the right.
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3.6 Sequential Circuits • This illustration shows a 4-bit register consisting of D flip-flops. You will usually see its block diagram (below) instead.
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3.6 Sequential Circuits General Diagram Combinational Circuit External Inputs External Outputs State Flip Flops
clock
General Schematic Diagram of a Synchronous Sequential Circuit
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3.6 Sequential Circuits Example: f D Q
y z
ck c
D = f = Q+y’z
yz Q 0 1
00,01,11 / 0
00
01
11
0 1
1 1
0 1
01 / 1
0
1
10
0 1
Next Q = D
00,01,10,11 / 1 Input / Output yz / Q
State Diagram
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Clicker 3.6 • Given a sequential circuit with 4 flips flops. What is the maximum number of states that can be reached from the state (0,0,0,0)? A: B: C: D: E:
1 4 8 16 None of the above
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Clicker 3.7 • Can a sequential contain 0 logic gates (in its combinational circuit block)? A: B:
Yes No
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Clicker 3.8 A mod-16 counter is a sequential circuit that counts from 0 to 15 and repeats, each time triggered by the clock. How many flip flops are needed to implement such a counter? A: B: C: D: E:
16 8 4 2 None of the above
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Clicker 3.9 Which of the following components in a processor is not a sequential circuit? A: B: C: D: E:
Program Counter ALU Register Control Unit None of the above
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