Chapter 4

Combinational Logic

4.1 Introduction

Logic circuits for digital systems may becombinational or sequential.

A combinational circuit consists of logic gateswhose outputsat anytime are determined

from only the present combinationof inputs.

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4.2 Combinational Circuits

Logic circuits for digital system Sequential circuits

contain memory elements

the outputs are a function of the current inputs and thestate of the memory elements

the outputs also depend on past inputs

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A combinational circuits 2possible combinations of input valuesn

Combination

al circuitsCombinatixnalxoxic Circuit

n inputm outputvariablesvariables

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Specific functions

Adders, subtractors, comparators, decoders, encoders,andmultiplexers

4-3 Analysis Procedure

A combinational circuit

make sure that it is combinational not sequential

No feedback path

derive its Boolean functions (truth table)

designverification

A straight-forward procedure

=AB+AC+BCF2

T1

=A+B+C

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1TT

2

3

1

=ABC =F2'T1

=T3+T2F

F 13221

) =AB+AC+BC)'(A+B+C+(ABC) =A'+B)('A'+C)(‘B'+C)('A+B+C+(ABC

) =A'+B'C)('AB'+AC'+BC'+B'C+(ABC =A'BC'+A'B‘C+AB‘C'+ABC

=T +T =F 'T +ABC

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The truth table

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4-4 Design Procedure

The design procedure ofcombinational circuits

State the problem (system spec.)

determine the inputs and outputs

the inputand output variables are assigned symbols

derivethe truth table

x erive the simplified Bool xan functionx

draw the logic diagram and verify the correctness

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derive the simplifi edb oolean functions

code conversion example

BCD to excess-3 code Thetruth table

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The maps

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The simplified functions z = D'

y = CD +C'D‘

x = B'C + B‘D+BC'D'

w = A+BC+BD

z = D'

y = CD +C'D' = CD + (C+D)'

x = B'C + B'D+BC'D‘ = B'(C+D) +B(C+D)'

w = A+BC+BD

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Another implementation

The logic diagram

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4-5Binary Adder-Subtractor

Half adder

0 + 0 = 0 ; 0 + 1 = 1 ; 1 + 0 = 1 ; 1 + 1 = 10

two input variables: x, y

two output variables: C (carry), S (sum)

truth table

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S = x'y+xy'

C = xy

the flexibility for implementation

S=xÅy

S = (x+y)(x'+y')

S = (C+x'y')'

C = xy = (x'+y')x

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S‘= xy+x'y'

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17Functional Block: Full-Adder

A full adder is similar to a half adder, but includes a carry-in bit from lower stages. Like the half-adder, it computes a sum bit, S and a carry bit, C.

For a carry-in (Z) of 0, it is the same as the half-adder:

For a carry- in(Z) of 1:

Z 0 0 0 0X 0 0 1 1 +Y +0 +1 +0 +1

C S 0 0 0 1 0 1 1 0

Z 1 1 1 1X 0 0 1 1 +Y +0 +1 +0 +1

C S 0 1 1 0 1 0 1 1

Full-Adder

the arithmetic sum of three inputbits

three input bits

x, y: two significant bits

z: the carrybit from the previous lower significant bit

Two outputbits: C, S

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19

S = x'y'z+x'yz'+ xy'z'+xyz

C = xy + xz + yz

S = zÅ (xÅy)

= z'(xy'+x‘y)+z(xy'+x'y)'

= z‘xy'+z'x'y+z(xy+x‘y')

= xy'z'+x'yz'+xyz+x'y'z

C = z(xy'+x'y)+xy

= xy'z+x'yz+ xy

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Binary adder

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Binary subtractor

A-B = A+(2’s complement of B)4-bit Adder-subtractor

M=0, A+B; M=1, A+B’+1

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Overflow

The storage is limited

Add two positive numbers and obtain a negative

number

Add two negative numbers and obtain a positive

number

V = 0, no overflow; V = 1, overflow

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Example:

4-6 Decimal Adder

Add twoBCD's

9 inputs: two BCD's and one carry-in5 outputs: one BCD and one carry-out

Design approaches

A truth table with 2^9 entriesthe sum <= 9 + 9 + 1 = 19

binary to BCD

useuinary full Adders

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BCD Adder: The truth Table25

Modifications are needed if the sum > 9C = 1

K = 1

Z

Z84Z =1

=18 2Z

moxification: - (10) dor +6 modification: -(10)dor +6

C = K +Z Z + Z Z8 4 8 2

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Block diagram27

Binary Multiplier

Partial products

–AND operations

fig. 4.15Two-bit by two-bit binary multiplier.

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4-bit by 3-bit binary multiplier

Digital Circuits

Fig. 4.16Four-bit by three-bit binary

multiplier.

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4-9 Decoder

A n-to-m decodera binary code of n bits = 2 distinct information

n

n input variables; up to 2 output linesonly one output can be active )high( at any time

n

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An implementation

Digital Circuits 38

Fig. 4.18Three-to-eight-line decoder.

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Combinational logic implementation

each output = a mintermuse a decoder and an external OR gate toimplement any Boolean function of n input

variables

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Demultiplexers

a decoder with an enable input

receive information in a single line and transmitsit in one of 2 possible output lines

n

Fig. 4.19Two-to-four-line decoder with enable input

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Decoder Examples

D0 = m0 = A2’A1’A0’

D1= m1 = A2’A1’A0

…etc

3-to-8-Line Decoder: example: Binary-to-octal conversion.

Expansion two 3-to-8 decoder: a 4-to-16 deocder

a 5-to-32 decoder?

Fig. 4.204 16 decoder

constructed with two3 x 8 decoders

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Decoder Expansion - Example 2 Construct a 5-to-32-line decoder using four 3-8-line

decoders with enable inputs and a 2-to-4-line decoder.

D0 – D7

D8 – D15

D16 – D23

D24 – D31

A3

A4

A0

A1

A2

2-4-line Decoder

3-8-line Decoder

3-8-line Decoder

3-8-line Decoder

3-8-line Decoder

E

E

E

E

Combination Logic Implementation

each output = a mintermuse a decoder and an external OR gate toimplement any Boolean function of n inputvariables

A full-adder

S)x,y,z(=S)1,2,4,7(C)x,y,z(= S (3,5,6,7)C)x,y,z(= S (3,5,6,7)

Fig. 4.21Implementation of a full adder with 1 decoder

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two possible approaches using decoder

OR(minterms of F): k inputs

NOR(minterms of F'): 2 - k inputs

In general, it is not a practical implementation

n

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4-10 Encoders

The inverse function of decodera decoder

1357

2367

4567

z D D D D

y D D D D

x D D D D

=

=

=

+ + +

+ + +

+ + +

The encoder can be implementedwith three OR gates.

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An implementation

limitations

illegal input: e.g. D 36

The output = 111 (¹3 and ¹6)=D x1

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Priority Encoder

resolve the ambiguity of illegal inputsonly one of theinput is encoded

DD

3

0

X: don't-care conditionsV: valid output indicator

has the highest priorityhas the lowest priority

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■ The maps for simplifying outputs x and y

fig. 4.22Maps for a priority encoder

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■ Implementation of priority

Fig. 4.23Four-input priority encoder

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0123

x =y=

D D

D D D

V D D D D

+¢+

= + + +

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3 1 2

4-11 Multiplexers

select binary information from one of many inputlines and direct it to a single output line

2 input lines, n selection lines and one output line

e.g.: 2-to-1-line multiplexer

n

Fig. 4.24Two-to-one-line multiplexer

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4-to-1-line multiplexer

Fig. 4.25Four-to-one-line multiplexer

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Note

n-to- 2 decodern

add the 2 input lines to each AND gateOR(all AND gates)

an enable input (an option)

n

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Fig. 4.26Quadruple two-to-one-line multiplexer

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Boolean function implementation

MUX: a decoders an OR gate

2 -to-1 MUX can implement any Boolean functionof n input variable

n

n of these variables: the selection lines

the remaining variable: the inputs

a better solution: implement any Boolean function

of n+1 input variable

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an example: F(A,B,C) = S(1,2,6,7)

Fig. 4.27Implementing a Boolean function with a multiplexer

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procedure:

assign an ordering sequence of the input variablethe rightmost variable (D) will be used for the inputlinesassign the remaining n-1 variables to the selection

0

determine the input lines

lines w.r.t. their corresponding sequ

consider a pair of consecutive minterms startingfrom m

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Lines with construct the truth table

Example: F(A, B, C, D) = S(1, 3, 4, 11, 12, 13, 14, 15)

Fig. 4.28 Implementing a four-input function with a multiplexer

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Three-state gates

A multiplexer can be constructed with three-stategatesOutput state: 0, 1, and high-impedance (open ckts)

Fig. 4.29Graphic symbol for a three-state buffer

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Example: Four-to-one-line multiplexer

Fig. 4.30Multiplexer with three-state gates

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