# Combinational Logic - Design and Analysis - Lecture Notes, Study notes for Digital Systems Design. Jaypee University of Engineering & Technology

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Combinational Logic, Combinational Circuits, Design Procedure, Binary Adder Subtractor, Decimal Adder, Binary Multiplier, Magnitude Comparator, Decoders are the key points in this lecture handout.
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Boolean Algebra

1

COEN 312 DIGITAL SYSTEMS DESIGN - LECTURE NOTES

Chapter 4: Combinational Logic

NOTE: For more examples and detailed description of the material in the lecture notes, please refer to the main textbook: Digital Design 3rd Edition, By Morris Mano, Publisher Prentice Hall, 3rd Edition All examples used in the lecture notes are from the above reference. Combinational Circuits

- A combinational circuit consists of input variables, output variables, and logic gates

that transform binary information from the input data to the output.

Combinational

circuit n inputs m outputs

- A combinational circuit cannot have any storage elements (registers) or any feedback

paths (connections from the output of one gate to the input of a second gate that

directly or indirectly affects the input to the first gate).

- As an example, consider the following combinational circuit with 3 inputs and 2

outputs (n = 3, m = 2):

F’2

A B C A B C

A B

A C

B C

T1

T2

T3

F2

F1

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- If there are more than two levels of gates to generate a function, label the output of

the corresponding gates and determine the Boolean expression for each of them (here,

we have used the labels T1, T , T , and F’2 3 2 to find the Boolean expression for F1).

231

123

2

1

2

.' ..

...

TTF TFT CBAT

CBAT CBCABAF

+= = =

++= ++=

- By using the properties of Boolean algebra, the expression for will be equal to: 1F

CBACBACBACBAF ..''..'.'.'.'.1 +++=

- The truth table for the outputs of this combinational circuit can be obtained by using

the above expressions obtained for and , or by using the labeled gate outputs in

the truth table and obtaining

1F 2F

CBCABAF ...2 ++=231 TTF += and for different

combinations of the input variables as follows:

A B C F FT T T F2 2 1 2 3 1

0 0 0 0 1 0 0 0 0

0 0 1 0 1 1 0 1 1

0 1 0 0 1 1 0 1 1

0 1 1 1 0 1 0 0 0

1 0 0 0 1 1 0 1 1

1 0 1 1 0 1 0 0 0

1 1 0 1 0 1 0 0 0

1 1 1 1 0 1 1 0 1

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[ ][ ]12 FFCBA =++- This is in fact the logic circuit for a full adder ( ).

Design Procedure

- Important design constraints include the number of gates, number of inputs to a gate,

propagation time of the signal through the gates, number of interconnections, etc.

Example

- Find a combinational circuit, which converts the binary coded decimal (BCD) to the

excess-3 code for the decimal digits.

Input BCDOutput Excess-3 Code

A B C D wx y z

0 0 0 0 0 0 1 1

0 0 0 1 0 1 0 0

0 0 1 0 0 1 0 1

0 0 1 1 0 1 1 0

0 1 0 0 0 1 1 1

0 1 0 1 1 0 0 0

0 1 1 0 1 0 0 1

0 1 1 1 1 0 1 0

1 0 0 0 1 0 1 1

1 0 0 1 1 1 0 0

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X

0

X

0

0

1

1

0

1

1

CD AB

00

01

00 01 11 10

D D D

C C

X

1

0

X

X

X

11

10

B

B

B

A

A

- Simplified expression: 'Dz =

X

1

X

0

1

1

1

0

0

0

CD AB

00

01

00 01 11 10

D D D

C C

X

1

0

X

X

X

11

10

B

B

B

A

A

- Simplified expression: ''.. DCDCy +=

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X

0

X

0

1

0

1

1

1

0

CD AB

00

01

00 01 11 10

D D D

C C

X

0

1

X

X

X

11

10

B

B

B

A

A

''..'.'. DCBDBCBx ++=- Simplified expression:

0

X

1

X

1

0

0

0

0

1

CD AB

00

01

00 01 11 10

D D D

C C

X

1

1

X

X

X

11

10

B

B

B

A

A

DBCBAw .. ++= - Simplified expression:

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- Using the above simplified expressions in sum of products, one can implement this

combinational circuit using seven AND gates and three OR gates.

- One can use the properties of Boolean algebra to change the expressions in a way that

some of the terms are shared between the output functions to make the

implementation even simpler (fewer gates) as follows:

).( ..

)'.()'.( ''..)'.( ''..'.'.

)'(. ''..

'

DCBA DBCBAw

DCBDCB DCBDCB DCBDBCBx

DCDC DCDCy

Dz

++= ++=

+++= ++= ++= ++=

+= =

- These expressions can be implemented using four AND gates and four OR gates and

one inverter (we assume that the inputs are available in both normal and complement

forms).

C+D

x

y

w

z

A

B

C D

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- A combinational circuit that implements the addition of two bits is referred to as a

- A combinational circuit that implements the addition of three bits (two significant bits

and a previous carry) is referred to as a full adder.

- A full adder can be implemented by using two half adders.

- Assuming that S and C denote the sum and the carry (the higher significant bit in the

binary sum) of the two input variables of a half adder, we will have:

x y C S

0 0 0 0

0 1 0 1

1 0 0 1

1 1 1 0

yxC yx

yxyxS

.

'.'.

= ⊕= +=

S

C

x y

x y

x y

x y S

C

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- For a full adder, where the third input z denotes the carry corresponding to the

addition of the previous lower significant position we have:

x y z CS

0 0 0 0 0

0 0 1 0 1

0 1 0 0 1

0 1 1 1 0

1 0 0 0 1

1 0 1 1 0

1 1 0 1 0

1 1 1 1 1

0

1

1

0

0

1

1

0

yz x

x

x

0

1

00 01 11 10

z z z

y y

- Simplified expression: or: zyxzyxzyxzyxS ..''..'.'.'.'. +++= zyxS ⊕⊕=

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0

0

0

1

1

1

0

1

yz x

x

x

0

1

00 01 11 10

z z z

y y

- Simplified expression: or: zyzxyxC ... ++= zyxyxzyxzyxyxC ).(..'.'... ⊕+=++=

- We will have the following logic circuits:

x y z x y z x y z x y z

S

x y

x z

y z

C

- Using the second expression given for S and C, we can implement the full adder with

two half adders and an OR gate as shown below:

x y

z

C

S

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- One can construct an n-bit binary adder with n full adders in series, by connecting

each output carry to the input carry of the next higher-order full adder.

- For example, a four-bit binary adder is given below:

FA

S3

A3 B3

C0

FA

S0

A0 B0

C1

FA

S1

A1 B1

C2

FA

S2

A2 B2

C3

C4

- For and 10110123 == AAAAA 00110123 == BBBBB , we will have:

Subscript i: 3 2 1 0

Input Carry 0 1 1 0 Ci Augend 1 0 1 1 Ai Addend 0 0 1 1 BBi

Sum 1 1 1 0 Si Output Carry 0 0 1 1 Ci+1

- Note that in this circuit we have nine inputs and , and

so the implementation of the corresponding circuit using the method of Chapter 3

would require a truth table with rows (if we ignore the input carry since

it must be zero, we will have 256 rows).

,,,,,,,, 32103210 BBBBAAAA 0C

0C5122 9 =

- The signal must propagate through the gates before the correct output sum is

available and the longest propagation delay time in an adder is the time it takes for the

carry to propagate through the full adders.

- The output of each full adder will not settle to its final value until the input carry is

available from the previous stage.

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A Pi i Bi

Ci

Ci+1

Si

Gi

- For a full adder, the carry propagates through an AND gate and an OR gate which

constitutes two gate levels, which implies that in an n-bit adder, the carry propagates

from input to output through 2n gate levels.

- Use faster gates with reduced delays.

- Reduce the carry delay time by increasing the equipment complexity properly.

- One of the popular methods for this purpose is to use carry lookahead as follows:

iii

iii

BAG BAP

.= ⊕=

- and are referred to as carry propagate and carry generate respectively. iP iG

- The output sum and carry can then be written as:

iiii

iii

CPGC CPS

.1 += ⊕=

+

- We now use iterations to obtain each output carry directly from the input carry

as follows:

1+iC

0C

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001230123123233

0012012122333334

0012012122001011222223

001011000111112

0001

.......... ).......(.

......)....(. ...)..(.

.

CPPPPGPPPGPPGPG CPPPGPPGPGPGCPGC

CPPPGPPGPGCPPGPGPGCPGC CPPGPGCPGPGCPGC

CPGC

++++= ++++=+=

+++=+++=+= ++=++=+=

+=

- Since all these expressions are in standard form, one can use a two-level circuit to

implement this combinational system.

- A 4-bit adder with a carry lookahead scheme is given below:

B3 A3

P3

G3

B0 A0

P0

G0

B2 A2

P2

G2

B1 A1

G1

P1

C0 C0

P0

P1 S1

P2

S0

S2

S3

C4 C4

C3

C2

C1

P3

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- Binary subtraction can be done by adding to the 2’s complement of BA A B

(which can be obtained by adding one to the 1’s complement).

- The logic circuit for a 4-bit adder subtractor is given as follows:

FA

S3

A3 B3

C0

FA

S0

A0 B0

C1 FA

S1

A1 B1

C2 FA

S2

A2 B2

C3

C4

M

- The mode input controls the output. M

- When M is 0, then and the input carry is 0 which means that the output of

the circuit is equal to

BB =⊕ 0

plus . A B

'1 BB =⊕- When is 1, we have M and the input carry is 1, which implies that the

output is equal to plus the 1’s complement of A B plus 1. This means that the output

is minus (provided that there is no overflow). A B

- Addition of two n-bit numbers which are both positive or both negative, may result in

an overflow, i.e., the result may exceed the capacity of an n-bit register. This is shown

for 8-bit registers below:

Carries: 0 1

+70 0 1000110

+80 0 1010000

+150 1 0010110

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14 Carries: 1 0

-70 1 0111010 -80 1 0110000 -150 0 1101010

- In general, an overflow can be detected by applying exclusive-OR operation on the

input carry and output carry of the sign bit position (the leftmost digit).

- Note that if the last two carries were equal, there would be no overflow.

- For example, for the 4-bit adder subtractor, we will have:

C4 C3

V

- If the numbers are unsigned (the numbers are considered positive and the leftmost bit

does not represent the sign), the circuit performs minus if A B BA ≥ or the 2’s

complement of minus if . B A BA

- For two unsigned numbers, the output carry of the leftmost bits detects a carry after

addition or a borrow after subtraction.

Carry: 1

70 1000110

80 1010000

150 0010110

- The most common way to code a decimal number is binary coded decimal (BCD),

where the decimal number is represented by encoding each digit in binary form.

- Applying two BCD digits to a 4-bit binary adder will result in a number ranging from

0 to 9+9+1=19 as shown in the following table:

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Binary Sum BCD Sum Decimal

K Z8 Z4 Z2 Z1 C S8 S4 S2 S1

0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 1 0 0 0 0 1 1

0 0 0 1 0 0 0 0 1 0 2

0 0 0 1 1 0 0 0 1 1 3

0 0 1 0 0 0 0 1 0 0 4

0 0 1 0 1 0 0 1 0 1 5

0 0 1 1 0 0 0 1 1 0 6

0 0 1 1 1 0 0 1 1 1 7

0 1 0 0 0 0 1 0 0 0 8

0 1 0 0 1 0 1 0 0 1 9

0 1 0 1 0 1 0 0 0 0 10

0 1 0 1 1 1 0 0 0 1 11

0 1 1 0 0 1 0 0 1 0 12

0 1 1 0 1 1 0 0 1 1 13

0 1 1 1 0 1 0 1 0 0 14

0 1 1 1 1 1 0 1 0 1 15

1 0 0 0 0 1 0 1 1 0 16

1 0 0 0 1 1 0 1 1 1 17

1 0 0 1 0 1 1 0 0 0 18

1 0 0 1 1 1 1 0 0 1 19

- The left side of the table gives the output of a 4-bit binary adder but we want the

output to be in a BCD form.

- From the entries of the table, it can be concluded that a conversion from binary sum

to BCD representation is needed if K=1 or Z8 and Z4 are both equal to 1 or Z8 and Z2

are both equal to 1.

- The Boolean expression to show the condition for a correction is:

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2848 .. ZZZZKC ++=

- It can be easily seen that a correction can be made by adding 0110 to the binary sum

and providing an output carry for the next stage.

- The following figure shows a BCD adder (the input and the output are BCD digits).

Output carry C

S8 S4 S2 S1

Z8 Z4 Z2 Z1

Carry in

0

- From this circuit it can be seen that if the output carry (C) is equal to 1, binary

number 0110 is added to the binary sum.

- The output carry (C) is, in fact, equal to the output carry generated from the bottom

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Binary Multiplier

- Multiplication of binary numbers is similar to the multiplication of decimal numbers

as shown below for a 2-bit binary number:

B BB1 B0 A A1 0 A . B A . B0 1 0 0

A . B A . B 1 1 1 0 C C C C3 2 1 0

- So, multiplication of two 2-bit numbers can be implemented by a combinational

circuit with two half adders as follows:

B1

B1

HA

HA

A0

A1 B0

B0

C0 C1 C2 C3

- Assume now that the multiplier has 3 bits and the multiplicand has 4 bits.

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B B B B B3 B2 B1 B0 A A A2 1 0

A . B A . B A . B A . B 0 3 0 2 0 1 0 0 A . B A . B A . B A . B 1 3 1 2 1 1 1 0

A . B A . B A . B A . B 2 3 2 2 2 1 2 0

C C C C C C C6 5 4 3 2 1 0

- In this case, we will need two 4-bit adders and 12 AND gates as follows:

A1 B0 B1 B2 B3

A2 B0 B1 B2 B3

Sum and output carry

Sum and output carry

A0 B0 B1 B2 B3

0

C0 C1 C2 C3 C4 C5 C6

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- In general, if the multiplier has bits and the multiplicand has J K bits, we will need

K-bit adders and AND gates and the product will have bits. 1−J KJ × KJ +

Magnitude Comparator

- A magnitude comparator is a combinational circuit that compares two numbers A and

B and determines whether A > B, A = B, or A < B.

- Comparison operation should start by comparing the most significant bits of A and B.

- For example, consider two numbers A and B as follows:

0123

0123

BBBBB AAAAA

= =

- Define the following functions (which are the bib-by-bit exclusive-NOR operations

on A and B):

3,2,1,0 for''.. =+= iBABAx iiiii

- It can easily be verified that:

1..........

1..........

1...

0 ' 01231

' 1232

' 233

' 3

' 00123

' 1123

' 223

' 33

0123

=+++⇒<

=+++⇒>

=⇒=

BAxxxBAxxBAxBABA

BAxxxBAxxBAxBABA

xxxxBA

- In the equations for inequalities, the most significant bits (with the same significance)

in the two numbers are compared and if one of them is 1 and the other one is 0, the

comparison is finished and only if they are equal, the next significant bits in the two

numbers will be compared (this is represented by ANDing the ’s with

corresponding terms in the inequalities).

ix

- A 4-bit magnitude comparator logic circuit is given below:

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(A=B)

(A>B)

(A<B)

x2 A2

B2

x3 A3

B3

x1 A1

B1

x0 A0

B0

Decoders

- Decoder circuits are used to decode encoded information.

- A decoder is a combinational circuit that has inputs and a maximum of outputs. n n2

- The truth table for a 3-to-8-line decoder is as follows:

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InputsOutputs

x y z D D D D D D D D0 1 2 3 4 5 6 7

0 0 0 1 0 0 0 0 0 0 0

0 0 1 0 1 0 0 0 0 0 0

0 1 0 0 0 1 0 0 0 0 0

0 1 1 0 0 0 1 0 0 0 0

1 0 0 0 0 0 0 1 0 0 0

1 0 1 0 0 0 0 0 1 0 0

1 1 0 0 0 0 0 0 0 1 0

1 1 1 0 0 0 0 0 0 0 1

- The logic circuit for this decoder is shown in the following figure, where each output

represents one of the minterms.

x

y

z

D0 = x’.y’.z

D1 = x’.y’.z

D2 = x’.y.z

D3 = x’.y.z

D4 = x.y’.z

D5 = x.y’.z

D6 = x.y.z

D7 = x.y.z

3×8 Decoder

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- Sometimes NAND gates are used instead of AND gates to generate complemented

outputs.

- One or more enable inputs are often used in the decoders to control the operation of

the circuit.

- A 2-to-4-line decoder consisting of NAND gates with an enable input is given below:

A

B

A B

A B

A B

E

D0

D1

D2

D3

- The corresponding truth table is as follows:

E A B D D D D0 1 2 3

X X 1 1 1 1 1

0 0 0 0 1 1 1

0 0 1 1 0 1 1

0 1 0 1 1 0 1

0 1 1 1 1 1 0

- In general, the enable input signal E may be activated with a 0 or with a 1.

- Some decoders have more than one enable inputs, which must be equal to a specific

binary number in order to enable the circuit.

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- A demultiplexer is a circuit that receives data from a single input line and directs it to

one of the output lines, which is determined by selection lines.

- One can build a demultiplexer by using a decoder with an enable line, as can be seen

in the above example.

- In the above example, E is the data input line and A and B are selection inputs which

determine which output will be the same as the input value E (this can easily be seen

from the circuit).

- One can connect multiple decoders with enable lines to build a decoder with a greater

number of outputs.

- For example, two 3-to-8-line decoders with enable lines can be connected to construct

a 4-to-16-line decoder as follows:

D8 to D15

D0 to D7 3×8

Decoder

E

3×8 Decoder

E

x y z

w

- The minterms 0000 to 0111 are generated by the top decoder, while the bottom one

generates minterms 1000 to 1111 as shown below:

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InputsOutputs

w x y z D D D D D D D D0 1 2 3 4 5 6 7

0 0 0 0 1 0 0 0 0 0 0 0

0 0 0 1 0 1 0 0 0 0 0 0

0 0 1 0 0 0 1 0 0 0 0 0

0 0 1 1 0 0 0 1 0 0 0 0

0 1 0 0 0 0 0 0 1 0 0 0

0 1 0 1 0 0 0 0 0 1 0 0

0 1 1 0 0 0 0 0 0 0 1 0

0 1 1 1 0 0 0 0 0 0 0 1

InputsOutputs

w x y z D D D D D D D D8 9 10 11 12 13 14 15

1 0 0 0 1 0 0 0 0 0 0 0

1 0 0 1 0 1 0 0 0 0 0 0

1 0 1 0 0 0 1 0 0 0 0 0

1 0 1 1 0 0 0 1 0 0 0 0

1 1 0 0 0 0 0 0 1 0 0 0

1 1 0 1 0 0 0 0 0 1 0 0

1 1 1 0 0 0 0 0 0 0 1 0

1 1 1 1 0 0 0 0 0 0 0 1

- In the first table D to D8 15 are all zero and D to D0 7 in the second one are all zero

(assuming that the decoder is constructed by AND gates not NAND gates).

- In general, in logic components, enable inputs are used for interconnecting two or

more components.

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- Any n-input m-output combinational logic circuit can be implemented by using an n-

to-2n-line decoder (to generate different minterms) and m OR gates (to generate sum

of products).

- For example, a full adder can be implemented as follows:

∑ ∑ =

=

)7,6,5,3(),,(

)7,4,2,1(),,(

zyxC

zyxS

3×8 Decoder

0 1 2 3 4 5 6 7

22 21 20

x

y

z C

S

- If the number of minterms in the Boolean expression of a function is more than half

of the number of the outputs of the decoder, it is easier to implement the complement

function and then use a NOR gate instead of an OR gate, which will result in the same

function with smaller number of inputs for the gate.

- If the decoder is implemented by NAND gates, one can use bulb technique and

implement the function with a NAND gate instead of an OR gate in the output of the

decoder.

Encoders

- An encoder’s operation is the inverse operation of a decoder.

- For example, an octal-to-binary encoder is given below:

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