Contents
Logic Implementation from specifications
Logic Implementation using Construction Design: Adders
Logic Implementation Using Decoders
Logic Implementation Using MultiplexersCombinational
Circuit
OutputsInputs
This is the Default Solution
Construct The Truth Table
K-map for each output
Implement the Simplified Equations
Any Gates
NAND ONLY
NOR ONLY
Logic Gates
Adders
Multiplexer
Implement Using Pre-Designed Blocks
Decoder
Implement by
Construction
Specifications Logic Implementation from specifications
Problem 3
Design a combinational circuit with three inputs and one output. The output is
1 when the value of the inputs is less than 3. The output is 0 otherwise.
DecXYZF
00001
10011
20101
30110
41000
51010
61100
71110
??????= � � + � �
11 1
00 01 11 10
0
0 1 3 2
1 4 5 7 6
Y
Z
XY Z
F
X
00 01 11 10
0
0 1 3 2
1 4 5 7 6
Problem 4
Design a combinational circuit with three inputs and one output. The output is
1 when the value of the inputs is even. The output is 0 otherwise.
DecXYZF
00001
10010
20101
30110
41001
51010
61101
71110
??????= �
1
1 1
Y
Z
X 1Z F
Design a combinational circuit with three inputs, x, y, z, and three outputs, A, B,
and C. When the binary input is 0, 1, 2, or 3, the binary output is two greater
than the input. When the binary input is 4, 5, 6, or 7, the binary output is three
less than the input.
Problem 5
DecXYZABC
0000010
1001011
2010100
3011101
4100001
5101010
6110011
7111100
A= Σ(2,3,7)
B= Σ(0,1,5,6)
C= Σ(1,3,4,6)
0 1
00
0 1
01
2 3
11
6 7
10
4 5
C= X Z+X Z
1
1
XY
Z
Z
Y
X
1
C= Σ(1,3,4,6)
0 1
00
0 1
01
2 3
11
6 7
10
4 5
B= X Y + Y Z+X Y Z
11
XY
Z
Z
Y
X
1
B= Σ(0,1,5,6)
0 1
00
0 1
01
2 3
11
6 7
10
4 5
A= X Y+Z Y
1
1
XY
Z
Z
Y
X
1
A= Σ(2,3,7)
1
1
Y Z
B
X
A
C C= X Z+X Z
B= X Y + Y Z+X Y Z
A= X Y+Z Y
Kindly note
In this section, we concentrated only on the
function and usage of the most important blocks.
Therefore, you need to study from the slides each
block details and any other unmentioned blocks.
ADDERS
Adders(Half-Adder)
DecABCoS
00000
10101
21001
31110
S=A ⨁ B
C = A BC
S
B
A
Adders(Full-Adder)
DecCiABCoS
000000
100101
201001
301110
410001
510110
611010
711111
011
11A1011
3B0011
14S1110
Full-Adder
Get the Equations &
Implement as in the Dr.’s Slides
Example
Design a 4-bit combinational circuit incrementor ( A circuit that adds one to
a 4-bit binary number). The circuit can be designed using four half-adders. BA
Co
S
Half-Adder
A0
S0
Vs
(Logic1)
BA
Co
S
Half-Adder
A1
S1
BA
Co
S
Half-Adder
A2
S2
BA
Co
S
Half-Adder
A3
S3
Cout
Co2Co1Co0
AA3A2A1A0
10001
A+11110
Example
Designacombinationalcircuitusingaminimumnumberoffulladdersto
decrementa4-bitnumberby2.Assume6-bitresult.Drawalogicdiagram
usingtheblockdiagramofafulladderasthebuildingblock.
Example
Designacombinationalcircuitusingfulladderstomultiplya4-bit
unsignednumberby2.Drawalogicdiagramusingtheblockdiagramofa
fulladderasthebuildingblock.
Problem 6
Theadder–subtractorcircuitdescribedatthelecturehasthefollowingvalues
formodeinputManddatainputsAandB.MAB
(a)001110110
(b)010001001
(c)111001000
(d)101011010
(e)100000001
Ineachcase,determinethevaluesofthefourSUMoutputs,andthecarryC
0 0111 0110
Binary Multiplier 2X2
4x4 Magnitude Comparator
DECODERS
Decoder
•A (m x 2
m
) decoder is a combinational circuit that decodes m bit binary
inputs into 2
m
output line.
•Each output line is numbered with a decimal number.
•When the input code has the same decimal number as an output line; the
output line is set to logic one and other outputs are reset to zero.
DecABCD0D1D2D3D4D5D6D7
000010000000
100101000000
201000100000
301100010000
410000001000
510100000100
611000000010
711100000001
Made Basically From AND gates + Inverters
DecABCD0D1D2D3D4D5D6D7
000001111111
100110111111
201011011111
301111101111
410011110111
510111111011
611011111101
711111111110
Made Basically From NAND gates + Inverters
Obtain the Outputs’ minterms
Use (#inputs x 2
#inputs
) active high Decoder
For Each output One OR gate is Needed
According to the minterms, the inputs of each
OR gate are taken from the decoder’s output
1
2
3
4
Logic Implementation Using Active High
Decoders (The Default)
Obtain the Outputs’ minterms
Use (#inputs x 2
#inputs
) active low Decoder
For Each output One NAND gate is Needed
According to the minterms, the inputs of each
NAND gate are taken from the decoder’s output
1
2
3
4
Logic Implementation Using Active Low
decoders
Problem7
A combinational circuit is specified by the following three Boolean
functions:
F1(A, B, C) = ∑(1, 4, 6)
F2(A, B, C) = ∑ (3, 5)
F3(A, B, C) = ∑ (2, 4, 6, 7)
Implement the circuit with a decoder constructed with AND gates
and OR gatesconnected to the decoder outputs.
A
B
C
3x8
Decoder
0
7
6
5
4
3
2
1
F1 F3F2
Y0
Y1
Y2
Y3
Y4
Y5
Y6
Y7 F1(A,B,C) = Σ (1,4,6)
F2(A,B,C) = Σ (3,5)
F3(A,B,C) = Σ (2, 4, 6, 7)
Example
A combinational circuit is specified by the following three Boolean
functions:
F1(A, B, C) = ∑(1, 4, 6)
F2(A, B, C) = ∑ (3, 5)
F3(A, B, C) = ∑ (2, 4, 6, 7)
Implement the circuit with a decoder constructed with NAND
gates and ANDgatesconnected to the decoder outputs.
Problem 8
Using a decoder and external gates, design the combinational circuit defined
by the following three Boolean functions
F1 = (Y’+X)Z
F2 = Y’Z’+XY’+YZ’
F3 = (X’+Y)Z
EquationTerm XYZDE
C
-Y’Z
0011
F1
1015
X-Z
1015
1117
EquationTerm XYZDEC
-Y’Z’
0000
F2
1004
XY’-
1004
1015
-YZ’
0102
1106
EquationTerm XYZDE
C
-YZ
0113
F3
1117
X’-Z
0011
0113
F1 = Σ(1,5,7)
F2 = Σ(0,2,4,5,6)
F3 = Σ(1,3,7) Z
Y
X
3x8
Decoder
0
7
6
5
4
3
2
1
F1 F2 F3
Y0
Y1
Y2
Y3
Y4
Y5
Y6
Y7
2
0
2
1
2
2
Multiplexers
•A (2
m
x 1) Multiplexer is a combinational circuit that connect one of 2
m
inputs with the output line according to the value of m selection lines.
•Each input line is numbered with a decimal number.
•To connect the input line number (x)
10(say x=3) with the output, put on
the selection inputs the value of x in binary (011).