1. Instructor: Alexander Stoytchev
CprE 281: Digital Logic
Instructor: Alexander Stoytchev

2. Multiplexers CprE 281: Digital Logic Iowa State University, Ames, IA
Copyright © Alexander Stoytchev

3. Administrative Stuff
HW 6 is due on Monday

4. Administrative Stuff
HW 7 is out
It is due on Monday Oct 4pm

5. 2-1 Multiplexer (Definition)
Has two inputs: x1 and x2
Also has another input line s
If s=0, then the output is equal to x1
If s=1, then the output is equal to x2

6. Graphical Symbol for a 2-1 Multiplexer
[ Figure 2.33c from the textbook ]

7. Truth Table for a 2-1 Multiplexer
[ Figure 2.33a from the textbook ]

8. Let’s Derive the SOP form

9. Let’s Derive the SOP form

10. Let’s Derive the SOP form
Where should we
put the negation signs?
s x1 x2
s x1 x2
s x1 x2
s x1 x2

11. Let’s Derive the SOP form
s x1 x2
s x1 x2
s x1 x2
s x1 x2

12. Let’s Derive the SOP form
s x1 x2
s x1 x2
s x1 x2
s x1 x2
f (s, x1, x2) =
s x1 x2 +

13. Let’s simplify this expression
f (s, x1, x2) =
s x1 x2 +

14. Let’s simplify this expression
f (s, x1, x2) =
s x1 x2 +
f (s, x1, x2) =
s x1 (x2 + x2)
s (x1 +x1 )x2 +

15. Let’s simplify this expression
f (s, x1, x2) =
s x1 x2 +
f (s, x1, x2) =
s x1 (x2 + x2)
s (x1 +x1 )x2 +
f (s, x1, x2) =
s x1
s x2 +

16. Circuit for 2-1 Multiplexers
(c) Graphical symbol
(b) Circuit
f (s, x1, x2) =
s x1
s x2 +
[ Figure 2.33b-c from the textbook ]

17. More Compact Truth-Table Representation1
(a)
Truth
table s x1 x2
f (s, x1, x2) 1
f (s, x1, x2) s x1 x2
[ Figure 2.33 from the textbook ]

18. 4-1 Multiplexer (Definition)
Has four inputs: w0 , w1, w2, w3
Also has two select lines: s1 and s0
If s1=0 and s0=0, then the output f is equal to w0
If s1=0 and s0=1, then the output f is equal to w1
If s1=1 and s0=0, then the output f is equal to w2
If s1=1 and s0=1, then the output f is equal to w3

19. Graphical Symbol and Truth Table
[ Figure 4.2a-b from the textbook ]

20. The long-form truth table

21. The long-form truth table[

22. 4-1 Multiplexer (SOP circuit)
f = s1 s0 w0 + s1 s0 w1 + s1 s0 w2 + s1 s0 w3
[ Figure 4.2c from the textbook ]

23. Using three 2-to-1 multiplexers to build one 4-to-1 multiplexerw w 1 1 f 1 w 2 w 3 1
[ Figure 4.3 from the textbook ]

24. Using three 2-to-1 multiplexers to build one 4-to-1 multiplexer

25. Using three 2-to-1 multiplexers to build one 4-to-1 multiplexer

26. Using three 2-to-1 multiplexers to build one 4-to-1 multiplexer

27. Using three 2-to-1 multiplexers to build one 4-to-1 multiplexerw0 w1 s1 f s0 w2 w3

28. That is different from the SOP form of the 4-1 multiplexer shown below, which uses less gates

29. 16-1 Multiplexer s f w [ Figure 4.4 from the textbook ] 1 3 4 2 7 8 113 4 7 12 15 2 f
[ Figure 4.4 from the textbook ]

30. Multiplexers Are Special

31. The Three Basic Logic Gatesx 1 2 × x 1 2 + x
NOT gate
AND gate
OR gate
[ Figure 2.8 from the textbook ]

32. Truth Table for NOT x 1 x

33. Truth Table for AND x 1 2 ×

34. Truth Table for OR x 1 2 +

35. Building an AND Gate with 4-to-1 Mux

36. Building an AND Gate with 4-to-1 Mux
These two are the same.

37. Building an AND Gate with 4-to-1 Mux
These two are the same.
And so are these two.

38. Building an OR Gate with 4-to-1 Mux

39. Building an OR Gate with 4-to-1 Mux
These two are the same.

40. Building an OR Gate with 4-to-1 Mux
These two are the same.
And so are these two.

41. Building a NOT Gate with 4-to-1 Mux1

42. Building a NOT Gate with 4-to-1 Muxy f 1
Introduce a dummy variable y.

43. Building a NOT Gate with 4-to-1 Muxy f 1

44. Building a NOT Gate with 4-to-1 Muxy f 1
Now set y to either 0 or 1 (both will work). Why?

45. Building a NOT Gate with 4-to-1 Mux1
Two alternative solutions.

46. Implications Any Boolean function can be implemented
using only 4-to-1 multiplexers!

47. Building an AND Gate with 2-to-1 Mux

48. Building an AND Gate with 2-to-1 Mux

49. Building an AND Gate with 2-to-1 Muxx2

50. Building an OR Gate with 2-to-1 Mux

51. Building an OR Gate with 2-to-1 Mux

52. Building an OR Gate with 2-to-1 Mux

53. Building a NOT Gate with 2-to-1 Mux1

54. Building a NOT Gate with 2-to-1 Mux1

55. Implications Any Boolean function can be implemented
using only 2-to-1 multiplexers!

56. Synthesis of Logic Circuits
Using Multiplexers

57. 2 x 2 Crossbar switch s x y x y [ Figure 4.5a from the textbook ] 1 1

58. 2 x 2 Crossbar switch
s=0 x y 1 1 x y 2 2
s=1 x y 1 1 x y 2 2

59. Implementation of a 2 x 2 crossbar switch with multiplexers1 y 1 1 s x 2 y 2 1
[ Figure 4.5b from the textbook ]

60. Implementation of a 2 x 2 crossbar switch with multiplexers

61. Implementation of a 2 x 2 crossbar switch with multiplexersy1 y2

62. Implementation of a logic function with a 4x1 multiplexer2 1 2 w 1 1 1 1 f 1 1 1 1 1
[ Figure 4.6a from the textbook ]

63. Implementation of the same logic function with a 2x1 multiplexer1 w 2 1 1 1 w w 2 2 1 1 f 1 1
(b) Modified truth table
(c) Circuit
[ Figure 4.6b-c from the textbook ]

64. The XOR Logic Gate
[ Figure 2.11 from the textbook ]

65. The XOR Logic Gate
[ Figure 2.11 from the textbook ]

66. Implemented with a multiplexer
The XOR Logic Gate
Implemented with a multiplexer f

67. Implemented with a multiplexer
The XOR Logic Gate
Implemented with a multiplexer x f y

68. Implemented with a multiplexer
The XOR Logic Gate
Implemented with a multiplexer x f y
These two circuits are equivalent
(the wires of the bottom AND gate are flipped)

69. all four of these are equivalent!
In other words,
all four of these are equivalent! x y f x y f w 2 w 1 x y f 1 f 1

70. Implementation of another logic functionw w w f 1 2 3 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
[ Figure 4.7a from the textbook ]

71. Implementation of another logic functionw w w f 1 2 3 w w f 1 2 1 w 1 3 1 w 1 3 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
[ Figure 4.7a from the textbook ]

72. Implementation of another logic functionw w w f 1 2 3 w w f 1 2 1 w 1 3 1 w 1 3 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
(a) Modified truth table w 2 w 1 w 3 f 1
(b) Circuit
[ Figure 4.7 from the textbook ]

73. Another Example (3-input XOR)

74. Implementation of 3-input XOR with 2-to-1 Multiplexers1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
[ Figure 4.8a from the textbook ]

75. Implementation of 3-input XOR with 2-to-1 Multiplexers1 1 w Å w 2 3 1 1 1 1 1 1 1 1 w Å w 2 3 1 1 1 1 1 1
[ Figure 4.8a from the textbook ]

76. Implementation of 3-input XOR with 2-to-1 Multiplexers1 1 w Å w 2 3 w 1 1 1 1 1 w Å w 2 3 1 1 f 1 1 w Å w 2 3 1 1 1 1 1 1
(a) Truth table
(b) Circuit
[ Figure 4.8 from the textbook ]

77. Implementation of 3-input XOR with 2-to-1 Multiplexersw3 1 1 w 2 w 1 1 w3 1 1 1 w 3 1 1 f 1 1 1 1 1 1 1 1
(a) Truth table
(b) Circuit
[ Figure 4.8 from the textbook ]

78. Implementation of 3-input XOR with a 4-to-1 Multiplexer2 3 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
[ Figure 4.9a from the textbook ]

79. Implementation of 3-input XOR with a 4-to-1 Multiplexer2 3 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
[ Figure 4.9a from the textbook ]

80. Implementation of 3-input XOR with a 4-to-1 Multiplexer2 3 w 3 1 1 1 1 w 3 1 1 1 1 w 3 1 1 1 1 w 3 1 1 1 1
[ Figure 4.9a from the textbook ]

81. Implementation of 3-input XOR with a 4-to-1 Multiplexer2 3 w 3 w 1 1 2 w 1 1 1 w 3 w 1 1 3 1 1 f w 3 1 1 1 1 w 3 1 1 1 1
(a) Truth table
(b) Circuit
[ Figure 4.9 from the textbook ]

82. Multiplexor Synthesis Using Shannon’s Expansion

83. Three-input majority functionw w w f 1 2 3 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
[ Figure 4.10a from the textbook ]

84. Three-input majority functionw w w f 1 2 3 w f 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
[ Figure 4.10a from the textbook ]

85. Three-input majority functionw w w f 1 2 3 w f 1 1 w w 2 3 1 1 w + w 2 3 1 1 1 1 1 1 1 1 1 1 1 1 1 1
[ Figure 4.10a from the textbook ]

86. Three-input majority functionw w w f 1 2 3 w f 1 1 w w 2 3 1 1 w + w 2 3 1 1 1 1 1 1 1 1 1 1 1 1 1 1
(b) Truth table w w 1 2 w 3 f
[ Figure 4.10a from the textbook ]
(b) Circuit

87. Three-input majority functionw w 1 2 w 3 f

88. Shannon’s Expansion Theorem
Any Boolean function can be rewritten in the form:

89. Shannon’s Expansion Theorem
Any Boolean function can be rewritten in the form:

90. Shannon’s Expansion Theorem
Any Boolean function can be rewritten in the form:
cofactor
cofactor

91. Shannon’s Expansion Theorem
(Example)

92. Shannon’s Expansion Theorem
(Example)
(w1 + w1)

93. Shannon’s Expansion Theorem
(Example)
(w1 + w1)

94. Shannon’s Expansion Theorem (In terms of more than one variable)
This form is suitable for implementation with a 4x1 multiplexer.

95. Another Example

96. Factor and implement the following function with a 2x1 multiplexer

97. Factor and implement the following function with a 2x1 multiplexer

98. Factor and implement the following function with a 2x1 multiplexer
[ Figure 4.11a from the textbook ]

99. Factor and implement the following function with a 4x1 multiplexer

100. Factor and implement the following function with a 4x1 multiplexer

101. Factor and implement the following function with a 4x1 multiplexer
[ Figure 4.11b from the textbook ]

102. Yet Another Example

103. Factor and implement the following function using only 2x1 multiplexers

104. Factor and implement the following function using only 2x1 multiplexers

105. Factor and implement the following function using only 2x1 multiplexers

106. Factor and implement the following function using only 2x1 multiplexers

107. Factor and implement the following function using only 2x1 multiplexers

108. Factor and implement the following function using only 2x1 multiplexers

109. Factor and implement the following function using only 2x1 multiplexersw3 w2 h w3 1 w2

110. Finally, we are ready to draw the circuitg w3 w2 f g h w1 h w3 1 w2

111. Finally, we are ready to draw the circuitg w3 w2 h 1 f w1

112. Finally, we are ready to draw the circuit2 1 w 3 f 1
[ Figure 4.12 from the textbook ]

113. Questions?

114. THE END