digital electronics chapter 4 combinational logic
TRANSCRIPT
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Digital Electronics
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Chapter 4
Combinational Logic
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Terminology
Combinational:Output is completely determined from the input(s) and does not depend on time
Sequential : Output depends on the input(s), previous history, and time
Analysis : A circuit is given and one must determine the Truth Table
Design: One must build a circuit whose output(s) are given as a Truth Table
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Analysis Problem
Set up the Truth Table
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Analysis: Truth Table
x y F
0 0 0
0 1 1
1 0 1
1 1 0
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Design or Synthesis
x y z F
0 0 0 0
0 0 1 0
0 1 0 0
0 1 1 1
1 0 0 0
1 0 1 1
1 1 0 1
1 1 1 1
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K-Map of Design Problem
y'z' y'z yz yz'
x'
x
1
1 1 1
F = x y + x z + y z
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Final Circuit Design
F = x y + x z + y z
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Binary Adder
Half Adder Truth Table
x y C S
0 0 0 0
0 1 0 1
1 0 0 1
1 1 1 0
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Implementation of Half Adder
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Full Adder
x y z C S
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
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Implementation of Full Adder
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Magnitude Comparator
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Comparator Theory
x is generated from XNOR and equals 1 if the two bits are equal
A = B if all the x’s are equal
A > B if the corresponding bit is greater as long as the previous bits are equal
A < B if the corresponding bit is smaller as long as the previous bits are equal
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3-to-8 Line Decoder
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Decoder Truth Table
Which output will be high when x =1, y = 1 and z = 0 ?
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Decoder Truth Table
Only line 6 will be high. The other 7 lines will be low.
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Decoder Application
Implement the Full Adder with a 3x8 Decoder
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Full Adder using 3x8 Decoder
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Important Note!
The actual 74LS138 decoder chip has inverted outputs … welcome to the REAL WORLD!!!
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Multiplexer
A multiplexer is a combinational circuit that selects binary information from one of many
input lines and directs it to a single output line.
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Multiplexer
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Multiplexer Application
Implement the function F(x,y,z) = Σ(1,2,6,7)
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F(x,y,z) = Σ(1,2,6,7) with a multiplexer
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VHDL
// A 2x4 Decoder with enable Emodule my_decoder (A,B,E,D); input A,B,E; output [0:3] D; assign D[0] = ~(~A & ~B & ~E), D[1] = ~(~A & B & ~E), D[2] = ~(A & ~B & ~E), D[3] = ~(A & B & ~E);endmodule
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Gate implementation of my_decoder
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More VHDL ...
//A 4-bit comparatormodule comp(A,B,ALTB,AGTB,AEQB); input [3:0] A,B; output ALTB,AGTB,AEQB; assign ALTB = (A < B), AGTB = (A > B), AEQB = (A == B);endmodule
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That’s All Folks!