ECE 2372Modern Digital System Design

Section 4.4

Mutiplexers - Demultiplexers

1

Multiplexers/Demultiplexers • Basic Structure: Multiplexing describes

any system that uses a single channel to carry more than one stream of information.

• Digital systems use time-division multiplexing.

2

Multiplexers/Demultiplexers • A basic structure for time-division multiplexing is:

Aout

Bout

Cout

Kout

Ain

Bin

Cin

Kin

Multiplexer Demultiplexer

Single Channel

• The switches are synchronized so that input A is connected to output A, etc. A multiplexer is an n-to-1 encoder.

3

Multiplexers/Demultiplexers • A 4-to-1 multiplexer takes the form:

S1 S0 Y 0 0 D0 0 1 D1 1 0 D2 1 1 D3

D0

Y D1

S1

4-to-1 Multiplexer D2

D3

S0

Selection Code

Y = S1S0D0 + S1S0D1+ S1S0D2+ S1S0D3 4

Multiplexers/Demultiplexers • With logic diagram:

S0

S1

D0

D1

D2

D3

Y

Y = S1S0D0 + S1S0D1+ S1S0D2+ S1S0D3 5

Multiplexers/Demultiplexers • A demultiplexer is a 1-to-n decoder of the form:

Inputs Outputs E S1 S0 D3 D2 D1 D0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 0 1 1 0 0 0 0 1 0 0 0 0 0 1 1 0 1 0 0 1 0 1 1 0 0 1 0 0 1 1 1 1 0 0 0

D0

E D1

S1

4-to-1 Demultiplexer D2

D3

S0

Selection Code

6

Multiplexers/Demultiplexers • A multiplexer – demultiplexer pair may be used as

follows:

D0 Y D1

S1

4-to-1 Multiplexer D2

D3

S0

D0

E D1 4-to-1 Demultiplexer D2

D3

7

Multiplexers/Demultiplexers • Quad Two Line – to – One Line

Multiplexer.• Used to select one of two 4 – bit words.

Enable Select Y 0 All 0’s 1 0 Select 4-bit word A 1 1 Select 4-bit word B

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A0

A1

A2

A3

B0

B1

B2

B3

S

E

Y0

Y1

Y2

Y3

Quad Two Line – to – One Line Multiplexer9

74150 16-to-1 Multiplexer

10

74150 16-to-1 Multiplexer

11

74150 16-to-1 Multiplexer

12

74150 16-to-1 Multiplexer

E0

W

E1

74150

E2

E3

E4

E5

E6

E7

E8

E9

E10

E11

E12

E13

E14

E15

A

B

C

D

G

13

74150 16-to-1 Multiplexer Select Strobe Output

D C B A G W

H H L L L L L E0 L L L H L E1 L L H L L E2 L L H H L E3 L H L L L E4 L H L H L E5 L H H L L E6 L H H H L E7 H L L L L E8 H L L H L E9 H L H L L E10 H L H H L E11 H H L L L E12 H H L H L E13 H H H L L E14 H H H H L E15

14

74157 Quad 2-to-1 Multiplexer

15

74157 Quad 2-to-1 Multiplexer

16

74157 Quad 2-to-1 Multiplexer

17

74157 Quad 2-to-1 Multiplexer

18

74157 Quad 2-to-1 Multiplexer

74157 4A

G S

4B

3A

3B

2A

2B

1A

1B

4Y

3Y

2Y

1Y

19

74157 Quad 2-to-1 Multiplexer

Enable Select Outputs G S 1Y 2Y 3Y 4Y H L L L L L L 1A 2A 3A 4A L H 1B 2B 3B 4B

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Applications of Multiplexers • Data-selection: A multiplexer may be used when one of

two data sources will be selected for a digital process.• This is in effect a digitally controlled selector switch.

Data

Select

D1

Output

Multiplexer

D2

Data

21

Applications of Multiplexers • Switching Functions: A multiplexer may be

used to generate minterms.

22

Applications of Multiplexers

A B C

0

1

2

3

4

5

6

7

a b c

f (c, b, a)

+ 5 V

f (c, b, a) = m1 + m4 + m6

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Multiplexer/demultiplexer

A B C

E0

E15

D A B C

0

15

D

Y G

C3

C2

C1

C0

74150 74154

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