Shankar Balachandran*
Associate Professor, CSE Department
Indian Institute of Technology Madras
*Currently a Visiting Professor at IIT Bombay
Digital Circuits and
Systems
Spring 2015 Week 3 Module 14
Multiplexers
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Shankar Balachandran* Associate Professor, CSE Department Indian Institute of Technology Madras
*Currently a Visiting Professor at IIT Bombay
Digital Circuits and
Systems
Spring 2015 Week 3 Module 14
Multiplexers
Multiplexers
Multiplexing means transmitting a large number of information
units over a smaller number of channels or lines.
A digital multiplexer ( MUX ) selects binary information from one
of many input lines and directs it to a single output line.
Data selector (2n:1 MUX). Inputs: 2n^ data inputs, n select lines. Output: 1 data output line.
s1 s0 Out 0 0 D 0 1 D 1 0 D 1 1 D
D0 D
0 1
D2 D
0
1
Out s1 s
s1 s (control)
D
D
D
D
Out
Internal Structure of a 4:1 MUX
A 2n:1 MUX needs 2n, ( n +1)-input AND gates for selection and a 2 n-input OR gate to generate the final output. AND/OR logic structure
Closeness to Decoders
En
D 2
D 3
D 1
D 0
Connect all the input lines of the multiplexer together (to make enable) and remove the OR gate to give you a decoder
Using 2:1 Muxes to Build a 4:1 Mux
0
w 0 w 1
0 1
w 2 w 3
0 1
f 0 1
s 1 s
Practical Application of Multiplexers
x 1 0 1
x 2 0 1
s
y 1
y 2
x 1 x 2
y 1 y 2
2x2 crossbar switch
s
MUX Based Logic
Advantages : Easier to design combinational circuits. Easier to debug circuits designed using multiplexers. Disadvantages : Multiplexers can become very large for a large number of inputs.
Good for small circuits.
Normally, any function with more than 4 variable is impractical for direct implementation (i.e., using a single large MUX). Use tree of small MUXes or using a variable as MUX data input or Shannon’s Expansion Theorem for implementing large functions.
Multiplexer Tree
A larger MUX can be implemented using a tree of smaller MUXes. Example : Implement the function using smaller MUXes instead of one 8:1 MUX.
F a ,b,c (^) 1 , 2 , 4 , 7