Digital-techniques-microprocessors

Digital Techniques and Applications - PART 7

07:24:00 Nandan Bhalwankar 0 Comments

e) Multiplexers

It is also commonly known as ‘Mux’. Depending upon the digital code applied at the selected inputs, the multiplexer selects one input from ‘n’ data sources and transmits to a single output channel.

If you note, Strobe input ‘G’ is incorporated which helps in cascading and its generally active low (digit 0). That means to enable the Mux, the Strobe (G) input needs to be 0 or low.

In the logic diagram given below, pins S0 and S1 are known as select pins. Based on their values, the respective input is selected.Basically Mux chose among 2n inputs where ‘n’ is the number of select pins. Looking at the basic 4:1 Mux, there are 4 inputs and 2 select pinsand single output.


Truth table for Multiplexer:

Select Pin 1 (S0)
Select Pin 1 (S1)
Output
0
0
Input 0
0
1
Input 1
1
0
Input 2
1
1
Input 3

As we can see in the truth table, combination of select pin inputs S0 and S1, Mux selects appropriate input in the output.

Realization of Multiplexer


As we see in the truth table, input X0 is selected when select pins A and B both are 0 (low). In the above realization, NOT gate is used to complement the select pins A and B i.e. the inputs to first AND gate will all be 1 and hence its output will be considered. In this case, the output of all remaining AND gates will be 0 (low). Basically, the AND gate with 1 (high) output passes the correct input among X0, X1, X2 and X3 as the output of whole Mux.



Advantages of Multiplexers:

1) Simplification of logic expression is not required
2) Minimization of IC package count
3) Logic design is simplified



f) De-Multiplexers

As the name indicates, it works opposite to Multiplexer. It is also commonly known as ‘De-Mux’. It accepts a single input and distributes it over several outputs.The select input code determines to which output the data input should be transmitted.
Have a look at equation n = 2m
Here, n = Number of output lines
m = Number of select lines

De-Mux has Enable input line which is generally active high (digit 1). That means to enable the De-Mux, the Enable (E) input needs to be 1 or high.



Truth table for De-Multiplexer

Select Pin 1 (S0)
Select Pin 1 (S1)
Output
0
0
A
0
1
B
1
0
C
1
1
D

As we can see in the truth table, combination of select pin inputs S0 and S1, De-Mux transmits input to appropriate output.


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