Digital-techniques-microprocessors
Digital Techniques and Applications - PART 7
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.
0 comments:
Post a Comment