Microsoft Word - Digital Logic Design v_4_6a

9.4. Multiplexer (MUX)/DeMultiplexer (DMUX) Design

Multiplexers (MUX) and DeMultiplexers (DMUX) are used to route signals between networks with unequal
number of signal lines. There are many applications that need one line for control or monitoring, but also
need to analyze the data in a more compressed format. The application can be in communication, power,
control, etc.

For example: You are building a security system that needs to control 200 entry ways. Each entry way
will provide one input (Open/Close). Instead of running the 200 wires to the control, we could DMUX it
into 8-bits (2 n = 256 when n=8). This means that only 8 lines are needed to go to the control instead of
200.

 An example of using a 1 to 8 DMUX and a 8 to 1 MUX

 Building a Large Scale MUX from Smaller MUXes

Typically use a cascading tree of MUXes to build a larger MUX :
 Identify the number of MUX-ed outputs needed:
n ≥ {ln(#input lines} / {ln 2} where n is the smallest integer that satisfies the equation and
indicates the number of outputs.
 If you have J-to-K MUX available then you will need n/K levels

 Example of implementing a 4-to-1 MUX using 2-to-1 MUXes

D 0 _i (^) MUX
D 1 _i

S 2 S 1 S 0

The signal can be used for DMUX

computation or reduce the

number of wires required to

communicate

Output Input

Connect one of D0-D7 to output

D 2 _i

D 3 _i

D 4 _i

D 5 _i

D 7 _i

D 6 _i

D 0 _o

D 1 _o

D 2 _o

D 3 _o

D 4 _o

D 5 _o

D 7 _o

D 6 _o

S 2 S 1 S 0

Selects the input line that will be

connected to the output.

Selects the output line that will be

connected to the input.