A multiplexer circuit accepts N inputs and outputs the value of one of those inputs. The selection of which input goes out on the output is determined by a set of M control inputs. A multiplexer with M control inputs can steer up to 2M inputs to a single output.
The I/O for a multiplexer is shown in the following figure.
I/O for the multiplexer
The truth table for a 2-to-1 multiplexer is shown in the following table.
| Control | Input0 | Input1 | Output | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 1 | 0 | 1 | 0 | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 1 | 0 | 1 | 1 | 1 | 1 | 0 | 0 | 1 | 1 | 1 | 1 |
It is often simpler to write these truth tables if we introduce the don't caresymbol X. This symbol is used in place of a logic 0 or 1 on an input when the input has no effect on the value of the output. With the multiplexer, for example, Input1 has no effect on the output when the Control input is 0. Similarly, Input0 has no effect when the Control is 1. Rewriting the multiplexer truth table with don't-cares results in the following table.
| Control | Input0 | Input1 | Output | 0 | 0 | X | 0 | 0 | 1 | X | 1 | 1 | X | 0 | 0 | 1 | X | 1 | 1 |
The logic circuitry for a 2-to-1 multiplexer is shown in the following figure.
These are the instructions to play with the circuit:
This circuit can be encapsulated in a single module, as can be seen in the following figure :
A 4 - bit multiplexer circuit is constructed in the next page
Last modified 1/1/2000 by Juan de Lara ( Juan.Lara@ii.uam.es, http://www.ii.uam.es/~jlara) need help for using this courses?.