# Using a 8-1 MUX only (no external gates) to perform digital logic

1. Oct 13, 2011

### bigoxygen

1. The problem statement, all variables and given/known data

Assuming the existence of 6 digital inputs and 1 digital output, design a schematic circuit diagram using any number of 8-1 muxs (i.e. no external gates) to satisfy the following requirements:
 The output is true (1) when (inputs 1 and 2 are not the same) and at least 2 of the inputs 3,4,5 and 6 are true (logic 1).

2. Relevant equations

3. The attempt at a solution

I have drawn out a truth table for the mux...

A B C as the selectors

with combinations of 000-111 for 8 inputs

I also attempted to write the boolean equation for the mux...
tried to simulate a mux in Quartus using VHDL and solve it that way (but my VHDL is not very good).

There are so many variables and I do not know exactly which path I should take...

Could you give me a clue?

Also, how do you use a digital circuit to simulate "at least #" are true or false?

2. Oct 14, 2011

### MisterX

An 8-1 mux can be used to implement any logic function with three inputs and one output. The logic function inputs would be connected to the selectors. The mux inputs would be constant. If true was high, then each minterm in the sum of products form of the logic function would have a high source connected to the mux input for that minterm. The other mux inputs would be low.

You're supposed to implement a logic function with six inputs and one output. That can be done using multiple logic functions with 3 inputs and one output.