# single bit FA using only MUXs

Discussion in 'Homework Help' started by micachu, Oct 14, 2014.

1. ### micachu Thread Starter New Member

Oct 13, 2014
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0
hi i'm slightly confused here.
the goal is to only use MUXs for a single-bit Full Adder.
when adding two single bits there's no carry in but a possible carry out
so im not sure how to set this up.

heres my attempt:

*not sure if im using this program correctly, im trying to use two 2x1 mux*

2. ### WBahn Moderator

Mar 31, 2012
17,716
4,788
If you are only allowed to use MUXes, then why do you have two other gates there. What are the actual rules?

On what basis do you claim that there's no carry in. You are asked to make a full adder and not a half adder. What is the difference between the two?

3. ### Papabravo Expert

Feb 24, 2006
10,136
1,786
One mux is used for the sum and the other mux is used for the carry out. There are three inputs so you need 8:1 muxes. Does that help?

4. ### micachu Thread Starter New Member

Oct 13, 2014
15
0
The word "single-bit" confused me.
FA truth table requires 3 inputs (A,B,Cin) and two outputs (S, Cout)
since its asking ONLY mux i should use two 8x1 mux?

5. ### micachu Thread Starter New Member

Oct 13, 2014
15
0
yes thank you

6. ### micachu Thread Starter New Member

Oct 13, 2014
15
0
for practice would this be correct if using 4x1's?

7. ### WBahn Moderator

Mar 31, 2012
17,716
4,788
Where are you getting Cin' from?

As for whether the implementation gives the correct output, you need to be able to figure that out for yourself. Treat it as the reverse problem where you are given the diagram and asked to come up with the resulting truth table. Do that and, if it doesn't match the desired truth table and you can't figure out what needs to change, post your truth table and then we can help you from there.