# using 8:1 multiplexers to add 4 bit numbers

Discussion in 'Homework Help' started by kingjamesthethird, Feb 20, 2015.

1. ### kingjamesthethird Thread Starter New Member

Feb 20, 2015
9
0
Hello

I have been asked to design a cellular logic system that uses two 8x1 multiplexers to add two 4 bit numbers. I can see how to create a full adder but am totally stumped on how to add two 4 bit numbers using only the mentioned mutliplexers. seems impossible!

Any ideas?

would be much appreciated, JJ

2. ### joeyd999 AAC Fanatic!

Jun 6, 2011
2,756
2,923
Considering that the sum of two 4 bit numbers is a 5 bit number, and two 8:1 muxs can only produce 2 bits, I'd say you are correct regarding it being impossible.

3. ### kingjamesthethird Thread Starter New Member

Feb 20, 2015
9
0
Yes thats what I thought but then I was wondering that perhaps there is a method that involves converting the parallel data inputs into some sequenced answer using the AB inputs as a clock. seems pretty abstract I know. Anyway its good to know i havnt missed something obvious!

4. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
Who's asked you to do this and what does it have to do with a cellular logic system?

Are you supposed to do it with ONLY the two multiplexers?

5. ### kingjamesthethird Thread Starter New Member

Feb 20, 2015
9
0
Apparently I was not limited to using one cell and therefore could use as many multiplexers as I wanted as long as it was 2 per cell. It was a poorly worded question. Thanks for the help anyways

6. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
Does that mean that you have it figured out?

I'm thinking that your use of the term "cellular" is not what I am taking it to be. Perhaps the term I might be thinking of for what you have in mind is "modular". What is meant by a "cell" in this problem?

7. ### kingjamesthethird Thread Starter New Member

Feb 20, 2015
9
0
yea I did it. Im no expert but I made 4 full one bit adders (each using 2 8-1 multiplexers) and stuck them together. I think its correct anyway

8. ### kingjamesthethird Thread Starter New Member

Feb 20, 2015
9
0
I am under the impression that each pair represents one ''cell'

9. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
But my question is, what is a "cell"? What is that term meant to encompass in this project?

10. ### kingjamesthethird Thread Starter New Member

Feb 20, 2015
9
0
basically a cell is part of a circuit that performs a simple operation. when lots of the same cell are attached together each can perform the operation on a particular part of the input and this combination substitutes for a more complicated circuit. thats what i learnt anyway. maybe where you come from they call it modular

11. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
Thanks. That's the conclusion I was coming to and, yes, here we would use a term like "module" or "subcircuit" instead of "cell", but that is not an unreasonable choice. Since we don't use it, however, I tend to think that you are talking about a "cell" in the same general vein as in a cellular phone system.