# Show how to make a 4-to-1 MUX, using an 8-to-1 MUX

Discussion in 'Homework Help' started by djburst, Nov 3, 2011.

1. ### djburst Thread Starter New Member

Nov 3, 2011
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Show how to make a 4-to-1 MUX, using an 8-to-1 MUX.

-I understand how to take smaller MUXes and construct multiple of them to construct a larger one. Such as 2 4-to-1's to make an 8-to-1, but how would I make a 4-to-1 MUX using an 8-to-1??

I honestly am completely stumped and so not even know where to start, So any help is greatly appreciated! Thanks!

2. ### justtrying Active Member

Mar 9, 2011
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398
well, you have 8 lines on an 8:1 and you need to use 4. Just think about how to take care of your unused inputs.

3. ### djburst Thread Starter New Member

Nov 3, 2011
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Are the unused inputs grounded? How can you not use 4 inputs? Doesn't the fact that 4 inputs are unused on a 8:1 make it a 4:1 by default??

4. ### Georacer Moderator

Nov 25, 2009
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The 8:1 MUX needs 3 selection pins. The 4:1 needs 2. Which of the 3 initial pins you must disable in order to be able to select only between 4 inputs of the 8:1 MUX?

5. ### Georacer Moderator

Nov 25, 2009
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1,266
After djsfantasi went all Ratch on my sorry @\$\$ (and for good reason), I 'll take some time to explain a bit better what I meant on the previous post, since what I said wasn't well-phrased.

Using 3 variables to in a MUX allows you to select 1 out of 8 inputs. If you make one of those 3 variables a constant, then only 2 variables are left to select an input, and that leaves only 4 possible selections.

For example, if you ground the LSB of the selection bits, then the only available inputs will be 0,2,4 and 6.

Is that clear?

6. ### djburst Thread Starter New Member

Nov 3, 2011
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Yes! Thank you it's clear now.. I wa thinking I had to change one of the 8 inputs, but now I understand it's all about the pins that select the inputs..

7. ### djsfantasi AAC Fanatic!

Apr 11, 2010
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"...went all Ratch"? (sorry, don't know that phrase) Oh my, I was hoping that my PM to you wouldn't be taken the wrong way

I was just curious if what I was thinking was correct. Sorry if I offended...

dj

8. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
No, no, none was taken, don't worry. I takes much more than that to remotely offend me. I like being corrected when I am mistaken, mostly because that justifies me to do the same thing, which I love.

Ratch was a past AAC member that was banned to oblivion for being a grammar Nazi and not letting a single word drop.