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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    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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    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
    2,794
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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
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    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.
     
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