Full adder design

Discussion in 'Homework Help' started by renegadegas, Apr 25, 2013.

  1. renegadegas

    Thread Starter Member

    Dec 2, 2012
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    0
    Hello everyone,

    I need help to progress to the next stage of my full adder design.
    I have attached the question and what iv'e done so far but i'm stuck.

    Any help would be deeply appreciated.
    Thanks
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    There are a couple of ways to proceed. First, have you been through the exercise of implementing an AND, and OR, and a NOT using only NAND gates? If so, then you could just implement your logic using 2-input AND, OR, and NOT gates and then replace each gate with its NAND implementation. This is very straight forward because going from a multi-input AND or OR to an equivalent that uses just 2-input ANDs or ORs is trivial.

    Another way to do it is to draw the circuit and then use bubble logic to transform it into just NAND gates. SOP form lends itself to this.
     
  3. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    You've constructed the expressions using the combinations of inputs that produce the desired output. Now, what about using the combinations of inputs that produce the compliment output and inverting it? Won't that give you the same thing? You can start by going back to your karnaugh maps and getting the expressions for the compliment. Can you guess an easy way to do that?
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    For the Cout, you are all but there. You already have all of the AND terms with just two inputs. You now just need to split the 3-input OR into two 2-input ORs. Once you have done that, you just need to apply DeMorgans (via bubble logic is the easiest) and you will end up with six two-input NAND gates, one of which is wired as an inverter.

    For the other one, you might recognize that as the checkerboard pattern associated with a particular type of logic function and realize you need two of those and that each can be implemented using four two-input NAND gates.

    So your solution can be done in no more than fourteen two-input NAND gates. I doubt it can be done in less, but I don't have a proof of that.
     
  5. renegadegas

    Thread Starter Member

    Dec 2, 2012
    89
    0
    Thank you all Wbahn and Brounout for your advice. I'll follow them through and let you know
    how I get on.
     
  6. renegadegas

    Thread Starter Member

    Dec 2, 2012
    89
    0
    Hello,

    I have attached the full adder circuit i designed.
    I have also included the 2 input NAND implementation of the full
    adder circuit.

    Please check to see if i'm on the right track so i can go ahead
    and cut out the unnecessary NAND gates from the design.

    Thank you.
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    Ah. Now your PM makes a bit more sense.

    I haven't walked through the logic in detail, but it looks like it has a good chance of being correct.

    But YOU should walk through it and see if it is correct! One of the skills that you HAVE to develop is the ability to verify the correctness of your own work.
     
    renegadegas likes this.
  8. renegadegas

    Thread Starter Member

    Dec 2, 2012
    89
    0
    thanks Wbahn
     
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