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2 bit half adder truth table?

  1. Oct 27, 2012 #1
    I had a cpre exam last night and this was the last question. Can some one tell me the answer, I'm curious if I got it right or not. The part that tripped me up was a 2 bit "half adder". I assumed I wasn't supposed to use a full adder. So I'm not sure what to do with the carry from the first adder.

    In this problem we will design a 2-bit half adder(add without carry) that adds two 2-bit number, denoted by ab/cd respectively and produces a 3 bit output (c,s1,s0)
     
  2. jcsd
  3. Oct 27, 2012 #2

    lewando

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    What did you come up with?
     
  4. Oct 27, 2012 #3
    i am sure you know that 2 H.A.= 1 F.A.
    and that carry 'travels' from one adder to the next one. All digital design books show you how it's done.
    right?
     
    Last edited: Oct 27, 2012
  5. Oct 27, 2012 #4
    You want to make sure it will produce the right answer, I think that should indicate what to do with the result of adding the first two bits together.

    A half-adder means there is no carry in. A 2-bit half adder is a box that adds two 2-bit numbers but there is no carry coming in.
     
  6. Oct 27, 2012 #5
    Here is what I got and my assumption of the circuit. Sorry in the drawing s1 is s1 and s2 is s0
     

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    Last edited: Oct 27, 2012
  7. Oct 27, 2012 #6
    this look right
     
  8. Oct 27, 2012 #7

    lewando

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    I think it is safe to say you got this one wrong. Your tables do not look right. For example, c should be a 1 only when ab + cd > 3.

    [edit-- fyi, a half adder has a carry output. A full adder has a carry input and a carry output.]
     
    Last edited: Oct 27, 2012
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