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Homework Help: A question in building a circuit using 2 full adders

  1. Mar 7, 2008 #1
    which produces a sum of 2 bit numbers x0x1 y0y1
    the output is C (carry)

    the problem is i know how to build a truth table and a KARNO table
    and even how to build this circuit

    i know how to build a full adder circuit

    the problem is i dont know how to build this task using 2 full adders
  2. jcsd
  3. Mar 7, 2008 #2


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    Staff: Mentor

    What does the truth table of a 2x 2-bit full adder look like?
  4. Mar 7, 2008 #3
    there is no 2 on two full adder
    there is only a 1 on 1 plus carry in adder (AKA full adder)

    i showed the formula of the sum and the carry out in the drawing


    what now??
    i dont know how to use this basic component in order to solve this

    as i say again
    there is no problem for me to build a truth table for all the
    possibilities and to make a KARNO table out of it
    and then to build a circuit

    my task is to build this fuction using 2 full adders
    Last edited: Mar 7, 2008
  5. Mar 7, 2008 #4
    A full adder has three inputs and two outputs. Can you name them?
  6. Mar 7, 2008 #5
    i showed it all in the drawing
    there is the input carrier
    the firstbit
    the secondbit
    the output sum
    the output carrier
    can you answer my question???
    Last edited: Mar 7, 2008
  7. Mar 8, 2008 #6
    Hi, Trans. This sounds like a homework question, so i'm following berkeman's lead in attempting to lead you to an answer, rather than giving it away.

    Binary addition is just like adding up decimal numbers longhand. It might help if you tried some examples. Like adding 0110 to 1011. It's binary so the carry-out only has one of two value-- zero or one. The carry out of the one's place is carried into the two's place. The carry out of the two's place is carried into the four's place, and so on.
  8. Mar 8, 2008 #7
    ye i understand that i can even show you the sketch in the book that explains that

    so like this story of herztel and gretel
    where is the next bread peace???
    cause i still dont have any idea of how to subtitute 2 adders in the circuit
    that i am supposed to build

    what is the next step in leading me for understanding this question??
  9. Mar 8, 2008 #8
  10. Mar 8, 2008 #9
    You can really think of all this as the same as adding columns of normal, base 10 numbers. Except that your doing base 2. Check it out.


    There's no carry into the ones place. Thats that same as having the carry into the one's place a zero. So you tie the first carry-input to ground.

    The last stage in the example is the hundreds-place. Its carry-out goes straight to the answer. It's the thousands-place digit.

    When they do this in an 8 bit CPU you can add up two 8 bit numbers and get a result that needs to fit in 9 bits. So they leave the last carry out in a special bit register, appropriately called the 'carry', so that you can still use it. It can become the carry-input for the next 8 bit addition you do. This allows you to add 16, 24, and 32 bit numbers 8 bits at a time.

    The biggest number you can add with two stages is 11 binary to 11 binary.


    Some thing as base 10. You have two 2-bit numbers coming in. But you get a three bit number coming out.
    Last edited: Mar 8, 2008
  11. Mar 8, 2008 #10
    so i understood that my last carry is the next bit place
    and that isntead of putting the first carry in sign 0
    i should have put a ground there.

    except that
    my solution is ok?
    Last edited: Mar 8, 2008
  12. Mar 8, 2008 #11
    So your "Cout" could be called S2. Bit zero's carry-in is a zero if your doing it on paper. If you're building it in hardware with positive logic you tie it low or tie it to ground.

    I that should do it.
  13. Mar 8, 2008 #12
  14. Mar 8, 2008 #13
    good luck
  15. Mar 8, 2008 #14


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    Gold Member

    I'll see your two adders and raise you a pit viper.

    Sorry... I'm having a slow day at work and had to chirp up. Carry on.
  16. Mar 8, 2008 #15


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    think about what you do when you add two binary numbers. which bits do you start with? what bits' results depend on what happens at the other bits?

    i'm surprized it isn't depicted in your text book.
  17. Mar 9, 2008 #16
    ok i think i got the consept
    do you have anything to say about my solution

    is it ok by you??
  18. Mar 9, 2008 #17
    i think that i depicted every thing
    i got the input bits
    i got the output bits
    i got the carry out bit
    which should have been the next bit in line on the third place

    what did i missed??
  19. Mar 9, 2008 #18


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    listen, what you have on your jpg are ad hoc notes about the operation of a single bit in the adder. what you need to do is think about how you would string together two of these single bit adders into a two bit adder.
  20. Mar 10, 2008 #19
    i dont know what is a two bit adder
    for me a have the basic operation of full adder
    to do that
    Last edited: Mar 10, 2008
  21. Mar 10, 2008 #20


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    a single full adder adds one bit (the "nth bit") from two binary numbers together with the Carry In bit (so the sum of these 3 bits can be 0, 1, 2, or 3 or 00, 01, 10, or 11 in binary). the result is the two bits i shown to the left. the left bit is the Carry Out bit and the right bit is the nth bit of the sum.

    what do you do with the Carry Out the nth bit of a full adder?
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