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Homework Help: Sequential Logic & JK Flip Flops

  1. Dec 9, 2012 #1
    1. The problem statement, all variables and given/known data
    Complete the truth table for the following
    http://forum.allaboutcircuits.com/image_cache/httpwww.freeimagehosting.netuploadsd537e2dd9a.jpg [Broken]

    2. Relevant equations
    JK Flip Flop Characteristic Table
    D Flip Flop Characteristic Table

    3. The attempt at a solution
    I understand how to make truth tables for logic circuits and I know the characteristic tables. But what is really stumping me is the AND gate that go to the JK Flip flop. It seems to me that whenever x and A are put together in the AND gate, it will always be zero except for the two entries where A and X are equal to one. I have the solutions manual and my answer is incorrect. It has 1's in row 2 and 4 but I have absolutely no idea how.
    Here is what I have down just for the A column.
    A B X A(t+1) B(t+1)
    0 0 0 0
    0 0 1 0
    0 1 0 0
    0 1 1 0
    1 0 0 0
    1 0 1 1
    1 1 0 0
    1 1 1 1

    Any help and/or explanation would be very much appreciated, my final is on Wednesday and I want to get this topic rock solid in my head.
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Dec 10, 2012 #2


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    If the circuit powers up with A=0, B=0 as far as I can see it's game over and nothing ever changes no matter what you do to X. JK is allways 00 = "no change" so A(t+1) = 0. D is allways 0 so B(t+1)=0.

    So I can't see how they get a different answer to you for row two.
  4. Dec 10, 2012 #3

    rude man

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    I make A(t+1) and B(t+1) both = 0 for rows 2 and 4.
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