Edge triggered JK flip flop

  1. Jan 29, 2007 #1
    1. The problem statement, all variables and given/known data
    I am unable to understand the working of a positive edge triggered JK flip flop. In the figure, there are 2 AND gates on the left (one over the other) and there are 2 NOR gates on the right (one over the other). I have used an RS flip flop to construct the JK flip flop. The inputs J and K are denoted as 2 & 5 respectively. Q’ is the complement of Q. The output Q’ is connected to input number 1 of upper AND gate and the output Q is connected to input number 6. The inputs 3 & 4 are connected to the clock pulse.
    Now consider the input condition J=0, K=1 and at this point the clock pulse makes a positive transition. The explanation given in my book is as follows:
    When J is low and K is high, the upper AND gate is disabled, so there is no way to set the flip flop. The only possibility is reset. When Q is high, the lower gate passes a reset pulse as soon as the next clock edge arrives. This forces Q to become low. Therefore, J=0, K=1 means that the next positive transition of the clock resets the flip flop.
    The lower gate sends a reset pulse which means it sends a low voltage signal. Why does this happen? Now, the lower AND gate has 3 inputs. At the time the positive edge of clock pulse arrives, K=1. So, two of the inputs of the lower AND gate is high. Suppose at this instant, Q was high. Then three of the inputs of the lower AND gate is high, which means the output would be high .i.e. S is high. A high at any of the inputs of a NOR gate gives a low output. Hence Q’ is low. Now, Q’ is one of the inputs of the upper NOR gate. As J=0, R=0. Hence the two inputs of the upper NOR gate is low. Hence, Q=1. There is something to do with the third input(1 & 6). I think digital electronics is tough if you don’t have good teacher. Someone please guide me!!!!!!!!!!!

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jan 29, 2007 #2
    Sorry, I forgot to attach the circuit diagram.

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  4. Jan 29, 2007 #3


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    Staff: Mentor

    Looks like you are missing a stage or two in your diagram -- maybe that's where the confusion is coming from. I went through HowStuffWorks to find a page about the JKFF for you:


    BTW, I think I'll move this to the EE forum, since it's not strictly homework, and more of a question about understanding how a general cicuit element curcuit works.
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