Question About Propagation Delay

Discussion in 'Homework Help' started by Alali, Apr 8, 2015.

  1. Alali

    Thread Starter New Member

    May 6, 2013
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    Hello everyone,

    So, I have homework for my Digital Systems class and I'm stuck in this question about propagation Delay.
    The question is:
    A latch is implemented with Nor gates, is the propagation delay for Q and Q* the same? Explain?

    My answer is yes they are the same and I'm thinking that they're the same because the propagation delay is the time it takes for the output to respond when there's a change in the input and that Q and Q* are both outputs, but I'm not sure if my answer is correct or not? I'm also not sure about my explanation. Can somebody help me out here?

    Thanks,
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    Your reasoning makes no sense. Based on your logic, the propagation delay for all outputs must be the same since they are all outputs. Does that make sense? Consider a case in which you have a circuit that has a single input and that input goes to two inverters The output of one of them goes to the Q' output and the other goes to another inverter and the output of that is the Q output. So this circuit simply takes an input and produced both a complemented and a non-complemented output (this is actually a useful circuit used in a lot of IC designed to drive address lines down an array). Will the propagation delay be the same for both of these outputs? If it isn't clear that they won't be, have the Q' output remain as the output from a single inverter but have the Q output go through not two inverters in series, but two hundred. Now it should be painfully obvious that the propagation delays won't be the same.

    In general you will have a propagation delay matrix with two columns for each output and two rows for each input. The reason you have two for each is that, in general, the propagation delay may be different if the input is changing LO-HI or HI-LO. Similarly, the prop delay may be different if the output is changing LO-HI or HI-LO.
     
  3. Papabravo

    Expert

    Feb 24, 2006
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    The proper analysis requires you to look at the signal path from each input to all of the outputs which are affected by that input.
     
  4. Art

    Distinguished Member

    Sep 10, 2007
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    I got a massive headache, but although set & reset alternate physical paths,
    the path each input takes is symmetrical which is a beautiful thing right?
     
  5. WBahn

    Moderator

    Mar 31, 2012
    17,768
    4,802
    There is an overall symmetry, but that does not mean that you get equal propagation delays for each input, only that there is an overall symmetry.
     
  6. Alali

    Thread Starter New Member

    May 6, 2013
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    Thanks everyone for trying to help and explain, but I still do not understand what the answer to this question should be and what is the explanation to it?

    Thanks,
     
  7. WBahn

    Moderator

    Mar 31, 2012
    17,768
    4,802
    Draw the schematic and then draw a timing diagram that goes through all of the possible transitions. Assume that the time it takes a signal to propagate from the input of a NOR gate to the output is the same regardless of which in put changed or how it changed (you have no real basis to make any other assumption). One easy way to do that is to draw your timing diagram on graph paper and have your inputs change at something like ten divisions apart (you can probably get by just fine with four or five divisions). Then assume that anytime an input to a gate changes that the change at the output, if there is one, occurs one division of the graph paper later.

    Do that and show your best attempt to work through it.
     
  8. Alali

    Thread Starter New Member

    May 6, 2013
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    0
    Thank you everyone for your help! I understand it now.

    Thanks,
     
  9. Art

    Distinguished Member

    Sep 10, 2007
    785
    61
    I misunderstood the question to mean is the action on S & R the same rather than outputs Q & Q bar the same!
     
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