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Switches - preset/clear features

  1. Feb 23, 2013 #1
    I have attached the switch. I thought I understood the 3 problems, until I got to question 35.
    My textbook says that the clear feature overwrites any input and sets the output to 0. So that seems to be the case with 33 and 34. But why not question 35?
     

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  2. jcsd
  3. Feb 24, 2013 #2

    mfb

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    R is 1 (do not clear) all the time in 35. Why do you expect any reset?
     
  4. Feb 24, 2013 #3

    CWatters

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    In question 35 the correct answer is that Q/Y will toggle. However that's not one of the multiple choice answers. The state of Q isn't specified before T falls so it's state afterwards is also unspecified. So "uncertain" is the answer.
     
  5. Feb 24, 2013 #4
    I think my thinking is wrong on these 3 questions. I'll start with the first question on the attached pic.
    Here is what I'm thinking:
    For question 33:
    X=0 R=0, then R changes to 1. Basically, Clear changes from 1 to 0.Which means do NOT clear. Thus Q=1 (the inverse of X)?
    This is obviously not the answer. Which part of my logic is erroneous?
     
  6. Feb 25, 2013 #5

    CWatters

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    "Do not clear" does not mean "Set" it means "do nothing".

    In Q33 there are two steps..

    Step 1, the initial conditions..

    X=0, R=0. R is connected to the clear input which is "active low". In other words R=0 means the clear input is active, so Q goes to 0 and Qbar = 1. Therefore Y = 0.

    Step 2

    X= 0, R -> 1. R is connected to the clear input which is active low. In other words R=1 means the clear input is inactive (or "do nothing"). So Q stays a 0 and Qbar stays a 1. Therefore Y stays a 0.
     
  7. Feb 25, 2013 #6
    Oh I see. Thanks for explaining this.

    So for #34:
    X=1 R=1
    R=1 => Clear=0 (do nothing)
    We don't know the previous Q value, so how would we know what Y or Z is?
    I guess we don't?

    then R changes to 0
    so Clear=1, so Q=0=Y, thus Z=1.


    For #35:
    X=1 R=1 => Clear=0 (does nothing for the whole time even if the input X changes).
    Since we don't know what Q was before, it's uncertain.


    Am I understanding this correctly?
     
  8. Feb 26, 2013 #7

    CWatters

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    Correct, we don't.

    Correct. The clear input makes Q=0 whatever it was before.

    Correct. The falling edge on X makes Q toggle, but since we don't know what it was before we don't know what it will be next. So answer is uncertain.

    Aside: Had they not said "Each question is independant" the answer to 35 would be different. In that case question 34 clears the latch (Q=Y=0) then question 35 toggles the latch (Q=Y=1).
     
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