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Latches and flip flops - how is stable state defined?

  1. Nov 3, 2013 #1
    Latches and flip flops - how is "stable state" defined?

    My textbook and professor both make numerous references to "stable state" of a latch of flip-flop, but never actually define it.
    It's not intuitive. For instance, if the present output Q is 0, and we input S=1 and R =0, the circuit's next state, Q+ , is 1, and this is a stable configuration, but the textbook says this is not a stable state.
    So what does "stable state" actually mean in this context?
  2. jcsd
  3. Nov 4, 2013 #2


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    I'm not aware of how the application of a steady state 1-0 or 0-1 input to an S-R latch (using nor2 or nand2 gates) can result in an unstable state except during the period of transition. But I expect the textbook is correct and I am missing some information. Perhaps it is referring to the transient state during which the output is 1-1 (nand) or 0-0 (nor) due to propagation delays.
  4. Nov 4, 2013 #3


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    I'd need you to sketch the gate arrangement to be sure, but I think you will find that a state which exists only while it is forced by the continued presence of a peculiar input is considered not a stable state. A stable memory state is one that will be maintained even when the inputs revert to their inactive/neutral level (the STORAGE state).

    I can't comment on any distinction re a "stable configuration" vs "stable state"
  5. Nov 4, 2013 #4


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    The word 'stability' can refer to several different things. This can lead to confusion.
    Stability, in this context usually refers to a situation in which, when a small perturbation is introduced, a system will return to its original state. In electronics, you can get stability with positive feedback (as with a simple 'bistable' circuit - Google gives dozens of hits). Stability in this sense requires some non-linearity. Once the bistable circuit is in one state (i.e. the output is hi or lo) then the input signal needs to cross a certain threshold value for the state to change - then the output state will swing to the other stable state. So the input output characteristic will be a 'step'.
    A Schmitt trigger is another circuit which has 'hysteresis' which is a decision making circuit that cuts out the effect of low level noise on a signal. Once the input reaches a certain level, the Schmitt 'decides' it's high enough in level so it 'flips', the input signal then needs to go to a significantly low level before it decides to flip back.
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