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edge vs level triggering

Discussion in 'Electronic Design' started by [email protected], Feb 26, 2007.

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  1. Guest

    Hi,

    Appreciate if anyone could explain me the significance of edge
    triggering vs level triggering. I know what they are but don't
    understand the significance except in interrupts. Where do we need
    edge triggering and where do we need level triggering. Is this
    applicable only for clock signals? or for data inputs as well? why?

    Thanks in advance
    mr
     
  2. Ross Herbert

    Ross Herbert Guest


    In simple terms, a voltage level may not be well defined for
    repeatable and reliable triggering, whereas an edge is an edge and is
    always well defined regardless of level. As long as the edge has a
    fast rise or fall time, we can select which edge we want to trigger on
    and always achieve repeatable results.
     
  3. John Barrett

    John Barrett Guest

    an edge triggered latch is probably the perfect example -- you only want to
    latch the data bit on the edge of the strobe, and ignore changes while the
    strobe is at high level, potentially because the input to the latch is
    effected by the output of the latch, and if the latch were level triggered,
    it would continue to accept changes in the input data -- think edge
    triggered as "leading edge triggered" and of level triggered as "trailing
    edge triggered" !!!

    another interesting example is the master/slave flip-flop -- data is latched
    into the master on the rising edge (output from the slave remains stable,
    further changes in the inputs ignored), and transfers the data to the slave
    and the outputs on the falling edge... just another way to make sure the
    data changes exactly when you want it to.
     
  4. Tam/WB2TT

    Tam/WB2TT Guest

    The simplest case I can think of is to compare a 74x74 edge triggered Dff to
    a 74x573 transparent latch. The '74 output will go to the state the input
    was in just before the clock edge. The '573 output will follow the input the
    whole time that the enable is active.

    Somebody mentioned master slave FFs. You can make one out of a pair of '573s
    running on opposite clock phases. But, when the level changes, that is
    really an edge as far as setup and hold times go.

    TYam
     
  5. krw

    krw Guest

    No, a "level triggered latch" is not "trailing edge triggered" (nor
    "triggered" at all). An edge-triggered flipflop will only change
    state on the specified edge of the strobe/clock and hold state at
    all other times. A level sensitive latch will change state
    whenever the enable is active and hold state when the enable is
    inactive.
    Take two level sensitive latches and put an inverted between the
    enables (with appropriate care for timing ;) and you get an edge
    triggered master-slave flipflop.
     
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