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
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.
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.
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
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.
-- Keith
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