RE: [SI-LIST] : Flip flop setup and hold time and sensing circuit

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Sun Nov 19 2000 - 14:33:22 PST


> (1) Can flip flop have negative setup and hold time ?
 
Yes, one at a time. Both negative at the same time? I don't think so.

> If so, what does that mean ?
 
Positive Setup time means the data input must be stable before the clock
event. Negative means it does not need to be stable until somewhat after
the clock. If, internally to the flip flop, the clock has more delay than
the data, that could cause it.

Positive Hold time means the data must remain stable until after the clock.
Negative means the data may start changing somewhat before the clock. If,
internally to the flip flop, the data has more delay than the clock, that
could cause it.

It is just a relative positioning of the "window" where the data input is
effectively being sampled by the flip flop. Usually both setup and hold are
positive and the window straddles the clock. One or the other can be
negative, meaning the window is shifted either before or after the clock.

Andy

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