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

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From: Wai-Ming Hung (whung@pericom.com)
Date: Fri Nov 17 2000 - 17:12:08 PST


I am new to this subsribe list. Here's the question I want some answers
(1) Can flip flop have negative setup and hold time ?
If so, what does that mean ?

(2) I have a circuit and want to keep track of switching signals.
What I mean is I want some control circuits to sense the switching
of input signals going from H->L or L->H, anybody has suggestions ?
I once keep track of this by comparing the phase difference between
input and output as there is always a delay for switching from input to
output, so once it switch, it has phase difference and I can make use
of such phase difference as a control signal for other parts of my
But the problem is that, it has to be a bit by bit searching for
switching which means
if I have a 32-bit circuits, then I have to search each bit if it
switches and then OR the
result to a control signal. However, I want to do it in a 2-bits basis
which means I want
to compare the input signals if they switch, then I generate a control
signal to other parts
of circuit, but I think it would be quite difficult to get control of
four signals on a 2-bits basis
since now I have 2 inputs and 2 outputs to handle with. I once played
around four input XNOR
gate to get either phase shift of two signals but the signals get messed
up easily and very noisy
if I switch 32-bit at the same time. Two bits at a time is fine.. so
anybody had suggestion on how
to accomplish this ?
Thanks a lot


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