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

From: Volk, Andrew M ([email protected])
Date: Sun Nov 19 2000 - 17:07:33 PST

Wai-Ming -

The sample window size of a F/F is finite in width. It can be a few
picoseconds wide, but it is a finite width. Therefore, a F/F cannot have a
setup and hold time which are simultaneously negative since this implies
hold time before setup time - data can change before it is settled.

As to a state change method, the one you are suggesting is about all I know
of. You can use magnitude comparators to reduce package counts or LSI
design complexity, but it is all about the same in the end.

Regards,

Andrew Volk

-----Original Message-----
From: Wai-Ming Hung [mailto:[email protected]]
Sent: Friday, November 17, 2000 5:12 PM
To: [email protected]
Subject: [SI-LIST] : Flip flop setup and hold time and sensing circuit

Hi,

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
circuits.
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