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

From: Michael Nudelman ([email protected])
Date: Fri Nov 17 2000 - 19:57:55 PST

Wai,

1. Yes, it can, though I never met this in real life, but if you delay clock
relative to data - this will do it. The question - why.

2. I did not understand everything, but a basic idea of sensing asynchronous
inputs is having a clock and an edge-detect-circuit (two flip-flops in
series and a logic element depending which edge you want to sense; if both -
XOR, etc ). And as the good theorem states, the clock is supposed to be at
least 2 times the frequency of the input.

Could you ...draw something to make it clear?

Mike.

Wai-Ming Hung wrote:

> 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
> anybody had suggestion on how
> to accomplish this ?
> Thanks a lot
>
> Kenneth

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