Hi all,

I thought I could find something about this via a Google search but so
far I have not found exactly what I need.

I'm having a disagreement with someone about how to handle a situation
where there are several processors (two NXP ARM-type microcontrollers,
a DSP, and two Freescale Power-PC microprocessors) on one PCB. To save
money, he wants to use one oscillator for all of these and distribute
it to them using clock distribution ICs.

I think that this is unwise unless he takes great care to length-match
the clock traces, control propagation velocity, etc. Otherwise, we
could end up with metastability problems because we will likely have
all of the processors operating in a pseudosynchronous mode and they
intercommunicate with each other. At boot time there will be a fixed
timing relationship established between them, and it could well be
that this ends up with two of them clocking together except for a tiny
amount of variation due to different propagation delays. It could then
be that every single bit communicated between them ends up right in
the middle of the metastability window, with the result that the
probability of bit errors is relatively high.

I am advocating that each processor be run with separate clocks. Even
if they are at the same frequency, the drift and jitter among them are
almost certain to be greater than the difference in propagation delays
so that instances where a transition violates setup and hold times for
the input synchronizers will be only rare, random events rather than
potentially every single transition.

He agrees that in principle I am right but he thinks that this is
really only a risk if you are trying to set up a synchronous digital
system.

My main question for all of you is this: where can I find some web
reference material which talks about clock hazards in nominally
asynchronous digital systems? Everything I can find seems to assume
that you are trying to set up all of the chips in lock-step
synchronous operation.

Secondarily, any opinions on this would be appreciated.

Thanks,

Sean
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