Sean Breheny wrote:
> 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.
>=20
> 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.

Frankly, I wouldn't worry about it. You don't say what kind of communicatio=
ns
is being used among the different processors, but as long as the interfaces
are designed to properly handle asynchronous signals (and most of them are)=
,
then pseudosynchronous signals cause no additional problems.

If you think about it, the only way you get the "persistent metastable stat=
e"
that you are hypothesizing is if the data lines change state on every singl=
e
CPU clock edge. As long as every data state lasts two or more clock CPU
periods at the pin, there will be no problem.

-- Dave Tweed
--=20
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