> The current impulses related to current surges at switching points is
> usually a much smaller effect in CMOS circuits.

I would have to disagree with that. At all times other than during
switching, there is no current flowing. Switching here means between
steady state conditions so it includes trace capacitance etc.
Supplying the current during switching is the whole point of the
decoupling caps.

> So we don't need much capacitance to fix this problem.

No argument with that. Otherwise we'd be back at square one.

> However, this problem would show up easily as ringing on the signal
> transitions. So it's still true that if the signal looks clean and sharp in
> the time domain, the problem is solved.

No. The opposite. If you had no available capacitance on the supply
then the current would have to be sourced from a long way away. That
would mean that the output of the pin would appear to rise slowly. At
the same time, the supply (local to the chip) is going to be pulled
down (and the ground pin, up) because of the loading. It is the
supply that shows the ringing and it is quite apparent on a scope.

Steve.

======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: steveb@tla.co.nz      fax +64 9 820-1929
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