> > You should. For example noise on the mains could come through the
> > transformer and exceed -0.3V. Bye-bye pin, possibly bye-bye PIC.
> > Been there

> even with that small amount of current, Joe?

> Been using dimmers with a 1Mohm straight to a 16F pin for zero-crossing
> =A0- nothing else. makes some 400uA... working for years.

> Are they that different?
> I can't see "how much current" those "diodes" (or whatever) can shunt...

Alas, all deices are potentially different. As someone else noted,
it's what happens to the current that matters. The protection diodes
will or may inject it into the substrate at points that were not
designed with operational current flow in mind. this may eg operate a
parasitic transistor that never normally functions or cause current
flow to some node that is "not expecting it" or ... . The problem is
that nobody know for sure. The IC designers COULD deign things so that
the system is moderately robust against such usage, but there's no
certainty that they have, and ongoing occasional indications that
sometimes they haven't done so well enough for a given application. I
got caught doing this with an RS232 input long ago, using a circuit
suggested by a kit supplier. The result was so intermittent and so
variable in effect that I'd decided to make it one of my life missions
ever since to warn people about it :-). It's clear from all data
sheets that it shouldn't be done during normal operation, and it's
quite likely that you MAY get away with doing it in many applications.
Sticking to 'data sheet says ..." us usually a good idea.

Note that even using a Schottky clamp MAY violate the spec by a small
amount. All experience I have had and heard of suggests that in fact a
Schottky clamp works OK. again YMMV but so far nobody's has :-) ).

One last 'trick' - in ultra low power circuits, a clamp diode to
supply can "pump up" the power supply voltage if the input circuit can
provide more current than the processor draws. That would be unusual
but worth being aware of. (One Microchip ap note appeared to do just
that - and also violated the body-diode conduction 'rule' above.
Several people argued vigorously that if uC did it in an ap note it
must be OK. I suspect the uC datasheet writers would have disagreed
:-)).



=A0 Russell

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