>
>Is there some mechanism for some PICs whereby the output pins themselves can
>"feed back" into the chip and cause a reset, or is it simply that the supply
>lines get spikes that cause a small brownout?  I noticed that the 16C71 (no
>onboard BOR) is not susceptible to this effect in my circuit, whereas simply
>replacing it with a 16C715 (onboard BOR disabled) causes the chip to reset
>frequently.

The input protection diodes dump excess current on the inputs into your
main supply, and that is what they are designed to do. If your supply
impedance is high enough relative to the current source, you may take VCC
outside the spec limits. There may also be problems with droop, immediately
after the transient where the regulator was shut down, and now has to turn
back on quickly.

I don't know your configuration, but series resistance in each I/O lead
will help prevent this from happening.

Another method to reset a processor, is to induce current into traces on
the PCB with rapidly varying magnetic fields. So far, I haven't found
anything in the normal world that will upset a bare processor (loop area
very small) but once you add real traces on a pcb, you get into the area
where EMI can cause induced current as above.

Also, weak oscillator design can get you there. It's very hard to diagnose
without special equipment.

For starters, take a small photo-strobe, and set it off repeatedly, near
the project. Move around till you find the part of the project that is most
sensitive to this disturbance.  If you need more flux, add a couple turns
of wire, about 1-2 inches diameter, in series with the flash
tube.  Basically, you're using the flash tube as a switch to discharge a
few joules into the indctance of it's wiring, or your inductor, and
creating a pulsed magnetic field.

Beware strobe mods, charged strobe caps can kill. Assume all strobe caps
are charged.

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