>For my (in-circuit) 16c84 programmer I'm considering to include a
>'power-short' feature for this kind of situation.
>Questions:
>- has anyone experienced such a lock-up where a reset is not sufficient
>- what caused it
>- does this situation ocur often enough to warrant a few extra components
>in my programmer

Oh, my.  I have spent the past two weeks working with this one.  I've put
hardware together to fix the problem, basically a daughterboard for the PIC
with a reset chip on it.

This situation, in our instance, is caused by a PIC running at 20MHz
(that's legal) at 5V most of the time.  The problem is that, during an AC
brownout, it is possible for the 5V rail to drop below 4.5V (very bad).
This will put the chip into a mode where the osc still runs, but the
outputs are locked in their last set state and no processing is being done.

Worse yet, the WDT won't trip either, so we have a totally useless device
out there.

And the brown-out detect circuitry (which may or may not be active,
depending upon which guy burned them :-( won't trip until the voltage hits
about 3.8V for a long-enough period of time.

To remedy the situation, we are requiring all future units to have a Dallas
DS1233-5 reset chip installed.  I investigated other chips (such as
Panasonic's from Digi-Key) and found it to have the best parameters.  It
trips very reliably at 4.5V (+/- 5% is the spec, but reality is much
tighter), generates a reset pulse a minimum of 300mS wide, and stays low
until power is up about 4.5V for 300mS.

It is very important that this mod be installed in our case.  We are having
to visit sites scattered around a several-hours-drive-wide area to
basically power cycle a unit to revive it.

A word to the wise.  Put the reset chip in.

Andy

==================================================================
Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865
          Hardware & Software for Industry & R/C Hobbies
        "Go fast, turn right, and keep the wet side down!"
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