First, thanks to everyone who replied with suggestions.

For the record, my original circuit (with 2N7000 instead of 2N3904,
making a logic level visible on the base of the latching transistor)
does work, basically.  The problem is that any decoupling you add
prevents it from ever *staying* off.  Once the chip's brownout detection
holds it in reset, the output goes hi-Z and the 2N7000 comes back on.
Interesting trick if you wanted a way to powercycle your board.  :)

On Tue, Apr 29, 2003 at 04:24:54PM -0400, Bob Ammerman wrote:
> Back to my original simple circuit. (Switch, 1 Transistor, 1 diode, 1C, 3R).
>
> If an  _open-drain_ PIC pin  is used then I don't think there is a power-off
> sneak current issue.

Actually you have to go look at the individual pin diagrams to know
whether it has protection diodes or not.  From the datasheets I have
on hand, RA4 on the 16F877 is open drain/protection to VSS only (note
that in that datasheet the protection diodes are in footnotes, not
in the picture).

On the 16F628 (my actual target in this case) the open drain output
RA4 has protection to VDD as well, while RA5 (the input-only pin which
is multiplexed with MCLR) does not.  Of course, being input-only it's
not going to work for this circuit.

The 12F675 GP3/MCLR is the same way.  Obviously anything multiplexed
with MCLR can't have a diode to VDD, but there doesn't seem to be a
corresponding "rule" that the open-drain outputs are only protected
on one side.

--
Ben Jackson
<ben@ben.com>
http://www.ben.com/

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