I'm new here, so hello to you all.

I have spent so long solving this problem, it has driven me crazy. I hope it
hasn't come up before on the list and I am posting this
simply to share what I have found in the hope that it might help someone
stay sane.

I am using PORTA 0..3 of the PIC for an LCD display data bus and PORTA 4 for
a logic level output. The output gets either set or cleared by the software
and then a suitable message is displayed. On this output is a pull-up
resistor, a series resistor and then a transitor switch to drive a relay.
Sorry if the ASCII "art" doesn't work, but I think you will know what I mean
anyway.

                       _ +5V
                       |
                       |
                      |  |  1k
                      |_|                                      /
                       |        ______              |- |/
RA4 O--------|-----|______|-------- |  |         BC109
                                  10k                  |_| \
                                                                \
                                                                 _|_ 0V



This all worked OK while it was on my bread board, but when I transfered it
to a PCB, strange behavior started. RA4 would only assert for a few 10's of
micro seconds, then return to zero. At first I suspected my code, then the
chip, then random resets. When I looked at RA4 with my 'scope, I found it
only had about 3 volts on it when it was on (with switching spikes when the
other pins were being changed). The reason for this was that I had miss-read
a 10k resistor  for a 1k, and both the pull-up and the base resistor were
10k, which if you ignore the base emitter drop of the transistor, was acting
like a potential divider.

What I think was happening is due to the read/ modify/ write cycle when a
port is assigned a new value, when my port was being read in, sometimes the
3V was being read as a 1, sometimes as a 0, (possibly depending on the
spike) and hence its state seemed to magically change.

I do hope this helps someone, I searched on the PIC archive before I fixed
the problem myself, and didn't turn up anything.

All the best....Mike.

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