If you want to save on the component count, there are no reasons to BOTH
have a pull-up AND a series resistor for the base of the transistor.

Simply put one resistor from base to VCC that will ensure proper
operation of the transistor. Then connect the open drain output to base.
When the output is driven low, the current through your resistor will go
directly to ground. The base voltage will be well below 500mV, and no ib
and ic will flow.


Regards
St=E5le Freyer



-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Mike
Sent: Tuesday, April 08, 2003 7:47 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: [PIC]:PORTA 4 of 16F84 changing its state - erratic behavior


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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