Hello,

I have a small problem with a big impact on my application.

I am using a 16F877A, running in a 5V environment with 10 MHz.
I am using a timer interrupt (with 25ms) to read PORTB, Bit5. This pin is
defined as input. There is no write at all to this pin by software.
The internal pull-up is disabled. I am using an external pull-up to 5V. This
pin is connected to an external electronic (PS_ON_m Signal from a computer
motherboard).

When the incoming signal is high, I am always reading a high (1) on this pin
by software. I have never seen, that a low was reed, when high was attached
to this pin.
But when the signal is low, it happens, that from time to time, this pin is
read as 1 instead of 0. I supervised the line with an osci, and I never saw
a glitch or spike or whatever. To be shure, I connected the pin direct to
GND. But again, from time to time, I read a wrong values. This may happen
after 30 seconds fro start or even after 20 minutes.

Well, we first build this application 18 month ago, and we never saw this
happen(with exactly the same firmware and revision number of the PIC. Now,
we had to build new boards, and now we have this issues.

I first though, we could could have a problem with the pic itself, so we
checked for datecodes. The very first (we no issue) had been from 2006, the
failing ones had been from 2007. So we decided to replace the pic on one
board. Now, with datecode from 2008, we still see this issue. And we see it
on all produced boards, meaing, we should not have shorts or bad soldering
(very unlikly to have the same fault on 5 boards).

Any ideas, what could happen here?

Best Regards 

Jens 

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