On 28 Jan 99, 11:09  Andrew Hall wrote:

> I want to use a 16F84 to monitor two independant frequencies via a
> couple of port pins (maximum frequency 60Hz). This requires the PIC to
> spot transitions on the pins. I would like to avoid polling as the PIC
> has other tasks to perform.

> I know there is an interrupt on PORTB change feature but I seem to
> remember a "piclister" saying there are problems with this technique.

The interrupt on change on port b works quite well, as long as you
do all your port B access (all bits, not just to those that might
trigger an interrupt) in the interrupt handler.

I fell into that trap just recently, when I monitored the pulses from a
bicycle generator (~300 hz) in a "port b change interrupt" routine,
but did all other stuff, including polling another pin, toggling yet
another one for sound, etc. in the main program.  I didn't get any
interrupts at all, it almost drove me crazy. I finally realized the
problem just by thinking it over (the documentation is quite unclear
about it, it just just vaguely recommends against using polling and
intterupt on change, without giving a reason). After moving almost
everything into the interrupt handler, and after moving the "check
port B status" out of the "first level" interrupt handler and calling it
from both the interrupt handler AND every port B access (including
output access!), everthing worked fine at the first attempt.

>
> Could anybody advise on this or any other technique which might be
> appropriate ?

In short: you have to take into account that you can and
occasionally will miss port B interrupts each time your programm
accesses the B port in any way. That's it. This has some
implications, not all of them obvious.

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