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PICList Thread
'[PICS] Interrupt on change'
1996\10\26@104426 by fastfwd

Bob Blick <> wrote:

> It's my understanding that if interrupt-on-change is enabled, it is
> enabled for all four pins(RB4,5,6,7), no choosing. Let's use a
> PIC16C72 as an example.
> I'm not clear on the use of those four pins if some are used solely
> as output pins. My questions are:
> If I use two of the pins(RB4,5) for outputs, the other two(RB6,7)
> for inputs, and enable interrupt-on-change, will my manipulation of
> the two output pins cause an interrupt, or otherwise mangle the
> correct operation of the interrupt?


   No.  The change-on-portb interrupt is only triggered when INPUT
   pins change states.

> Second question: will reading port B mess with the use of the
> interrupt-on-change feature?

   Probably.  If a change occurs just as a read operation is being
   performed on port B, no interrupt will be generated.

   Worse, it seems that even WRITING to port B can make the PIC
   miss interrupts, since ALL register accesses (even MOVWF, etc.)
   perform a read first.

> Do I need to use the first four pins(RB0,1,2,3) of port B
> output-only if I enable interrupt-on-change? Must I read port B only
> when servicing the interrupt?

   Doing those things will HELP the situation, but they won't
   guarantee that you'll never miss interrupts.  In general, the only
   time the change-on-portb interrupts are truly reliable is when
   they're used to wake the processor from Sleep Mode.

   As I said in the answer to a similar question posted to my
   company's web page, "Sucks, don't it?"

   Sorry to be the bearer of bad news.


=== Andrew Warren -                 ===
=== Fast Forward Engineering - Vista, California          ===
===                                                       ===
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