You are not being overly paranoid.

Use a shadow register!

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

----- Original Message -----
From: "Bob Barr" <bob_barr@HOTMAIL.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, May 18, 2001 2:09 PM
Subject: [PIC]: Port B Change Interrupt - Looking for Trouble?


> Hi all,
>
> I'm about to use the Port B change interrupt for the first time in a 16F84
> project.
>
> Because of the warnings that I've seen in the Microchip docs, I've tried
to
> minimize reads of Port B. All of my inputs (other than the interrupt
> sources) are on Port A and all of my outputs on Port B. I don't have to
read
> Port B except after the interrupt occurs.
>
> To re-use some existing code, I'd be using bsf and bcf instructions to
> control my Port B outputs. Since these instructions operate as a
> read-modify-write, I'm concerned that using them may cause a conflict with
> the interrupt-on-change operation.
>
> If need be, I can maintain a shadow register and only do pure writes to
Port
> B. Using bcf/bsf would be much easier. Is a shadow register necessary or
am
> I just being overly paranoid about mixing read-modify-writes with
> interrupt-on-change?
>
> (I'm operating on the ASSumption that the read of port B allows some
window
> of time when the change interrupt could be missed. Is this a valid
> assumption or am I missing something obvious here?)
>
> Thanks, Bob
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