Brah,

 I believe nothing would happen, and you would not get an interrupt.
 But if it were me, instead of spending so much time trying to figure
 out would would happen in theory, I'd write the code to make it
 happen and try it out.   That way, I'd know for sure.

 And if by some chance an interrupt did occur, I'd turn of the
 interrupt capability until after I had written to the port.
 One of the nice things about microcontrollers in general, and PIC's
 in particular, you can reconfigure a lot of things on the fly.
 This makes then very versatile, as I'm sure you're already aware.

                                               Regards,

                                                 Jim




On Thu, 28 December 2000, brah wrote:

>
> I'm presently using portb to drive 3 seven seg displays, multiplexed
> with common drives from porta 0,1 & 2.
> I need an external interrupt, and I've tried using RB7, with poor
> results, mainly due to determining the polarity of the interrupt signal.
> I'd like to try RB0 as an interrupt, because of it's settable edge
> triggering facility. (I need to interrupt on a falling edge only.)
> However, when RB7 is not used as an interrupt, or anything else - it's
> not connected - I only have to copy a full byte to the port, and it's
> done. What happens to RB7 doesn't matter.
> OK - but if I, say, configure RB0 as an input, the rest outputs, setup
> RB) as an interrupt pin, set the edge triggering correctly, then get my
> byte of data for the 7 segment led, and RLF once (garbage in bit0) and
> then write it to portb, I'll be OK on the leds, but what's the effect on
> RB0?
> I've read a lot on this, and I may have attention deficiency syndrome
> (an accute possibility) but I can't find out exactly what happens if you
> write to an port bit that's configured as an input.  E.G. if the port
> bit is held up, a byte is written to the whole port, and the bit in
> question just happens to be low.  Does the port bit input buffer go low
> momentarily, then get pulled back high again once the write operation is
> over?  If it doesn't then OK, but if it does, would that trigger an
> interrupt?
> However, even if I don't have an interrupt problem looming, I'd like to
> know what happens anyway.
> Regards to you all,
> Howard.
>
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jim@jpes.com

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