Am I just dense? I've read the Microchip docs, a fat book by Myke Predko,
and I've looked through the FAQ. But I have what appear to be bizarre
problems trying to write to the various port latches on the '877.

I've been working on interfacing a parallel LCD to a PIC 16F877. I
succeeded, but only after noticing a (possible) problem with port A. If I
execute the following instructions (all of port A is configured as output):

        bsf     porta,0
        bcf     porta,1
hold    goto    hold

then both RA0 and RA1 will be cleared (RA0 will be set momentarily). This
was causing the assertion of RS on the LCD to fail (or at least not last
long enough), because I was asserting RS via a port a pin, then pulsing E
via a different port a pin. I solved the problem by using port B pins
instead, and the same exact code (with the exception of the different port)
started to work.

At first I had RS connected to RA4, an open-drain output, so I moved both RS
and E to RA0 and RA1, but I kept having the same bsf/bcf problem.

Is there something simple I've overlooked? I also tried two different '877
chips, in case one was damaged, but they both fail the same way.

I also tried the following. I've put all the code so you can see exactly
what I've done. I have a bunch of LEDs wired to the pins of port C, through
a 100-ohm resistor to Vcc (each), so that the LEDs would light if a pin goes
low.

        list    p=16f877

        include    "p16f877.inc"

        org        0x000
        nop

start
        bsf      status,rp0
        movlw    b'00000000'
        movwf    trisc

        bcf      status,rp0
why     movlw    0xFF           ;   Can change this to 0x00
        movwf    portc

prob
        movlw   0x00
        movwf   portc

circle  goto    circle

        end

Up to the label "prob", the LEDs remain unlit. I would expect them to light
after "prob" executes. They don't. However, if I change the 0xFF at "why" to
0x00, then the LEDs are lit (and I can then change the 0x00 at "prob" to
0xFF, and they'll remain lit).

Thanks for any help,

Rick

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