Francois Robbertze <piclist@mit.edu> wrote:

> I found the following in a asm file for a 16f84a
> Can someone tell met what is the function of the
> IO_MASK=0FFH &~(1<<IO_PIN)

    Francois:

    It sets IO_MASK to all 1's except for a "0" at the bit
    location specified by IO_PIN.  Since IO_PIN is defined
    as 7, IO_MASK is 01111111 binary, or 0x7F.

    This lets you perform operations on IO_PIN with byte-
    oriented instructions: ANDing PORTB with IO_MASK will
    clear the IO_PIN bit, ORing with the complement of IO_MASK
    will set the bit, XORing with the complement will toggle
    the bit, etc.

> I have trouble understanding the port setup
> first 0xFF is moved to W

    Correct.

> then TRIS Port A&B  wich will make them all inputs?

    Correct.

> then W is moved to PortA&B  - will this drive an input high?

    No, but it will pre-load the port output latches with
    1's so that if/when the pins ARE made outputs by the
    firmware, they'll drive high.  It appears from the
    IO_MASK comment that IO_PIN is used as both input and
    output, so this makes sense.

> and then again PortB7 is set - what is this function?

    It's redundant, probably either left over from earlier
    editing of the source code or maybe the start of whatever
    the software is doing with IO_PIN.

> (The option command is obvoisly setting the TMR0 to 1:256
> ratio)

    Well, no... It's assigning the prescaler to the WDT and
    setting it to 1:128.  It also (probably more importantly)
    turns on the PORTB weak pullups.  Read the datasheet; these
    details are important.

    -Andy

=== Andrew Warren - fastfwd@ix.netcom.com

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