I think Keong is looking for something slightly different.

        movfw   port
        xorwf   old_sh,W
        xorwf   old_sh,F
        andwf   old_sh,W        ;low2high ? 0=no/1=yes
        iorwf   flags,F         ;OR resulting flags

Where bits in flag have to be cleared by another
procedure after reading and executing.


WBR Dmitry.



Olin Lathrop wrote:
>
> > Ok. Now I am using the PIC16CE624 to monitor 8 data
> > lines by software polling. Memory location called
> > Result is used to store the states of the I/O pins
> > with a special condition. In any event say data line 2
> > is toggled from a low to a high, bit 2 of Result will
> > be toggle to a '1'.
>
> So you want a 1 when the previous value of a bit was 0 and the new value is
> 1.  You have 4 possible cases as shown by this truth table:
>
>   old  |  new  |  out
>   -----|-------|-----
>     0  |    0  |    0
>     0  |    1  |    1
>     1  |    0  |    0
>     1  |    1  |    0
>
> One way (there are others) to describe what you want is OUT = (-OLD and
> NEW).  Here is a brute force example with bank selection left as an exercise
> to the reader:
>
>      movf   portb, w      ;get the NEW bits into W
>      movwf  temp          ;temp save them
>      andwf  nold, w       ;AND the NEW bits with -OLD bits
>      movwf  out           ;save OUT bits
>      comf   temp, w       ;get -NEW bits
>      movwf  nold          ;save as -OLD bits for next time
>
> This can probably be optimized (Dimitry? Scott?) but I'm trying to
> illustrate the algorithm and show how it implements the truth table above.
>
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