If all you need is a simple delay, here's what I use:


Small delay, up to 769 cycles:

        movlw   n               ;}1 cycle
        movwf   TEMP            ;}1 cycle
        decfsz  TEMP            ;/\
        goto    $-1             ;__> 3n-1 cycles

c=3n+1
where:
c = cycles
n = literal at first instruction, 1 =< n =< 256 (0x001
=< n =< 0x0100, where 0x000=0x0100)


Nested delay, unlimited cycles:

        movlw   n               ;}1 cycle
        movwf   TEMP            ;}1 cycle
loop    (nested delay loop)     ;/\
        decfsz  TEMP            ;  > nd+3n-1 cycles
        goto    loop            ;_/

c=nd+3n+1
where:
c = cycles
n = literal at first instruction, 1 =< n =< 256 (0x001
=< n =< 0x0100, where 0x000=0x0100)
d = number of cycles in nested delay loop

To calculate the length of delay, find the number of
instruction cycles it will complete, and divide that
by the number of instructions per second your PIC is
running at (equal to the frequency of the internal
oscillator divided by four).  Chances are you will
have to use a nested loop if what your are doing is a
visual effect, where you can just plug in the first
delay above into the second (max 197,633 cycles, about
2ms at 4MHz).  If more cycles were needed, you could
then take this delay, and nest it once again in
another loop (max 50,594,817 cycles, about 51s at
4MHz, just make sure that the TEMP register is
different in each loop.  If all you are doing is
blinking lights, delays this long will not matter, but
in other applications this would be a considerable
amount of dead potential processing time, and other
methods of timing would be used.  For more time
critical applications, if there are no possible
combinations of literals to get the exact number of
cycles you need, you would simply find the combination
with the closest number of cycles possible below the
desired number, then add a few nop's.

--Nick


--- Michael Johnston <stoney40@EARTHLINK.NET> wrote:
> Hi guys
>   I wrote a program that i adaptive with help from a
> friend to flash 12 leds tied to  the 12 I/O Pins on
> c54. I have four routines that rotate the bit
> pattern through porta and portb forwards and
> backwards.  My problem is that when the routine is
> going from porta to portb forward ra3 flashes so
> fast and jumps to portb bit0 so fast you can not see
> this happening. It also does this when going
> backwards from portb to porta. But it portb bit 7
> that you dont see flashing. My friend told me the
> reason this was happening was because of the part i
> am using so i ported it to F84 and the program did
> the same thing in this port. I need so type of a
> delay but everything i try seems to mess my code up.
> Thanks
> Mike Johnston
>
> --
> http://www.piclist.com hint: To leave the PICList
> mailto:piclist-unsubscribe-request@mitvma.mit.edu


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