> > I've got to set a pin every 24 hours plus or minus 10 minutes.  I need the
> > timer to work while I'm doing a bunch of other things (I guess this means
> > I've got to use interrupts).
> Or you have to have a main loop that always takes the same amount
> of time, that is, the same amount of instructions, no matter what
> "else" it is doing.  Sometimes that's practical, usually not.

Make a subroutine that adds the contents of the RTCC to several registers
then resets the RTCC. The subroutine has to be called often enough to
keep the RTCC from overflowing. This means including a call to it inside all
loops. The tricky part is accounting for the time it takes for the subroutine
to execute.

Here's an example that creates a 24 bit timer:

TIMER
; add RTCC contents to 24 bit value (RTIMEL, RTIMEM, RTIMEH)
; This strange looking addition is designed to execute in exactly
; the same number of cycles regardless of the number of carries.
        movfw   RTCC        ;
        addwf   RTIMEL,F    ;
        btfss   STATUS,C    ;
        goto    $+2         ;
        incfsz  RTIMEM,F    ;
        goto    $+2         ;
        incf    RTIMEH,F    ;
;*  reinitialize RTCC
        movlw   $+3-TIMER   ; this the number of instruction cycles between
                            ; the time that the RTCC is read and the time when
                            ; it is written, plus 2 more to adjust for the
                            ; time that is required for the RTCC to restart
                            ; after being written
        movwf   RTCC        ;
;*  return
        retlw   0           ;

This routine will always take exactly the same time to execute.

Unfortunately, using a prescaler with this technique will introduce error
because subroutine can be called at varying times relative to the
prescaler's interval.

I've used this with great success to generate long and accurate timers on
PICS without interrupts. It can also be used to get very high resolution (as
fine as 0.2 microsecond with a 20mhz pic) measurement of external events
on PICS that do have interrupts, while still maintaining long counts.

It is also sometimes convenient to generate slow, non-critical timings by
monitoring single bits within this 24 bit timer.  For example, you might
blink an LED by finding a bit that changes at a frequency close enough to
what you want (like maybe every 1/2 second) and writing it to an IO pin
whenever convenient (like maybe once in each iteration of your main loop,
or maybe attach this to the TIMER routine itself).  The timing won't be
perfectly consistent, but the inconsistency will be too small for human
perception.

You can modify this technique to create longer times.  The 24 bits in this
example won't be enough for your 24 hour timer, unless you run your pic
extremely slow.

This timer routine can also be a convenient place to put routine tasks like
resetting the WDT (may or may not be a good idea, depending on your
application), or re-writing your tris registers in case they get corrupted.


---
                                        Peace,
                                        William Kitchen
                                        bill@iglobal.net

The future is ours to create.