If variable from 1-300, and assuming that that means integers,
I'd say use TMR0 and divide accordingly in the ISR to get 1-sec
pulses.  Then maintain 2 counters -- one set to 26 and the
other to 35.  On every 1-sec pulse, decrement counter A and
perform whatever action when it hits 0.  Also reset it to 26.
ALSO, on the same 1-sec pulses, do the same for counter B and
reset to 35 when it hits 0 and you perform the necessary
actions.

Can't say how this would work using the WDT, but I don't feel
that it was designed for this purpose, where as TMR0 etc was.

Cheers,
-Neil.



-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Adam Smith
Sent: Tuesday, June 11, 2002 2:16 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: [PIC]: Multitasking and timing


Hello,

I am trying to make a PIC16LF872 do multiple things at variable
intervals.

Let's suppose that I have eventA and eventB.  I want eventA to occur (it
could be making an output pin high, calling a procedure, whatever...)
every 26 seconds and eventB to occur every 35 seconds.  How do you guys
suggest I do this?

I want the PIC to be asleep when the events are not being fired.  Also,
the times given above (26 and 35 seconds) are just for illustration,
these will be variable from 1-300 seconds.

I was thinking of using the WDT with the prescaler and use some
computations to add a postscaler in accordance to the requirements, but
given the inaccuracy of the WDT (7-33 ms) it isn't very feasible.

Given the incredibly fast advancement of the timers (even with a
prescaler), this option doesn't sound very appealing either.  (The chip
will be running on a 4MHz clock.)

The only reasonable option I've come up with is a 555 timer giving me a
1Hz pulse to count off of.  The big downfall here is the power
consumption.  Not only will this be running off of 4 AA batteries, these
periodic events will be occurring 24 hours a day.

Any suggestions?

Thanks,
Adam Smith

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