> you misunderstoot my post. TIMER0 is what i want to use for this new
function, i'm already using, or are planning
> on using, TIMER1 and TIMER2

OK, that's a little bit more work.  To recap, you want an interrupt every
250,000 instructions and you want to not use timer 1 or timer 2 for this
purpose.

One way or the other, you are going to get multiple timer 0 interrupts for
each of your 250mS periods.  There are lots of ways to set up the details,
however:

1  -  How accurate does it need to be?  250,000 / 256 = 976.5625.  The
simplest approach is to do the 250mS processing every 977 timer 0
interrupts.  That will be every 250,112 instructions, or only .045% off.  If
you can tolerate a little more error and want to use less cycles, you can
increase the prescaler value.  With a 2:1 prescaler setting, you do the
250mS processing every 488 interrupts for .058% error.  Since 488 is
divisible by 8, you can use a prescaler of 16:1 and do the processing every
61 interrupts for the same .058% error.  If you can tolerate .058% error,
this is the best way to go because it is the simplest and requires the least
overhead.

2  -  Note that 250,000 is 256 * 976 + 144.  This means that each period you
can take 976 "whole" timer 0 interrupts and adjust the timer so that one
period ends up only being 144 cycles long.  256 - 144 = 112, which is the
amount you want to add to timer 0 to adjust it to a 144 cycle period between
interrupts.  However, you actually need to add 2 less, since timer 0
increments are inhibited for the two instructions following the add.

3  -  If you can tolerate some jitter for each interrupt but don't want
accumulating long term error, do a Bresenham to determine the number of
timer 0 interrupts per period.  This will bounce between 976 and 977, with
no long term error.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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