Hartung, Greg wrote:
>
>    My ISR routine takes a very long time to service.  About 70ms, longer
> than the lowest frequency I can engineer with the dividers on a 20Mhz
> crystal (20m/4/65536=.  I would like to call the ISR on a nice even
> frequency of 10hz so the sampling is easy and intuitive to graph later.  The
> dividers will reduce by up to 65,536, so...  a crystal of 10x? or a 555 of
> 10x? or is there an easier way?  I don't think I can do it the traditional
> way of just interrupting more often and keeping a counter because my ISR
> will skip interrupts on certain iterations.  Olin suggested trimming my ISR
> code, but everything there needs to get done before the next iteration, so
> that doesn't seem like the solution.


Hi Greg, a good way to handle this is to use a fast
timer interrupt routine, which checks the timer and
makes an "event" every 100mS. This one-sec timer
routine at:
http://www.ezy.net.au/~fastvid/one_sec.htm

can be easily adapted to do that. Then with such
a long (70mS) task I suggest doing it OUTSIDE the
int routine, so the int routine just sets a bit, then
in your main non-int code you detect that bit and
start the 70mS task. That way the timer int keeps
perfect time while the task is being performed.
:o)
-Roman

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