At 04:02 PM 5/3/2005 -0700, you wrote:

>My confusion comes in as follows:  I'm trying to use the internal RC osc
>
>(  _INTRC_OSC_NOCLKOUT  ) which I believe ends up at 1 us
>
>execution cycles.  To achieve a 38KHz flash rate I need an interrupt every
>
>26.3 us.

13.15usec per half cycle.  I don't think you have enough
time to save and restore context and reload timer0 at 4MHz, let alone
keep track of other timing.

If you run the PIC from a 10/20MHz resonator you'll get a more accurate signal
(because a typical resonator is +/-0.5% guaranteed) and you'll get a
resolution of 200ns (and, crucially, a lot more cycles to play with for
each interrupt). If you use 33/66 cycles for each half-cycle, the error
will be -.3% (not counting oscillator error), which doesn't sound too bad.

At 4MHz, you should be able to get it to work by writing it as an
isochronous loop rather than using interrupts, but it will severely limit
what else you can get the PIC to do at the same time, and how you have
to do it.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com




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