26us will give you ~38.5khz which works well with a 38khz IR receiver mod=
ule,=20
though I expect the range may not be as great as if you get 26.3khz.  I=20
currently do this in a calculated loop (not in the ISR), but my app is a=20
keyfob transmitter with only a few different signals.  If you plan to do =
much=20
more in the PIC, you will probably need to run it at a higher frequency.

Cheers,
-Neil.


On Tuesday 03 May 2005 06:02 pm, Tom Wrighton scribbled:
> ...  My project is to make a small IR
> beacon as part of a robotic competition for navigation.  I would like t=
o
> execute a small flashing LED countdown (one pulse each for 5 seconds)
> and than modulate an IR LED for 20 seconds.  I think I have the TMR0
> interrupt figured out and I can execute seconds timing and LED flashing.
> My confusion comes in as follows:  I=92m 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 ev=
ery
> 26.3 us.  This calculates to loading the TMR0 with something like
> decimal 242 +/- 4 cycles for a half cycle at 38KHz.  This does not leav=
e me
> very many cycles left for the ISR.  At this point I have tried to measu=
re
> the
> frequency of the flashing LED with a Craftsman DMM that has a freq
> measurement capability.  The best I can do so far is 32 and a fraction =
KHz.
> At this point I am wondering if I need to use a crystal osc. And run it=
 at
> 10 MHz instead of the 4Mhz of the internal?  I have followed the PICLIS=
T
> ...



--=20
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