> Yes, the PWM module cannot achieve the low duty cycles required while
> maintaining adequate resolution.  If I recall correctly I think if I
was
> willing to tolerate 10ms pulse spacing (instead of 20) then it would
have
> functioned but with very low resolution and I was told the short pulse
> spacing may have upset some servos.  (Don't quote me, I'm speaking
from
> memory - but I'd wanted to use the PWM module and was forced to reject
that
> idea)
>=20
> The way I do it now is to set the TMR0 prescaler to 256, which gets me
> something like 12 or 13ms until the interrupt.  The first time it
interrupts
> it resets the prescaler to 128 and returns which delivers the
remaining 7 or
> 8ms.  The second time the interrupt is triggered it sets all the servo
lines
> high for 1ms followed by an additional 0 to 1ms.  In this way I get
8bit
> pulse width resolution but still have the processor yielded for
> communications about 90% of the time.  I'm not saying this is the best
> way... but it was the only way I could come up with to combine control
of
> multiple servos, good resolution, serial communication, 6 ADCs for
> measurements, and fast 20MHz clock for computation.
>=20
> Drew

OK, I have a feeling you could possibly select a better chip for the job
- seriously.

The chip you have is not a nanowatt part - from what I can make out from
the Microchip site, it won't let me look at the page for that chip
currently. There are PIC24 chips that are nanowatt, and I suspect you
probably want to be saving all the power you can in a pilotless drone.
This may or may not be a consideration, it is not clear to me if you are
attempting a solar powered drone or not.

As indicated above you are having problems with PWM resolution. PIC24
PWM channels have 16 bit resolution in place of the 5/6/7/8/10 bit that
that the 16F series do. And they have a heap of hardware channels that
will do it for you - as Olin said, set and forget. The largest chips do
have 5 hardware PWM channels and 4 UART channels. There is also a number
of SPI channels that could be used (with a bit of thought) as PWM
channels, with 16 stages of resolution, by transmitting a 16 bit word
with the appropriate bits set, at a timer interrupt determined interval.
Set the SPI clock to make the 16 bits 2.5mS long if that gives you
enough resolution, or stuff a number of 16 bit words into the SPI Tx
FIFO with a suitably faster clock to achieve better resolution.

You are already using a 40/44 pin part, so going to a large pin count
PIC24 won't be a hardship. Use the best tool for the job, instead of
make do and mend.
--=20
Scanned by iCritical.

--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.