>I'm working on a project (currently using the
>PIC16F84) in which I would like to be able to set the
>servo (1-8) to a specific setting (1=Extreme
>left/128=center/255=extreme right -- or whatever)

I wrote a program that controls a single servo, adjusting the pulse widths
for jitter-free operation at any setting along its range of motion,
controlled with a potentiometer input to the A/D.  With only 3 locations
predefined, it should be fairly simple.  What's your input source to
determine the servo positions?

The only thing you really have to worry about is the resolution.  If you're
off by ~5us in pulse widths, you're going to jitter back and forth.

>figure this would have to be table driven, but just
>can't get this to work.  The timing is something else

At 20MHz, you should be a little better off than I was at 4MHz, as you can
tolerate more variation in the instruction cycle counts for pulse timing.
At 50Hz cycles to the servo, you could just put the delay time for the high
pulse in the table for each of your 3 positions and send the line low for
the remainder.  As for handling 8 servos simultaneously, you'd have to poll
each servo's delay timing to shut it off at the right time.  This is where
you might run into trouble with jitter.  But with 8 servos and 20ms between
cycles for each servo, you could pulse one servo, wait for the 1-2ms high
pulse, then move on to the next servo.  You'd run through all 8 in only
16ms, leaving 4ms for any other calculations you might want to do.  At
20MHz, that's quite a few instructions.  Or, better yet, you can set up an
interrupt to go off at your 1-2ms pulse time and send the line low, if 4ms
isn't enough to do whatever you have to do between pulses.  If you keep the
same values going through the loop (only 3 distinct servo positions), you
probably won't have any problem with jitter.

You should be able to do this with an '84 without a problem, if you don't
want to shell out the expense for the '877.

And if any of this advice doesn't seem right, ignore it... I'm having bad
luck today :)

--Andrew
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