I've done both in C, but not assembly...  if you'd like to see the servo
C code from my first attempt (warning:ugly, amateur C code!) you can see
it at https://www.botkin.org/dale.  There is a .LST file that has the C
and ASM code in the same listing, if that helps.

What I do is set up a regular interrupt using TMR0, usually around 1ms.
In the interrupt service routine I increment a counter.  When the counter
hits the refresh interval of 20ms (or a little longer), I generate a pulse
with its period determined by the value of a variable.  So all I have to
do is make that variable represent the position (or speed, in your case)
desired for the servo, and let the ISR do the work.

I have long suspected that the servo will hit max speed with a pretty
small offset in an application like yours.  I have done some poking around
inside Futaba and Airtronics servos with a 'scope, it's pretty
interesting.  Been meaning to use a couple for a robot drive for a year or
so now, but I keep getting side--tracked on other projects.  Maybe on '02.

By the way, if anyone has not seen it yet, the /TAB BOOKS BUILD YOUR OWN
ROBOT KIT/ is available in the US at Barnes & Noble.  Awesome deal!!
Designed by Myke Predko & Ben Wirz, two names that should be very familiar
to anyone on this list.  It's a complete PIC controlled robot kit in a
box, with a socket for a Basic STAMP II and Parallax App Mods.  Price is
$60 and B&N had 20% off when I got one for my 16 year old son.  The web
site at http://www.tabrobotkit.com/ has complete source listings and
schematics, so it's a hack-friendly platform.  My *only* wish is that the
wheels were big enough for carpet use.  Anyway, #3 Son will be opening it
some time today or tomorrow, I'll have more info on it then.  I just hope
someone took my blatant hints and got one for me too!

Dale
--
"Curiosity is the very basis of education and if you tell me that
curiosity killed the cat, I say only the cat died nobly."
          - Arnold Edinborough


On Mon, 24 Dec 2001, Prince Anamalech wrote:

>
> Hello fellow PICSter's
>
> I am new to the pic scene but have gained sufficient knowledge to get me through so far!
> i am currently building a robot using a PIC16F84 . I am using two hacked servos for the motors, and am hoping to use two
> ultrasonic transducers (40Khz) to detect obstacles.
> I have hacked the servo's using a transistor bridge (making it think it is always in the centre position). I am trying to
> test out the servos by writing some assembly code with mplab, but i am having no success.
> I have connected the servo's control wire to a pic pin. PINS A.0 and A.1. Left and Right respectively.
> i have tried manipulating some code on the piclist archive but with no success.
> COULD anyone please help me out with some assembly code PLEASE?
> also does anyone have any ideas or schem's/code to help me out with my ultrasonic's??

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