Hi Peter,
Picaxe's are great fun and so easy to use, it'll take you about 1 hour at the most to understand it if you never knew anything about mcu's. its designed so school students can start with mcu's very early on, but its still very powerful.
I've been playing with them for over year and have built a picaxe based
stepper controller for RA tracking and slewing. so far, i've gotten away with 2 min exposures and no star trails without autoguiding. main culprit is my gear train which i'm working on.
the X2 picaxes (except 20x2) have pins for optional external resonators or crystals.you can get them from
www.microzed.com.au.
they start from $4. the 28X2 would be ideal.
as below, the line for stepping a motor for tracking is very simple, but after experimentation i found pulsout to be a much more accurate method than pwm as pwm can't go too low.
if you have a gear reduction of around 700, to get sidereal rate, you'll need a clock of around 60Hz or so (roughly) at 1/16 microstepping.
you can't get that with pwm on the picaxe, so just use this simple method.
track:
pulsout c.1,100
pauseus 100
goto track
it basically sends 100us pulses on pin c.1, pauses for 100 micro seconds and loops. just change values for the pauseus and you get different frequencies. the pulsout pulsewidth can remain constant.
i've been experimenting with picaxes for generating a trapezoidal acceleration profile as well for
stepper motors for high speed slews. that way you can get it to run at the steppers highest rated speed with the picaxe.
and for motor drivers, I've switched to the TI DRV8824.
this supports 1/32 microstepping, 2A rated.
only issue is this comes in the SMD format, so soldering is not easy.
but works really well.
send me a PM if you need more info.
this is a vid of my early prototypes with picaxes for high speed slew, low speed slew, and tracking.
http://www.youtube.com/watch?v=oO68oe5R-Ws
recent positioning test with a DRV8824 driver at 1/32 microstepping.
the picaxe sends 6400 pulses, pauses, toggles the direction bit, sends 6400 pulses and repeats.
i just wanted to test the accuracy of 1/32 microstepping.
http://www.youtube.com/watch?v=53tgJ4pwn1w
one very good thing about the picaxes is the software simulator. you can simulate almost all functions in the free software, it shows you how the pins toggle, what the variable values are and so on. so you learn how to improve your code.