I've got a dandy problem using microstepping in a stepper motor.

As you know, microstepping involves sinusouidally varying the voltage on a
pair of stepper motor windings (bipolar stepper motor, in this case, 2
windings)  so as to continuously move the motor between steps.

I'm driving a TEA3718 step motor driver with a PIC.  The TEA3718 has a handy
Vref pin, that is used as a reference to compare the output current with a
known value.  Varying the voltage on Vref sinusoidally causes a nice smooth
action on the motor.  So far so good.

EXCEPT when I change paolarity.  After 180 degrees of sine wave on one coil
(and 180 degrees of cosine wave on the opposite coil) the motor has moved
essentially one step's worth (or is it a half--- )  anyway, at that point
you must change the polarity of the coil.   Vref is zero, so changing
polarity on the copil should couse it to begin to swing the other way, as
soon as Vref begins to increase again.

Except it doesn't.  The motor moves about 1/4 step whenever you change
polarity.  This negates all the nice advantages of microstepping, like
turning smoothly.  Even when Vref is zero, theres movement.

Anybody seen anything like this?

-- Lawrence