david@PAPDEV.COM wrote:
>
> I have a question for all the PIC experts out there:  I am trying to
> interface a quadrature decoder to a PIC which currently drives some
> stepping motors.  I can easily figure out how to build an external up/down
> counter and "roll my own" absolute position decoder-  but has anybody
> successfully used the PIC itself to  do this?
>
> I was thinking along the lines of using one externally clocked counter to
> count all pulses, and another to count only those in one direction.
> Subtracting these two give a relative number of steps in a particular
> direction.

I threw together a few latches and XOR gates to convert from quadrature
to tachometer and direction.  The tach input drove one of my interrupt
on change pins.  Whenever I got that interrupt, I'd just poll the
direction pin.  I needed to use 16 bit maths, but that wasn't too
horrible.  The only problem (?) with it was that when the direction
changed, my circuit produced a one cycle latency.  Not too tough to fix
in software.  I can't remember the circuit offhand, but I seem to recall
that it used two XORs, two latches, and nothing else.  I'll dig it up if
you'd like.

Jason ( # -e^(pi*i) ) <g>