Olin Lathrop wrote:

> > So, any suggestions on getting 2x analogue
> > output from a small PIC at high speed??
>
> The easy solution is to use a PIC with two PWM modules or use external D/As.
> However, you might be able to do this if you're very careful.  With 20MHz
> clock and 32,000 analog out updates per second, you have 156 instructions
> per update.  Doing one PWM at this rate isn't too hard, especially if there
> are no other interrupts.
>
> Doing two is going to be tricky, because handling of one will result in
> latency (and therefore wrong value) in the other.  You'd pretty much have to
> dedicate the PIC to doing nothing else.  In that case you probably need
> another PIC to do the stuff this PIC can't do, in which case it would be
> cheaper and easier to just get a PIC with two PWM outputs in the first
> place.


Thanks Olin, I think I have a final solution and I came
to the same comclusion that you did, that PWM is best.

My problem is that the PWM voltage is used for current
regulation, and needs to be fairly ripple free. So that
means filter RC at least at 10x the PWM period, so that
meant VERY high speed PWM if I need to get 32,000 actual
current levels/sec, that's maybe 320,000 Hz PWM.

I was looking at resistor ladder solutions to give the
different analogue outputs and found I was looking at it
from the wrong direction. To get 2-phase 8 microsteps
I don't need two high resolution DACs, I need one
DAC with 9 steps and 2 outputs. I can do that with 9
PIC pins and a bunch of resistors. I even had circuit
and parts values worked out, but this pin count forced
me to F87x series parts, which I didn't want (and lots
of resistors).

Once I had the 9-stage 2-output thing happening, I thought
about PWM again. Why use PWM modules? Why even use the
TIMER? Why limit PWM to the 256 count fixed frequency
concept?

What about something like this:

;-----------------------------------------------
; Stage 1 of 9 (example)

stage_1_loop:

        movlw b'00000011'       ; both AB phases on
        movwf PORTB             ; change output pins
        nop                     ; delays as needed
        nop                     ;

        movlw b'00000010'       ; turn phase A off
        movwf PORTB             ; change output pins
        nop                     ; delays as needed
        nop                     ;

        movlw b'00000000'       ; turn phase B off
        movwf PORTB             ; change output pins
        nop                     ; delays as needed
        nop                     ;
        nop                     ; delays as needed
        nop                     ;
        nop                     ; delays as needed
        nop                     ;

        goto stage_1_loop       ; loop forever
;-----------------------------------------------


This tight loop gives *very* high freqeuncy PWM, so
I can meet my spec of small RC constant/ripple and
also give very fast response.

Using infinite loop gives rock solid timing for
accurate PWM voltage of both motor phases. I can use
interrupt on change feature on the "step" control
input pin to simply count up or down and then goto a
new code loop, one loop for each of the 9 voltage
levels I need, to control BOTH phases in each tight
loop. Extending the theory, I can handle all four
quadrants for all 32 possible 8-microstep variations,
using four PIC pins. So I no longer have to use one
set of pins for current and four more pins for phase
direction switching, I can just use the 4 PWM pins
to directly contol the h-bridge halves. Nice. Parts
count just went down again.

To get my required 9 sets of voltages, I think every
voltage can be done using hardcoded blocks of about
20 to 30 instructions. So although there will be
different pwm FREQUENCY for the differing 9 stages,
it gives all required voltage variances at a fast
enough speed. At 20MHz the 30 instruction loop still
gives 170,000 Hz pwm, and this lets me use a small
enough RC to get the result. Another advantage is
that the interrupt on the "step" input command,
then jumping to tight hardcoded loops gives fairly
instant control of the PWM voltages...

Now my next challenge is to read up on hardware
errors re fast port changing and pin capacitance,
but I can design it to only write to the 4 pins
(the whole port) in one hit, and use the other 4
pins as inputs, and hope this will keep me out of
trouble if I don't load the pins too much.

Many thanks to everyone who offered help with this
one! :o)
-Roman

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