If you can live with 256 steps of resolution then you can use something
like:

        clrf    Fref
        incf    Fref,f          ; Fref = 1

pwm_again:
        incfsz  Fref,f
        goto    ($+2)
        clrf    Fio             ; all channels are 0

        ; first channel
        movlw   CH0_MASK        ; has a 1 in the o/p of that mask
        incfsz  Fch0,f          ; increment channel data directly
        goto    ($+2)
        iorwf   Fio,f           ; when crossing 0 set the bit

        ; next channel
        movlw   CH1_MASK
        incfsz  Fch1,f
        goto    ($+2)
        iorwf   Fio,f

        ; ... more like this

        ; code to load etc channels here (your choice)

        goto    pwm_again       ; forever

The code has constant run time and is very flexible, for example you can
replace the iorwf with xorwf or you can implement full stroke
(differential) PWM to achieve +/-5v excursion by using two io pins for a
channel and activating them 180 degrees out of phase, f.ex. by replacing
the clrf at the top of the loop with a suitable load and the iorwf of th
channel with a xorwf so both bits toggle together.

There will be a small bias (offset) added to each channel because of its
position in the loop (wrt the clrf).

If you allow the loop to run 256 times then you do not need to reload the
pwm counters in despite of their incrementing. You can also sequence the
channels (must if your PSU is crummy) by replacing the incfsz with decfsz
on certain channels.

The Fref contains a running phase reference that can be useful.

The loop timing is Tloop = 4 + 5 * channels + Your_Overhead + 2 cycles.
This times 256 is the PWM frequency. For 20 msec PWM period (standard
servos) and 4MHz clock and no interrupts (bit banged) you have 'time' for
14 channels in the loop. You can also eliminate the loop and have a fast
timer ISR run the code (but probably not at 4MHz).

If you reload the channel data at an unfortunate time the respective
output may skip one output pulse. To avoid this reload when Fref == 1 at
the bottom of the loop (or 0 at top).

hope this helps,

Peter

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