Scott, if I use the INFSNZ/BRA method you showed, do I still need to AND
right before it? I've confused myself sufficiently to not be able to
figure it out. Help! I like the NEGF version...I will investigate that
once I get the problem I just mentioned clear in my head.

Help!

Josh
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Scott Dattalo wrote:
> If you're using the 18F series then  you can save an instruction:
>
> Instead of doing this (at the end of the routine)
>
>         XORLW   11111111b       ;Toggle the current state.
>         INCFSZ  rising_edge,F   ;If the rising edge counter has not
>          XORLW  11111111b       ;rolled over then toggle them again.
>                                 ;Double toggle == no effect. However,
>                                 ;if the rising edge counter does roll
>                                 ;over then a single toggle will turn
>                                 ;the pwm bits on, unless of course the
>                                 ;pwm counter has just rolled over too.
>
> You save the instruction by:
>
>        INFSNZ   rising_edge,F
>         BRA     UpdatePWMs      ;When the rising edge counter rolls over
>                                 ; then update everything.
>
>        MOVWF   pwm_state       ;Save the state
>        MOVWF   PWM_PORT        ;update the outputs
>
>        RETURN
>
> Then:
>
> UpdatePWMs:
>
>   ; (note the reversed order from above - this ensures the
>   ; pwm_port is written isochronously).
>
>        MOVWF   PWM_PORT        ;update the outputs
>        MOVWF   pwm_state       ;Save the state
>
>   ; copy the current duty cycles into counters
>
>        MOVFF    pwm0DC,pmw0
>        ...
>
>        RETURN
>
> Everytime the "rising_edge" counter rolls over, 17 extra cycles are spent
> updating the PWM counters. However, the counters don't need to be updated
> if nothing has changed (they just roll over). So you could save 15
> instructions by adding a flag:
>
> UpdatePWMs:
>
>   ; (note the reversed order from above - this ensures the
>   ; pwm_port is written isochronously).
>
>        MOVWF   PWM_PORT        ;update the outputs
>        MOVWF   pwm_state       ;Save the state
>
>        BTFSS   ShouldUpdate,bitn
>         return
>
>        BCF     ShouldUpdate,bitn
>
>  ; copy the current duty cycles into counters
>
>        MOVFF    pwm0DC,pmw0
>        ...
>
> ----
>
> You can also dynamically adjust the PWM. However, this runs the risk of
> creating an abrupt discontinuity. See the comments in the code (on my web
> page) on how this can be done.
>
> ----
>
> BTW, If you're using the 18F there's a way to save many instructions:
>
>        DCFSNZ    pwm0,F           ;When the pwm counter reaches 0
>         BTG      PWM_LATCH,0      ;toggle it's output
>
>       ....
>
>        DCFSNZ    pwm7,F
>         BTG      PWM_LATCH,7
>
>        DCFSNZ    rising_edge,F
>         NEGF     PWM_LATCH,F      ; tricky - drive all ouput highs.
>
> 18 cycles versus 23.
>
> The NEGF works because all of the outputs are cleared by executing a
> single BTG (bit toggle) instruction every 256 iterations. At the end of
> 256 cycles, all 8 outputs are cleared and the NEGF will turn them all back
> on. Of course, this assumes that when the PWM engine was started that the
> output latch was initialized to all ones.
>
> A slightly better ending:
>
>        DECFSZ    rising_edge,F
>         RETURN
>
>        NEGF      PWM_LATCH,F
>
>        ; update the PWM's:
>
>        BTFSS    ShouldUpdate,bitn
>         return
>
>        BCF      ShouldUpdate,bitn
>
>        MOVFF    pwm0DC, pwm0
>        ...
>
>        RETURN

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