Getting out of an interrupt service Routine

[emufambirwi]

Some PIC assembly fragments of code to do PLL based mains sync and 255-level output phase control.
I wrote this for a 16C73 about 20 years ago.. Most later stuff is in C rather than assembly.
[Using a 20MHz clock; timing based on that].

All main product and customer-specific code removed..



Put the control value in "outphase".

You need a few other memory locations like "clockref", "phase", "mainsref", "stat_temp" etc.

The timing reference input looks to have been a direct pickup from AC via a resistor to a port A pin (hz).
It uses each transition either way and compares against the last input state at each interrupt.
You need to change the mains00 / main10 parts if you use a different type of sync input, eg. trigger on the negative [high to low] transition only and bypass the routine [exitint] (but still set the image flag high) on low to high, with the full-wave opto setup.

mainsref,7 is the bit that the normal program loop monitors to count cycles & do low speed timing, read it and reset when seen in that.
(Also used by the PLL phase comparison, so be careful if you change that).


PORTB,s_oe is the triac trigger ouput.
PORTB,mref is a spare pin to monitor the program zero detection.

[Edit - typos & clarification].

Some initialisation stuff..

;
; PAGE 0 Register setup ***
;
                bcf     STATUS,RP0      ; Select page 0
                movlw   000             ; Set port B to
                movwf   PORTB           ; $00 = all o/p off.
                movlw   D'001'          ; Init. timer
                movwf   TMR0            ; to avoid w/d
;
; Clear workspace
;
                movlw   021             ; Clear RAM
                movwf   FSR             ; Set indirect ptr
                movlw   05E             ; Byte Count
                movwf   iomode          ; Save Count
;
begin2          clrf    INDF            ; Clear ram via indirect
                incf    FSR,f           ; next addr
                decfsz  iomode,f        ; Loop till done
                goto    begin2          ;
                clrf    iomode            ;
;
; All ram clear.
;
; Set o/p on phase
; (Higher value = shorter firing pulse)
;
                movlw    d'160'            ; Triac firing pulses turned off
                movwf    outphase        ; at ??/256 from end of each half cycle.
;
; Set master Timer value
;
                movlw    D'050'            ; 256 - 50 = 200 Cycles
                movwf    clockref        ;
                movwf    TMR0            ; Reset Timer
                bcf        INTCON,2        ; Reset int flag.
;
; Enable Interrupts.
;
                bsf     INTCON,T0IE     ; Enable timer int
                bsf     INTCON,GIE      ; & global int.

The interrupt routine that does all the work:

;
;
; Interrupt driven routines
;
; Executed at interval of (200nS * 200) = approx. 40uS
;
intrpt          movwf   w_temp          ; Save W
                swapf   STATUS,w        ; Save context
                bcf     STATUS,5        ; Bank 0
                movwf   stat_temp       ;
                movf    PCLATH,w        ; Get latch
                movwf   pcl_temp        ; save it
;
;Timer counts UP, Int at rollover 255->0
;
                movf    clockref,w      ; Get timer value
                movwf   TMR0            ; reload timer
                bcf     INTCON,2        ; reset ctr int flg
;
; Shift Phase counter
;
                decf    phase,f            ; Next step
;
; Do Phase Control outputs
;
outangle        movf    phase,w            ; Get Phase
                subwf    outphase,w        ; Compare to o/p value
                btfsc    STATUS,C        ; Negative result?
                goto    out_off            ; No, turn OFF
out_on            bcf        PORTB,s_oe        ; Yes, turn ON
                goto    endphase        ;
out_off            bsf        PORTB,s_oe        ; Set outputs OFF
;
; Do mains Zero-Crossing detection        ;
;
;endphase        movf    phase,w            ; Look at phase
;                subwf    D'64'            ; 3/4 way through?
;                btfsc    STATUS,Z        ;
;                bsf        mainsref,7        ; Show zero crossing occurred for async. program.
;
endphase        btfsc    mainsref,0        ; Look at present value
                goto    mains10            ;
;
; Last was 0, has it changed?
;
mains00            btfss    PORTA,hz        ; Still 0?
                goto    exitint            ; Yes.
                bsf        mainsref,0        ; Update flag,
;                bsf        PORTB,mref        ;
                goto    mainsx            ;
;
; Last was 1, has it changed?
;
mains10            btfsc    PORTA,hz        ; Still 1?
                goto    exitint            ; yes.
                bcf        mainsref,0        ;
;                bcf        PORTB,mref        ;
;
; Zero crossing occurred.
;
; Update phase values from output values.
;
; Look at Phase counter (counts down from 255 - 0).
; Adjust master clock to get as near 255 counts as possible per sequence.
;
;
; Do clock timing.
;
mainsx            movf    phase,w            ; Look at count
                btfsc    STATUS,Z        ; Is it Zero?
                goto    mainsnul        ; Yes, no change.
                btfsc    phase,7            ; Look at phase counter
                goto    mainsdec        ;
;
; Phase has not overflowed, shorten timer interval.
;
mainsinc        incf    clockref,f        ; Clock will run faster.
                goto    mainsnul        ;
;
; Phase has overflowed, lengthen timer interval.
;
mainsdec        decf    clockref,f        ; Clock will run slower.
;
mainsnul        movlw    0FE                ; Start just below 255, so 100% on possible.
                movwf    phase            ; Restart phase counter
                bcf        clockref,7        ; Ensure clock can't get too fast...
;
                bsf        mainsref,7        ; Show zero crossing occurred for async. program.
;
;
; Restore context & return
;
exitint         movf    pcl_temp,w      ; Restore saved regs.
                movwf   PCLATH          ;
                swapf   stat_temp,w        ;
                movwf   STATUS             ;
                swapf   w_temp,f        ;
                swapf   w_temp,w        ;
                retfie                  ; Back to prog.
;
; End of base program.
;

Thank you so much, there is so much to learn