PIC18 interrupt handler, where to save W etc?

[Bruce]

Hi Darrel,

With RCON,IPEN = 0 it's in compatibility mode. I.E. no priority levels on any interrupts, and it's just as easy as say setting up & using interrupts on a 16F part.

There's no GIEH, GIEL, or any priority high/low bit settings to worry about. So, it's actually a bit easier than setting up & using high & low priority interrupts.

Here's an example;

    list p=18f452
    include "p18f452.inc"
 
    CONFIG OSC=XT,OSCS=OFF,WDT=OFF,WDTPS=128,BOR=ON,BORV=45,LVP=OFF
 
    CBLOCK 0x08
      MyVar
    ENDC
 
    ORG    0x000
    BRA    Init
 
    ORG    0x008           ; all ints vector here in compatibility mode
    BRA    TMR_INT         ; and high-pri vectors here when enabled
 
    ORG    0x018           ; low-pri int vector, but only when priority
    BRA    LOW_PRI         ; interrupts are enabled
 
Init
    CLRF   MyVar           ; clear MyVar
    CLRF   PORTB           ; Clear PORTB
    CLRF   TRISB           ; PORTB all outputs
    MOVLW  0x07
    MOVWF  ADCON1          ; disable A/D
    MOVLW  B'11000111'     ; TMR0 on, 8-bit, prescaler, 1:256
    MOVWF  T0CON
    BCF    RCON,IPEN       ; disable priority interrupts
    BCF    INTCON,TMR0IF
 
    ; since RCON,IPEN=0, we don't need to worry about any priority
    ; bit settings for any interrupt source, and we refer to GIE,PEIE
    ; VS GIEH,GIEL because we're in "compatibility" mode. We don't
    ; have to mess with anything related to high or low priority
    ; interrupts. Compatibility mode is just as easy as interrupts on
    ; 16F parts.
 
    BSF    INTCON,TMR0IE   ; enable Timer0 interrupt
    BSF    INTCON,PEIE     ; peripheral ints enabled
    BSF    INTCON,GIE      ; global enabled
 
Main
    MOVFF  MyVar,PORTB     ; show count on PORTB LEDs
    BRA    Main
 
TMR_INT                    ; default int vector for all interrupts
    INCF   MyVar           ; when in compatibility mode
    BCF    INTCON,TMR0IF
    RETFIE FAST
 
LOW_PRI                    ; never gets here because priority ints
    INCF   MyVar           ; are not enabled when RCON,IPEN = 0
    BCF    INTCON,TMR0IF
    RETFIE FAST
    END

And you still have the benefit of fast return with auto context restore even with low pri interrupts.

Maybe I should have said - I use NO priority levels or compatibility mode VS only low priority interrupts. The only time I enable priority levels is when I have more than one interrupt, and one or more needs to interrupt the less important interrupt assigned to the low vector.

Otherwise it's easier to just use compatibility mode with no priority levels to worry about.

I know you're already aware of this, but I figured it would be good to post so folks reading this didn't get confused between using priority levels - I.E. high & low VS compatibility mode where there are no priority levels enabled.