> Here's my ISR.  I'm fairly sure I've covered all the bases, but I'm new to
> this.  Pick away.
>
> Pulse                                            ; called every 1.0ms on
> Timer0 timeout
>                                                     ; note to self: must
be
> <900 instructions total (incl. worst-case sub/rt's)
>  BCF             INTCON,T0IF       ; clear Timer 0 interrupt flag
>  BSF             INTCON,T0IE       ; re-enable Timer 0 interrupt (if
> necessary)

It is pointless to disable then immediately re-enable the timer 0 interrupt.

>  BCF             STATUS,RP0
>  BCF             STATUS,RP1         ; Bank 0

This corrupts STATUS before it is saved.  It has the effect of randomly
trashing the bank bits in the foreground code (it hates that).

>  MOVLW     d'008'                      ; reset Timer 0 by reloading TMR0
reg

This randomly trashes W from the foreground code's point of view (it really
hates that).  Very bad unless your foreground code is the equivalent of "10
GOTO 10".

>  MOVWF      TMR0                      ; FFh-248d=8d (req'd for 1.00ms
> overall Timer 0 rate)

It is better to add a value into timer 0 during the interrupt than to reset
it to a fixed value.  The add is independent of relative position from the
start of the interrupt, and accumulates no long term error due to interrupt
jitter, disabled interrupts, etc.  Check the manual about timer 0 stopping
for a few cycles after it is written to.

While there is nothing wrong with using timer 0 to generate a periodic
interrupt, it is easier to use timer 2 and its period register if you are
using a machine that has timer 2.

>  MOVWF      W_TEMP                ; save W contents during ISR

Too late, W already contains 8.

>  SWAPF        STATUS,W             ;
>  CLRF            STATUS                 ; select Bank 0 and clear IRP
>  MOVWF       STATUS_TEMP     ; save status to Bank 0 temp reg
>  MOVF         PCLATH,W                     ;
>  MOVWF     PCLATH_TEMP        ; save PCLATH in temp reg
> .
> .
> .

You need to carefully read the section about interrupts and how to
save/restore state in the ISR.  PICs don't do anything special on an
interrupt except to clear INTCON,GIE and CALL 4.  The rest is left as an
exercise to the coder.  You have to save/restore everything you use in
software including W and STATUS.  This is a little tricky since everything
uses W, including any method of saving STATUS, but you can't set the bank
without altering STATUS.

I have a template interrupt routine and lots of other stuff at
http://www.embedinc.com/pic.  Take a look at QQQ_INTR.ASPIC.  It is an
example of the right way to get into and out of an interrupt.  It also
automatically saves/restores PCLATH if running on a machine with more than
one code page, and lets you set an assembly time switch to save/restore FSR
in case your interrupt routine uses FSR.


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(978) 742-9014, http://www.embedinc.com

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