Thanks very, very much for the response.

> You say "random" but are the noise spacings multiples of 1ms or
> 73ms ? The actual T0 IRQ looks to be 1,000,000 / 4 / 256 = 976us.
> Is that clashing with the 73ms routine somehow ? Are you context
> saving / restoring in the ISR ?
>
I really can't tell the extent of the "randomness": my dinosaur scope only
indicates that the weirdness is not periodic.

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)
 BCF             STATUS,RP0
 BCF             STATUS,RP1         ; Bank 0
 MOVLW     d'008'                      ; reset Timer 0 by reloading TMR0 reg
 MOVWF      TMR0                      ; FFh-248d=8d (req'd for 1.00ms
overall Timer 0 rate)
 MOVWF      W_TEMP                ; save W contents during ISR
 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
 BTFSC         FLAGS3,4                 ; test the "Initialization In
Progress" flag
 GOTO         _EXIT_PULSE           ; if it's set, bypass all the usual
routines
<"the usual routines"=a list of calls to short counter update routines (left
out for brevity)>
_EXIT_PULSE
 CALL           Supervisor                   ; prevent system supervisor
from causing a reset
 BCF             STATUS,RP0
 BCF             STATUS,RP1              ; select Bank 0
 MOVF         PCLATH_TEMP,W     ; restore PCLATH
 MOVWF     PCLATH                      ;
 SWAPF       STATUS_TEMP,W     ; set STATUS reg to its original state...
 MOVWF     STATUS                      ; ...and put it back where it goes
 SWAPF       W_TEMP,F                 ; engage in some chicanery...
 SWAPF        W_TEMP,W               ; ...to restore W
 RETFIE                                            ; return and re-enable
interrupts

> ===================================================
>
> Love the Subject btw, very clever. You sure you haven't got a bunch
> of sexually dysfunctional chips who are just noisy boys?

Perhaps.  Would this do the trick, then?
MOVFW    _DYSFUNC
SKPZ
CALL        Water_Cannon
<etc.>

Rob Robson

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