>     This is what I don't know how to do

Using TMR1 is very simple (when you know how to do it)

Here are some jottings. E&OE. Watch the banking, I didn't look
any up for this so you may need more or fewer banksels. The aim
is for the IRQ system to be reset after the 76th interrupt. As I
mentioned, the '19264' will need to be decreased by the number
of cycles needed to do the load. TMR1 can be stopped during
the load and restarted after, remember to take those instruction
cycles into account as well

Note that if a 19.6608MHz had been used the ISR would have
been quite a bit simpler, just a 75 count

The timing system could be done quite a different way. For example
load TMR1 with 50,000 (-ish) every time and count 100 interrupts
to get the 5,000,000. This does give you some consistency of
execution in the ISR as there's no clean-up 77th interrupt

Joe

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

;make a flag in a RAM location

#define  last_irq  flags,0

;configure TMR1

         banksel t1con
         movlw   b'00000000'
;                  -
;                   0           not T1osc
;                    00         pre-scaler =3D 1:1
;                      0        T1osc off
;                       0       internal source
;                        0      internal source
;                         0     T1 off
         movwf   t1con

;initialise interrupt counter to - 76

         banksel irqs
         movlw   -.76
         movwf   irqs
         bcf     last_irq       ;initialise "last interrupt" flag

;enable TMR1 interrupts

         banksel pie1
         bsf     pie1,tmr1ie
         bsf     intcon,peie
         bsf     intcon,gie

         clrf    tmr1l
         clrf    tmr1h

         bsf     t1con,tmr1on  ;start TMR1

here   goto   here

;=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

;ISR

irq      bcf     pir1,tmr1if    ;clear irq flag
         btfsc   last_irq       ;test if last interrupt
         goto    last           ;yes

         incfsz  irqs           ;increment, skip after 76 interrupts
         retfie                 ;exit

         banksel tmr1l
         movlw   low(.19264)    ;load TMR1 for last interrupt period
         movwf   tmr1l
         banksel tmr1h
         movlw   high(.19264)
         movwf   tmr1h

         banksel flags
         bsf     last          ;set "last interrupt" flag
         retfie                ;exit

last    banksel  irqs
         movlw   -.76          ;re-initialise counter
         movwf   irqs
         bcf     last_irq       ;and "last interrupt" flag

         movf    portb,w       ;read portb
         xorlw   b'00000001'   ;toggle portb,0
         movwf   portb         ;write portb
        =20
         retfie                ;exit
--=20
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