I've spent way too much time on this today trying to figure out
what's going on. Maybe someone can see something I can't

Code for a s/w UART on an 18F4550 should have been straight-
forward, but TMR0 isn't doing what I expected

Fosc is derived from a 3.2768MHz crystal, and instruction cycles
are being measured as 101.72ns, so the PIC is running at 39.3216

The loading I have for TMR0 should make about 104us, which
is the bit time for an 8MHz 18F1320 set to receive 9615 baud
(eventually the 4550 will be bumped up to 48MHz for USB and
the s/w UART will also be transmitting at 9615 baud)

I calculate

$10000 - $0FC02 = $003FE = 1022, * 0.10172 = 104us

But TMR0 puts out 757.85us. I can't figure out what needs to
be done. It's not a recognisable multiple (~ 7.28), so counted
out a pre-scaler. The msdelay does put out the calculated 1ms,
so Fosc is good

No interrupts or WDT enabled. The h/w UART is comming
with an external device at 9600 baud, no problems

Any ideas ? Please ?

TMR0 set-up

         mov     b'000001000',t0con
;                  0          t0 off
;                   0         16-bit
;                    0        internal clock
;                     x
;                      1      no pre-scaler
;                       xxx

Test transmission

txa5     movlw   0x55
         call    txw
         call    msdelay
         movlw   0xaa
         call    txw
         call    msdelay
         bra     txa5

;================================================
;        Transmit byte from W - set dtx high/low (inverted)
;================================================

txw      movff    wreg,temp
         mov      8,cnt0
         bcf      dtx          ;Start bit
         call     bitdel

byte_out btfss    temp,0
         bra      zerobit
onebit   bcf      dtx
         call     bitdel
         bra      nextbit
zerobit  bsf      dtx
         call     bitdel

nextbit  rrncf    temp
         decfsz   cnt0
         bra      byte_out

         bsf      dtx          ;Stop bit
         call     bitdel
         return

; 1 bit delay, 104.004us

bitdel   mov      0x02,tmr0l
         mov      0xfc,tmr0h
         bcf      intcon,tmr0if
         bsf      t0con,tmr0on ;timer on
         btfss    intcon,t0if  ;wait for rollover
         bra      $-2
         bcf      t0con,tmr0on ;timer off
         return

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