Hi Jinx,

I think you have to write TMR0H first which is a latency byte only and it
updated when you write TMR0L. Reading is the opposite way, TMR0H updated
when you read TMR0L.

Regards,
Tamas


On 6/27/07, Jinx <joecolquitt@clear.net.nz> wrote:
>
> 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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