Hi Brian.

Nobody knows for sure what black magic is :) Perhaps...

It would be strange to expect 1 cycle precision from 3 cycle bit test loop.

2nd thing - in what internal clock ( out of 4 ) TMR1 is incrementing (write)
and bit test command reads the bit value. Take a closer look in datasheet
that might explain why you miss first three.

One magic word is interrupt :) TMR1 is first in the queue.

You might also want to change prescaler from 1 to say 4 to make life easier.
( in many situations prescaler=4 is good enough when main clock is 20 MHz )

Out of curiosity - what burst of pulses you are trying to generate with
19.6608 MHz crystal ? :) ( looks like something serial to me .. :)


WBR Dmitriy.



Brian Harris wrote:
> 
> Greetings,
> 
> I'm using a 16f628's TMR1 At 19.6608 MHz to generate bursts of pulses.
> 
> With TMR1H staying at 0xff,  I've noticed that changing   TMR1L
> from 0xff  to 0xfe or 0xfd makes no difference to the loop time between
> i_val and goto load in this segment.
> 
> When  observed programmed, on  a logic analyser or on MPLab's stopwatch,
>  the loop time is the same for 0xff , 0xfe and 0xfd=4.069010us=20IC's
> It changes at 0xfc--0xfa to 4.37962us=23IC's f9--f7=26IC's, f6--f4=32IC's
> and 0xac--0xae=101IC's etc etc.
> I've noticed this in another 16f870 TMR1 circuit too.
> I'm wondering if anyone knows why it's skipping.
> In threes by the look of it.
> It's perplexing because the application needs to be able to use the finer increments to generate evenly spaced bursts over the range of 1-230 pulses/ms.
> Any insight greatly appreciated.
> 
> Thanks,
> 
> Brian Harris
> 
> load               means get new values for cnt and int_
> 
> i_val               bcf       t1con,tmr1on    ;stop tmr1
>                       movfw   int_h               ;load interval between pulses
>                       movwf   tmr1h
>                       movfw   int_l
>                       movwf   tmr1l
>                       movlw   b'00001001'  ;prescaler=1
>                       movwf    t1con            ;start tmr1
>                       btfss     pir1,tmr1if       ;wait for Timer1 overflow
>                       goto      $-1
>                       bcf       pir1,tmr1if
> 
>                       bsf    porta,1                ;output pulse
>                       nop
>                       nop                              ;*****timed at 2 nops******
>                       bcf    porta,1
> 
> ;Count out bursts
> 
>                    decfsz    cnt,f
>                    goto   i_val
> 
>                     goto    load
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