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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