why trm0 preset don't work in this position?

[HenrikOlsson]

The toggle does effectively halve the frequency but neither time or the timer stops during the actual time it takes for the instructions to execute.

Clearing the interrupt flag takes instruction(s), toggle the output takes multiple instructions, reloading the timer takes instruction(s). Each instruction takes time. By the time you're actually presetting the timer to 128 it's already continued counting and you're "pulling it back" ever so slightly every time, so the frequency drops. No secrets, nothing special, just the way it works. Think about it.

Here are two different versions you can try to see which one is the most accurate for your needs, then you can always tweak the value.

mainloop:

if INTCON.2=1 THEN 
  TMR0 = 128 'yes work 
  INTCON.2=0
  toggle portb.2 
endif

goto mainloop

mainloop:

if INTCON.2=1 THEN 
  TMR0 = TMR0 + 128 'yes work 
  INTCON.2=0
  toggle portb.2 
endif

goto mainloop

/Henrik.

[lutherblisset]

thanks, now the frequency is 484 hz