timer1 too slow??

[kloppy]

i want to produce a variable delay by using timer1 (i can't use delay_us)
i need to adjust the delay between about 20us and 500us

what i did is to preset timer1 with (0xFFFF-mydelay) and then let it overflow. to test i inverted a0 at every overflow.

test-code looks about like this:

[Mark]

The extra time is from the int handler over head. Try adding a value to timer1 to cancel the effect.

timer1 += (0xFFE7);

Be aware of overflow, your timer would have to roll through a complete cycle before you get an int.

[ckielstra]

This is the classic problem that everyone starting to work with interrupt based timers stumbles across.

You are assuming that an interrupt is an 'instant' action, requiring no time at all to start. The thing is that before entering your interrupt handler function the compiler has added several instructions for saving all the possible registers that you might change in your function.
On exiting the interrupt the compiler has again added code for restoring the previously saved registers.
On a PIC16 it takes about 18 instructions to enter the interrupt routine the same to exit again. The PIC18 has more registers and so takes even longer with approximately two times 36 instructions.

The easiest solution is to change your interrupt routine by not setting timer1 with a fixed value, but reading timer1 first and then subtract your delay from this.