problem with timer0 int and program size

[eliberg]

Hi,
I have a really big problem with the timer0 interrupt.
I'm using a 18F4431 PIC, and wuold like to have an int0 every 30ms.
If my program is short (under 30% ROM usage), no problem, the int works fine.
( setup_timer_0(RTCC_INTERNAL|RTCC_DIV_256);
enable_interrupts(INT_RTCC);
set_timer0(64950);

Inside the int routine of course I set the timer again
)
But my program is very long. With a 61% of rom usage, the int0 fires every 167ms, even if the set_timer is always 64950, and if I compile the whole program (65% of rom usage) int0 fires every 250ms.
It seems that the interrupt timing depends on the program size.
Is it possible?
What can I do?
Thanks
eli

[PCM programmer]

[asmallri]

Here are a couple of things to look at:

Make sure you do not call a function from both within the interrupt handler and in the main code body. If you do do this then it is likley the PIC is adding disable interrupts around the call in the main body of code with the result that you miss interrupts.

Some functions, such as writing to the internal EEPROM, automatically disable interrupts during the process however this does not explain the large delays you are seeing.

Another common mistake is to code functions inside the interrupt handler - such as printf or delay_ms(xxx) in which case you are staying too long in the handler and missing interrupts.
_________________
Regards, Andrew

http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!!

[Ttelmah]

Try something radical. Make the code 'grow' to the larger size, by writing a macro, to do something simple, like loop a few dozen times, and repeat this into the main code a lot of times.
I suspect you will find that the interrupt remains at the same frequency.
What is probably happening, is that some individual command in the main, is resulting in interrupts being disabled, and you are spending longer in this state as your code grows...

Best Wishes