Arduino - need reassign Timer 0 overflow (TIMER0_OVF_vect) to different ISR - how?

Question

On a generic Arduino (ATMEGA386P) -based application, I need Timer 0's overflow interrupt (normally goes to TIMER0_OVF_vect) to point to my own code, not the existing Arduino code. I realize this will cause the Arduino runtime functions dependent on the normally-functioning 1 kHz interval to be unusable. I am working within an existing Arduino-based application and cannot readily move out of the Arduino environment (example, to AVR-GCC) as there are some other Arduino dependencies I need to retain. I cannot harness the other timers (Timer 1 or Timer 2) as they are in use for other purposes.

My question is this - how do I (or can I) reconfigure Timer 0 so that it's Overflow interrupt vector points to my own code's ISR entry point?

Thank you.

ADDING INFORMATION 13 DEC 2021 per amike comment 10 Dec 2021

I created ISR(TIMER0_OVF_vect) (that I would use as a substitute for the one present in wiring.c)

ISR(TIMER0_OVF_vect)
{
   // MY SUBSTITUTE CODE WILL GO HERE
}

While the code appears to get through the compiler, it fails during linking with the following error message (from the Arduino IDE

wiring.c.o (symbol from plugin): In function __vector_16': (.text+0x0): multiple definition of __vector_16' sketch\MYTESTPROGRAM.ino.cpp.o (symbol from plugin):(.text+0x0): first defined here collect2.exe: error: ld returned 1 exit status exit status 1 Error compiling for board Arduino Uno

This problem is a show-stopper for a larger project I'm working on, so I'd be very grateful for any further advice on whether I can replace the interrupt service routine (without altering or recompiling the Arduino libraries.)

Thanks again

Answer 1

This topic is treated in arduino.cc.

Nevertheless, I am using a workaround to obtain the same result. This was tested on an Arduino Micro with ATmega32U4 but should be straight-forward to translate to your chip.

Instead of using the timer0 overflow whose ISR routine is already defined by Arduino, I am using the output compare register B and set it to OCR0B = 0xFF. This will make the Output Compare Match B Interrupt to fire on overflow of timer0.

To avoid any interference, you should disable the TOIE0 bit in TIMSK0 so Arduino's ISR(TIMER0_OVF_vect) is never executed. Instead you set the OCIE0B bit:

TIMSK0 = (1 << OCIE0B); // Disables TOIE0 and enables OCIE0B

Your code can then be exectued within the interrupt

ISR(TIMER0_COMPB_vect)
{
 // Your code
}

To give the whole operation some meaning, you'd also want to change the clocking in TCCR0B register by setting the CS02:0 bits as required.

Obviously, you wont be able to use the arduino-native millis() / micros() / delay() functionality. You could also use the output compare register A instead, but I like to keep the option of defining OCR0A as TOP for the counter, thus using B.