Timer interrupt won't work if the ISR is not set

Question

I am using an Arduino Mega 2560.

I have a code in a c file, that looks like this:

 #include <avr/io.h> /* for DDRB, PORTB, etc. */
#include <util/delay.h> /* for _delay_ms () */
#include <arduino.h>
int main(void){
 DDRB |= ((1 << DDB7)); // Define pin 13 as output
 noInterrupts();
//set timer1 interrupt at 1Hz
 TCCR1A = 0;// set entire TCCR1A register to 0
 TCCR1B = 0;// same for TCCR1B
 TCNT1 = 0;//initialize counter value to 0
 // set compare match register for 1hz increments
 OCR1A = 15624;// = (16*10^6) / (1*1024) - 1 (must be <65536)
 // turn on CTC mode
 TCCR1B |= (1 << WGM12);
 // Set CS12 and CS10 bits for 1024 prescaler
 TCCR1B |= (1 << CS12) | (1 << CS10); 
 // enable timer compare interrupt
 TIMSK1 |= (1 << OCIE1A);
 interrupts();
 while(1){
 asm("nop");
 }
return 0;
}
 ISR(TIMER1_COMPA_vect) // timer compare interrupt service routine
{
 PORTB ^= ((1 << PB7)); // toggle LED pin
 PINB |= _BV(7);
 PINB = PINB | _BV(7);
}

Which is called from an ino file, like this:

 extern "C"
 {
 void foo ();
 }
void setup ()
 { 
 foo ();
 } // end of setup
void loop ()
 {
 } // end of loop

However, if I set up another timer interrupt (ex Timer Interrupt 0 or 2, the blink will stop happening, for some reason...

For example:

//set timer0 interrupt at 2kHz
 TCCR0A = 0;// set entire TCCR2A register to 0
 TCCR0B = 0;// same for TCCR2B
 TCNT0 = 0;//initialize counter value to 0
 // set compare match register for 2khz increments
 OCR0A = 124;// = (16*10^6) / (2000*64) - 1 (must be <256)
 // turn on CTC mode
 TCCR0A |= (1 << WGM01);
 // Set CS01 and CS00 bits for 64 prescaler
 TCCR0B |= (1 << CS01) | (1 << CS00); 
 // enable timer compare interrupt
 TIMSK0 |= (1 << OCIE0A);

Now, the strangest part happens that if all the three (or just the same two) timer interrupts are set, and with everything defined in the INO file, it works fine...

//this code will enable all three arduino timer interrupts.
//timer0 will interrupt at 2kHz
//timer1 will interrupt at 1Hz
//timer2 will interrupt at 8kHz
//storage variables
boolean toggle0 = 0;
boolean toggle1 = 0;
boolean toggle2 = 0;
void setup(){
 //set pins as outputs
 pinMode(8, OUTPUT);
 pinMode(9, OUTPUT);
 pinMode(13, OUTPUT);
cli();//stop interrupts
//set timer0 interrupt at 2kHz
 TCCR0A = 0;// set entire TCCR2A register to 0
 TCCR0B = 0;// same for TCCR2B
 TCNT0 = 0;//initialize counter value to 0
 // set compare match register for 2khz increments
 OCR0A = 124;// = (16*10^6) / (2000*64) - 1 (must be <256)
 // turn on CTC mode
 TCCR0A |= (1 << WGM01);
 // Set CS01 and CS00 bits for 64 prescaler
 TCCR0B |= (1 << CS01) | (1 << CS00); 
 // enable timer compare interrupt
 TIMSK0 |= (1 << OCIE0A);
//set timer1 interrupt at 1Hz
 TCCR1A = 0;// set entire TCCR1A register to 0
 TCCR1B = 0;// same for TCCR1B
 TCNT1 = 0;//initialize counter value to 0
 // set compare match register for 1hz increments
 OCR1A = 15624;// = (16*10^6) / (1*1024) - 1 (must be <65536)
 // turn on CTC mode
 TCCR1B |= (1 << WGM12);
 // Set CS12 and CS10 bits for 1024 prescaler
 TCCR1B |= (1 << CS12) | (1 << CS10); 
 // enable timer compare interrupt
 TIMSK1 |= (1 << OCIE1A);
//set timer2 interrupt at 8kHz
 TCCR2A = 0;// set entire TCCR2A register to 0
 TCCR2B = 0;// same for TCCR2B
 TCNT2 = 0;//initialize counter value to 0
 // set compare match register for 8khz increments
 OCR2A = 249;// = (16*10^6) / (8000*8) - 1 (must be <256)
 // turn on CTC mode
 TCCR2A |= (1 << WGM21);
 // Set CS21 bit for 8 prescaler
 TCCR2B |= (1 << CS21); 
 // enable timer compare interrupt
 TIMSK2 |= (1 << OCIE2A);
sei();//allow interrupts
}//end setup
ISR(TIMER0_COMPA_vect){//timer0 interrupt 2kHz toggles pin 8
//generates pulse wave of frequency 2kHz/2 = 1kHz (takes two cycles for full wave- toggle high then toggle low)
 if (toggle0){
 digitalWrite(8,HIGH);
 toggle0 = 0;
 }
 else{
 digitalWrite(8,LOW);
 toggle0 = 1;
 }
}
ISR(TIMER1_COMPA_vect){//timer1 interrupt 1Hz toggles pin 13 (LED)
//generates pulse wave of frequency 1Hz/2 = 0.5kHz (takes two cycles for full wave- toggle high then toggle low)
 if (toggle1){
 digitalWrite(13,HIGH);
 toggle1 = 0;
 }
 else{
 digitalWrite(13,LOW);
 toggle1 = 1;
 }
}
ISR(TIMER2_COMPA_vect){//timer1 interrupt 8kHz toggles pin 9
//generates pulse wave of frequency 8kHz/2 = 4kHz (takes two cycles for full wave- toggle high then toggle low)
 if (toggle2){
 digitalWrite(9,HIGH);
 toggle2 = 0;
 }
 else{
 digitalWrite(9,LOW);
 toggle2 = 1;
 }
}
void loop(){
 //do other things here
}

Is anybody able to explain why it works in the first case, doesn't work in the second, and in the third, it works as well?

Answer 1

I found out that it's not enough to just define the interrupts, there must also be an ISR method with the compare vector...

 ISR(TIMER0_COMPA_vect) // timer compare interrupt service routine
{
 PORTB ^= ((1 << PB6)); // toggle LED pin
 PINB |= _BV(6);
 PINB = PINB | _BV(6);
}

There was no error at compile, though.

• Nick Gammon: there must also be an ISR method with the compare vector - that's correct - you have told the processor to jump to an ISR vector, and the default vector jumps to the start of your program. There was no error at compile, though. - you have not made any syntactical errors.