user22898: I am trying to find a simple timer0 interrupt example, but none of those work. Neither this code which I tried to run:
boolean toggle0 =0;

void setup() {
pinMode(8, OUTPUT);
  cli();

  // Set timer1 interrupt at 1 Hz
  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 1 Hz 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);

  sei();
}

ISR(TIMER0_COMPA_vect){  // Change the 0 to 1 for timer1 and 2 for timer2
  if (toggle0){
    digitalWrite(8,HIGH);
    toggle0 = 0;
  }
  else{
    digitalWrite(8,LOW);
    toggle0 = 1;
  }
}

void loop() {

}

What am I doing wrong?

user23841: The main problem is you are using the wrong interrupt handler. You are setting up Timer1, but the interrupt handler is for Timer0:
boolean toggle0 = 0;

void setup() {
pinMode(8, OUTPUT);
  cli();
//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);

  sei();
}

ISR(TIMER1_COMPA_vect){  //change the 0 to 1 for timer1 and 2 for timer2
  toggle0 = !toggle0;
  digitalWrite(8, toggle0);
}

void loop() {

}


22898: <p>I am trying to find a simple timer0 interrupt example, but none of those work. Neither this code which I tried to run:</p>

<pre><code>boolean toggle0 =0;

void setup() {
pinMode(8, OUTPUT);
  cli();

  // Set timer1 interrupt at 1 Hz
  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 1 Hz increments
  OCR1A = 15624;// = (16*10^6) / (1*1024) - 1 (must be &lt;65536)

  // Turn on CTC mode
  TCCR1B |= (1 &lt;&lt; WGM12);

  // Set CS12 and CS10 bits for 1024 prescaler
  TCCR1B |= (1 &lt;&lt; CS12) | (1 &lt;&lt; CS10);

  // Enable timer compare interrupt
  TIMSK1 |= (1 &lt;&lt; OCIE1A);

  sei();
}

ISR(TIMER0_COMPA_vect){  // Change the 0 to 1 for timer1 and 2 for timer2
  if (toggle0){
    digitalWrite(8,HIGH);
    toggle0 = 0;
  }
  else{
    digitalWrite(8,LOW);
    toggle0 = 1;
  }
}

void loop() {

}
</code></pre>

<p>What am I doing wrong?</p>
 23841: <p>The main problem is you are using the wrong interrupt handler. You are setting up Timer1, but the interrupt handler is for Timer0:</p>

<pre><code>boolean toggle0 = 0;

void setup() {
pinMode(8, OUTPUT);
  cli();
//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 &lt;65536)
  // turn on CTC mode
  TCCR1B |= (1 &lt;&lt; WGM12);
  // Set CS12 and CS10 bits for 1024 prescaler
  TCCR1B |= (1 &lt;&lt; CS12) | (1 &lt;&lt; CS10);
  // enable timer compare interrupt
  TIMSK1 |= (1 &lt;&lt; OCIE1A);

  sei();
}

ISR(TIMER1_COMPA_vect){  //change the 0 to 1 for timer1 and 2 for timer2
  toggle0 = !toggle0;
  digitalWrite(8, toggle0);
}

void loop() {

}
</code></pre>
