msec counter for avr.

millis.c

/*
Calculated for 20MHz:
(((1sec/20000000) *  8  )*  250  )*       10     =0.001sec
          freq     presc   ticks     multiplayer    1msec

so at freq 20mhz, we need to prescale with 8, count to 249, and then multiple by 10 to get 1msec out of the counter.
This needs some calculations for each different frequency.
Adjust prescalar and ticks until you get the "multiplayer" to be an integer!
 */

//the counter
volatile unsigned long milCounter = 0;

ISR(TIMER0_COMPA_vect){
  milCounter++;
}

void initMillis(void){
  //start timer
  TCCR0A = (1<<WGM01); //set to compare (CTC)
  TCCR0B = (1<<CS01); //8prescalar
  OCR0A = 249; //ticks
  
  //enable interrupt on compare
  TIMSK0 = (1<<OCIE0A);
  sei();
}


unsigned long millis(void){
  unsigned long millis;

  unsigned char sreg_backup = SREG;
  cli();
  millis = milCounter;
  SREG = sreg_backup;

  return millis/10;//returns 1msec, by dividing by multip
}