Internal Timers of Arduino

1/*
2This program turns on and off a LED on pin 13 each 1 second using  an internal timer
3*/
4
5int timer=0;
6bool state=0;
7void setup() {
8  pinMode(13,OUTPUT);
9    
10  TCCR0A=(1<<WGM01);    //Set the CTC mode   
11  OCR0A=0xF9; //Value for ORC0A for 1ms 
12  
13  TIMSK0|=(1<<OCIE0A);   //Set  the interrupt request
14  sei(); //Enable interrupt
15  
16  TCCR0B|=(1<<CS01);    //Set the prescale 1/64 clock
17  TCCR0B|=(1<<CS00);
18
19 
20    
21}
22
23void  loop() {
24  //in this way you can count 1 second because the nterrupt request  is each 1ms
25  if(timer>=1000){
26    state=!state;
27    timer=0;
28  }
29  
30  digitalWrite(13,state);
31  
32}
33
34ISR(TIMER0_COMPA_vect){    //This  is the interrupt request
35  timer++;
36}
37
38