Since you talk about the watchdog I presume you want timer interrupts. (There are also I/O interrupts, for instance when the level of a pin changes state.)
Like Russell McMahon says the watchdog is not a good source. The ATmega has several timers (8-bit and 16-bit) which serve your exact purpose. Let's view some example code:
// timer interrupts
// adapted from code by Amanda Ghassaei
// June 2012
// http://www.instructables.com/id/Arduino-Timer-Interrupts/
/*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 3 of the License, or
* (at your option) any later version.
*
*/
// timer setup for timer0, timer1, and timer2.
// For arduino uno or any board with ATMEL 328/168.. diecimila,
// duemilanove, lilypad, nano, mini...
// this code will enable all three arduino timer interrupts.
// timer0 will interrupt at 2kHz
// timer1 will interrupt at 1Hz
// timer2 will interrupt at 10kHz
// 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 10kHz
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 = 199;// = (16*10^6) / (10000*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(); // enable 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)
toggle0 = HIGH - toggle0;
digitalWrite(8, toggle0);
}
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)
toggle1 = HIGH - toggle1;
digitalWrite(13, toggle1);
}
ISR(TIMER2_COMPA_vect) { // timer1 interrupt 10kHz toggles pin 9
// generates pulse wave of frequency 10kHz/2 = 5kHz (takes two cycles
// for full wave- toggle high then toggle low)
toggle2 = HIGH - toggle2;
digitalWrite(9, toggle2);
}
void loop() {
// do other things here
}
This sets up 3 different timers: one at 500 µs, one at 1 s, one at 100 µs. For the details about each timer, look up the registers in the datasheet.
With each timer goes a timer interrupt service routine (ISR). These perform the action required when an interrupt occurs. It's a good practice to exit an ISR as quickly as possible. Only do the things which are really time-critical, but for the rest set a flag to indicate that the interrupt occurred. Then in the main loop you continuously check that flag, and when it is set, perform the necessary actions. Don't forget to clear the flag when you're done.
