//timer interrupts
//by Amanda Ghassaei adapted by T.Nilsson July 2019 :)
//June 2012
//https://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.
*
*/

// ATMEGA2560 has 4 16 bit timers: 1,3,4 and 5


//storage variables
boolean toggle4 = LOW;

void setup(){
 
 //set pins as outputs
 pinMode(13, OUTPUT);
 pinMode(2, INPUT_PULLUP);


cli();//stop interrupts

//set timer3 interrupt at 6KHz
 TCCR3A = 0;// set entire TCCR1A register to 0
 TCCR3B = 0;// same for TCCR1B
 TCNT3  = 0;//initialize counter value to 0

 // set compare match register for 6000hz increments
 //OCR3A = 2665;         //((16*10^6) / (6000)) - 1. = 2665
 OCR3A = 5332; // this will equal 3khz
 // turn on CTC mode
 TCCR3B |= (1 << WGM32);

 // Set CSn2, CSn1 and CSn0 bits for prescaler. (Uncomment the one you need)
 TCCR3B |= (1 << CS30);                                 // No prescaler
 //TCCR3B |= (1 << CS31);                                 // CLK/8
 //TCCR3B |= (1 << CS31)| (1<< CS30);                     // CLK/64
 //TCCR3B |= (1 << CS32);                                 // CLK/256
 //TCCR3B |= (1 << CS32) | (1<< CS30);                    // CLK/1024
 
 // enable timer compare interrupt
 TIMSK3 |= (1 << OCIE3A);

sei();//allow interrupts

}//end setup

ISR(TIMER3_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)

 digitalWrite(13,toggle4);
 toggle4 = !toggle4;

}
 
void loop(){
  //Serial.println(digitalRead(2));
if (digitalRead(2) == 0) {
  //TIMSK3 |= 1 << OCIE3A;        // set the bit - enables the interrupt
  //TIMSK3 &= ~(1 << OCIE3A);     // Clear the bit - disables the interrupt
  TIMSK3 ^= 1 << OCIE3A;        // toggle
}
}