/*
 * Low resolution implementation of timer using a 8 bit clock with prescaler, 
 * etc.  Clicks approximately every millisecond.  Microseconds are not available, 
 * and any queries to the function will just return milliseconds * 1000.
 * You can use this file when you don't need high resolution timing, and don't care
 * about slight clock drift, inaccuracy, etc.  For timing most human-related activities,
 * though, it should be more than sufficient.
 */
#include "timer.h"

static volatile uint64_t _timer_millis;

/*
 * Initializes the timer, and resets the timer count to 0.  Sets up the ISRs 
 * linked with timer0.
 */
void timer_init(){
	//Set up the timer to run at F_CPU / 256, in normal mode (we reset TCNT0 in the ISR)
	TCCR0A = 0x0;
	TCCR0B |= _BV(CS02);
	
	//Set compare value to be F_CPU / 1000 -- fire interrupt every millisecond
	OCR0A = F_CPU / 256 / 1000;
	
	//Enable compare interrupt
	TIMSK0 = _BV(OCIE0A);

	//Reset count variables
	_timer_millis = 0;
	
	//Enable interrupts if the NO_INTERRUPT_ENABLE define is not set.  If it is, you need to call sei() elsewhere.
#ifndef NO_INTERRUPT_ENABLE
	sei();
#endif
	
	DDRB = 0xFF;
}

/*
 * Returns the number of milliseconds which have elapsed since the 
 * last time timer_init() was called.  Overflows after about 49 days.
 */
uint64_t timer_millis(){
	return _timer_millis;
}

/*
 * Returns the number of microseconds which have elapsed since the 
 * last time timer_init() was called.  Overflows after about 71 minutes.
 */
uint64_t timer_micros(){
	return (_timer_millis * 1000);
}


/* 
 * The ISR for timer0 overflow.  Increment the _timer_count here, and do the calculcations
 * to increment _timer_millis as needed.
 */
ISR(TIMER0_COMPA_vect){
	TCNT0 = 0;
	_timer_millis++;
}