/*
* timer_hw.c
*
* Created: 25-04-2013 18:44:43
*  Author: pf
*/

#include <stdio.h>
#include <avr/io.h>
#include "timer_hw.h"

#define NUMBER_OF_PRESCALERS (sizeof(prescalers)/sizeof(prescalers[0]))


int8_t set_timer(uint32_t frequency)
{
	/* Possible values for counter prescaler */
	const uint16_t prescalers[] = {0,1,8,32,64,128,256,1024};
	
	uint32_t divider;
	uint8_t i=1;
	
	/*
	 * Handle special case when frequency is zero.
	 * 
	 * A zero value for frequency means stopping the timer 
	 */
	if(frequency == 0)
	{
		/* Stop timer */
		TCCR0 = 0;
		return 0;
	}

	/* 
	 * Compute overall divider (from oscillator to output)
	 * This divider is defined by
	 *   divider = prescaler * max_count
	 * where prescaler is counter0 prescaler and
	 * max_count is the MAX value of the counter.
	 *
	 */
	divider = XTAL_FREQ / frequency;
	
	/* find a prescaler value */
	/* try to find a suitable value, starting from the */
	/* first non-zero prescaler value */
	while(prescalers[i]< divider/255)
	{
		i++;

		if(i>=NUMBER_OF_PRESCALERS)
		{
			/* There is not a large enough prescaler value */
			/* Stop the counter */
			TCCR0 &= 0xF8;

			/* Signal error and leave */
			return -1;
		}
	}
	/* i holds the index to the selected prescaler value and
	* it will be the value to write to CS02:0 in TCCR0 */
	
	/* Reset counter0 value */
	TCNT0 = 0x00;
	
	/* Program Output Control Register */
	OCR0 = divider / prescalers[i];
	
	/* Write the CS02:0 bits in TCCR0 (select prescaler) */ 
	/* and WGM01:2=2 (force CTC mode) */
	TCCR0 = (1 << WGM01) | i;
	
	return 0;
}