#include <stdint.h>
#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h>

//X has to be between 1 and 129
void clock_prescale_set(uint8_t x)
{
    if(x == 0)
    {
        return;
    }
    else if(x == 1)//Disable prescaler
    {
        XDIV = 0;
    }
    else if(x <= 129)
    {
		XDIV &= ~_BV(XDIVEN);
        XDIV = (129 - x) | _BV(XDIVEN);
    }
}

uint8_t clock_prescale_get(void)
{
    if(bit_is_clear(XDIV, XDIVEN))
    {
        return 1;
    }
    else
    {
        return (129 - (XDIV & 0x7F));
    }
}

ISR(TIMER1_OVF_vect)
{
	static int i = 0;
	static uint8_t prescaler = 1;
	uint8_t prescalerRead;
	PORTB ^= 0xFF;

	i++;
	if(i > 10)
	{
		i = 0;
		prescaler++;
		if(prescaler > 129)
			prescaler = 1;
		clock_prescale_set(prescaler);
		prescalerRead = clock_prescale_get();
		if(prescalerRead != prescaler)
			asm("break");
	}
}

int main(void)
{
	DDRB = 0xFF;
	sei();

	//Enable timer 1
	TIMSK |= _BV(TOIE1);
	TCCR1B |= _BV(CS10) | _BV(CS11);

	for(;;)
		asm("nop");


	return 0;
}