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

#define USART_BAUDRATE 4800
#define BAUD_PRESCALE (((F_CPU / (USART_BAUDRATE * 16UL))) - 1)

void icp_init(void);
void usart_init(void);
void usart_transmit(unsigned char);
unsigned char usart_receive(void);

volatile unsigned char sync;
volatile unsigned char end;
volatile unsigned int  start_pulse;
volatile unsigned int  end_pulse;

int main(void)
{
	DDRD = 0xFE;
	usart_init();
	icp_init();

	while (1) {
		if (sync == 2) {
			usart_transmit(0xFF);
		} else  if (sync == 1) {
			usart_transmit(end_pulse>>3);
		}
		sync = 0;
	}
}

ISR(TIMER1_CAPT_vect) {
	if (end == 0) {
		start_pulse = ICR1;
		end = 1;
	} else {
		end_pulse = ICR1 - start_pulse;
		end = 0;
		if (end_pulse > 2100) {
			sync = 2;
		} else  if (end_pulse > 900) {
			sync = 1;
		}
	}
}

void icp_init(void) {
	// set ICP pin as input
	DDRB &= ~(1<<PB0);
	// timer clock = system clock, rising edge trigger
	TCCR1B = 0xC1;
	// clear ICF1 clear pending interrupts
	TIFR1 = 1<<ICF1;
	// enable timer1 captuer event interrupt
	TIMSK1 = 1<<ICIE1;
	// set local var for start of pulse
	end = 0;
	// enable global interrupts
	sei();
}

void usart_init(void) {
	// set baud rate
	UBRR0H = (unsigned char) (BAUD_PRESCALE>>8);
	UBRR0L = (unsigned char) BAUD_PRESCALE;
	// enable receiver and transmitter
	UCSR0B = (1<<RXEN0) | (1<<TXEN0);
}

void usart_transmit(unsigned char data) {
	// wait for empty transmit buffer
	while ( !(UCSR0A & (1<<UDRE0)) );
	// put data into buffer, sends the data automatically
	UDR0 = data;
}

unsigned char usart_receive(void) {
	// wait for data to be received
	while ( !(UCSR0A & (1<<RXC0)) );
	// get and return received data from buffer
	return UDR0;
}