#include <avr/io.h>
#include <stdlib.h>
#include <avr/interrupt.h>

//#define USART_U2X_EN

#define F_CPU 20000000UL
#include <util/delay.h>
#define USART_BAUD 2400
#define UBRR_VALUE_DS 520

#ifndef USART_U2X_EN
#define UBRR_VALUE (F_CPU/16/USART_BAUD-1)
#else
#define UBRR_VALUE (F_CPU/8/USART_BAUD-1)
#endif

#if !(UBRR_VALUE == UBRR_VALUE_DS)
#warning "Calculated UBRR value differs from the value given in the datasheet: " 
#define UBRR_VALUE UBRR_VALUE_DS
#endif

int cnt;

void
usart_init(void)
{
	UBRR0H = (unsigned char)(UBRR_VALUE >> 8);
	UBRR0L = (unsigned char)UBRR_VALUE;
#ifdef USART_U2X_EN
	UCSR0A |= _BV(U2X0);
#endif
	UCSR0B = _BV(TXEN0) | _BV(TXCIE0);
	UCSR0C = _BV(UCSZ00) | _BV(UCSZ01);
	//UCSR0C = _BV(UCSZ00) | _BV(UCSZ01);
	cnt = 0;
	sei();
}


ISR(USART_TX_vect)
{
	if(!cnt)
	{
		UDR0 = '\n';
		cnt++;
	}
	else
	{
		cnt = (cnt + 1) % 10;
		UDR0 = '0' + cnt;
	}
}

int main(void)
{
	DDRC |= _BV(PC5);
	usart_init();
	while(!(UCSR0A & _BV(UDRE0)));
	UDR0 = '0';
	while(1)
	{
		PORTC ^= _BV(PC5);
		_delay_ms(500);
		PORTC ^= _BV(PC5);
		_delay_ms(1000);
		PORTC ^= _BV(PC5);
		_delay_ms(500);
	}
	return 0;
}