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

static inline void uart_init();
int main()
{
    /* serial */
    uart_init();

    /* data input */
    PORTB &= ~0x0F;
    DDRB  |=  0x0F;

    for (;;)
    {
        while ((UCSRA & (1 << RXC)) == 0);
        PORTB ^= UDR & 0x0F;

        while ((UCSRA & (1 << UDRE)) == 0);
        UDR = PORTB & 0x0F;
    }

    return 0;
}

static inline void uart_init()
{
    UCSRB |= (1 << RXEN)  | (1 << TXEN);
    UCSRC |= (1 << UCSZ0) | (1 << UCSZ1);
#define BAUD 38400
#include <util/setbaud.h>
    UBRRH = UBRRH_VALUE;
    UBRRL = UBRRL_VALUE;
#if USE_2X
    UCSRA |= (1 << U2X);
#else
    UCSRA &= ~(1 << U2X);
#endif
}