#include <avr/io.h>

#include <avr/interrupt.h>

#include <avr/sleep.h> 

#ifndef F_CPU

//define cpu clock speed if not defined

#define F_CPU 3686400

#endif

//set desired baud rate

#define BAUDRATE 19200

//calculate UBRR value

#define UBRRVAL ((F_CPU/(BAUDRATE*16UL))-1)

void USART_Init()

{

    //Set baud rate

    UBRRL=UBRRVAL;      //low byte

    UBRRH=(UBRRVAL>>8);   //high byte

    //Set data frame format: asynchronous mode,no parity, 1 stop bit, 8 bit size

    UCSRC=(1<<URSEL)|(0<<UMSEL)|(0<<UPM1)|(0<<UPM0)|

        (0<<USBS)|(0<<UCSZ2)|(1<<UCSZ1)|(1<<UCSZ0); 

    //Enable Transmitter and Receiver and Interrupt on receive complete

    UCSRB=(1<<RXEN)|(1<<TXEN)|(1<<RXCIE);

    //enable global interrupts

    set_sleep_mode(SLEEP_MODE_IDLE);

    sei();

}

ISR(USART_RXC_vect)

{

    //defien temp value for storing received byte

    uint8_t Temp;

    //Store data to temp

    Temp=UDR;

    Temp++;//increment

    //send received data back

    // no need to wait for empty send buffer

    UDR=Temp;

}

int main(void)

{

    USART_Init();

    while(1)

    sleep_mode();

    //nothing here interrupts are working

    return 0;

}