#include<avr/io.h>
 
#define USART_BAUDRATE 9600
#define BAUD_PRESCALE (((F_CPU/(USART_BAUDRATE*16UL)))-1)
 
int main(void){
 char recieved_byte;
  
 UCSR0B |= (1<<RXEN0)  | (1<<TXEN0);
 UCSR0C |= (1<<UCSZ00) | (1<<UCSZ01);
 UBRR0H  = (BAUD_PRESCALE >> 8);
 UBRR0L  = BAUD_PRESCALE;
  
    for(;;){
  // wait until a byte is ready to read
  while( ( UCSR0A & ( 1 << RXC0 ) ) == 0 ){}
 
  // grab the byte from the serial port
  recieved_byte = UDR0;
   
  // wait until the port is ready to be written to
  while( ( UCSR0A & ( 1 << UDRE0 ) ) == 0 ){}
 
  // write the byte to the serial port
  UDR0 = recieved_byte;
    }
    return 0;   /* never reached */
}