Arduino uart AVR-GCC

main.c

#ifndef F_CPU
#define F_CPU       16000000UL
#endif

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

int main(void) {

  uint16_t baudrate = 9600;

  /* calculate and set the baudrate */
  uint16_t baud = (F_CPU / 8 / baudrate - 1) / 2; 
  UBRR0H = (uint8_t) (baud >> 8);
  UBRR0L = (uint8_t) (baud & 0x0ff);  

  /* activate transmitter, receiver and receiver interrupt */
  UCSR0B |= (1 << 3); /* TXEN */
  UCSR0B |= (1 << 4); /* RXEN */
  UCSR0B |= (1 << 7); /* RXCIE */
  
  /* set 8 data bits */
  UCSR0C |=  (1 << 1); /* UCSZ0 = 1 */
  UCSR0C |=  (1 << 2); /* UCSZ1 = 1 */
  UCSR0B &= ~(1 << 2); /* UCSZ2 = 0 */
  
  /* set parity NONE */
  UCSR0C &= ~(1 << 4); /* UPM0 = 0 */
  UCSR0C &= ~(1 << 5); /* UPM1 = 0 */
  
  /* set 1 stopbit */
  UCSR0C |=  (1 << 3); /* USBS = 1 */
  
  while(1) {
     UDR0 = 0x56; /* sende ein grosses V */ 
     _delay_ms(100);
  }
    
  return 0;
}

/* UART Empfangs Interrupt Routine */
ISR(USART0_RX_vect){
    uint8_t data = UDR0;
    /* Hier kannst du dann selbst überlegen was du mit dem empfangenen Byte machst ;-) */
}

The first thing I found with the correct registers of atmega328p.