#define F_CPU 12000000UL
#define BAUD 9600
#include "util/setbaud.h"
#include <util/delay.h>


#include <avr/io.h>

int main(void)
{
	
	// uart konfigurieren
	// baud rate
	UBRRH = UBRRH_VALUE;
	UBRRL = UBRRL_VALUE;
	
	
	UCSRA = 0x00;
	UCSRB = (1<<RXEN) | (1<<TXEN);
	UCSRC = (1<<UMSEL) | (1<<UCSZ1) | (1<<UCSZ0);
	
	// ports konfigurieren
	DDRD |= (1<<PD1);
	
	// UDR => lesen/schreiben; buffer
	
	DDRC|=0xff;
	
	



    while(1)
    {
        
        
        // byte lesen
       // unsigned char data = "br";
        
      
		
		
	
		if ( PINA & (1<<PA0) )
		{
			// pin ist high (Taster NICHT betätigt)
			//UDR = 'o';

		}
		else
		{
			// pin ist low (Taster betätigt)
			_delay_ms(100);
			UDR='9';
			_delay_ms(100);
		}

        
        // byte senden
       // UDR = data;

    }
}




ARDUINO CODE

int led = 13;
char c = 'a';

void setup() {
  Serial.begin(9600);
  Serial1.begin(9600);
  pinMode(led, OUTPUT);
}

void loop() {
 // if (Serial1.available() > 0)
  while(Serial1.available() > 0)
  {
     c = Serial1.read();
   // digitalWrite(led, HIGH);
    
   if (c == '5')
    digitalWrite(led, HIGH);
    
    if (c=='o')
    digitalWrite(led, LOW);
    
    Serial.write(Serial1.read());
      
     
      
    
  }
  
}