UART falsch beim Systemtakt von 2MHz

#include "main.h"

int main( void )

{

  clock_init_2MHz();

  //clock_init_1MHz();

  USART_Init(25); // 9600bps bei 2MHz

  //USART_Init(12); // 9600bps bei 1MHz

  __enable_interrupt();

  for(;;)

  {

      Usart_Tx(0x5A);

  }

}

void clock_init_1MHz(void)

{

  CLKPR = (1<<CLKPCE);        // set Clock Prescaler Change Enable

  // set prescaler = 8, Inter RC 8Mhz / 8 = 1Mhz

  CLKPR = (1<<CLKPS1) | (1<<CLKPS0); 

}

void clock_init_2MHz(void)

{

  CLKPR = (1<<CLKPCE);        // set Clock Prescaler Change Enable

  // set prescaler = 8, Inter RC 8Mhz / 4 = 2Mhz

  CLKPR = (1<<CLKPS1); 

}

void USART_Init(unsigned int baudrate)

{

    // Set baud rate

    UBRR0H = (unsigned char)(baudrate>>8);

    UBRR0L = (unsigned char)baudrate;

    // Enable 2x speed

    UCSR0A = (1<<U2X0);

    // Enable receiver and transmitter

    UCSR0B = (1<<RXEN0)|(1<<TXEN0)|(0<<RXCIE0)|(0<<UDRIE0);

    // Async. mode, 8N1

    UCSR0C = (0<<UMSEL0)|(0<<UPM00)|(0<<USBS0)|(3<<UCSZ00)|(0<<UCPOL0);

}

void Usart_Tx(char data)

{

    while (!(UCSR0A & (1<<UDRE0)));

    UDR0 = data;

}