Sekundenzeiger läuft zu schnell

BODLEVEL = 2V7

RSTDISBL = [ ]

DWEN = [ ]

SPIEN = [X]

WDTON = [ ]

EESAVE = [ ]

BOOTSZ = 1024W_3C00

BOOTRST = [X]

CKDIV8 = [ ]

CKOUT = [ ]

SUT_CKSEL = EXTXOSC_8MHZ_XX_16KCK_14CK_65MS

EXTENDED = 0xFD (valid)

HIGH = 0xDA (valid)

LOW = 0xFF (valid)

/*

* USART.c

*

* Erstellt: 17.07.2014 03:08:12

*   Author: sschultewolter

*/

#define F_CPU 16000000UL

#include <avr/io.h>

#include <avr/interrupt.h>

#include <util/delay.h>

#include <stdlib.h>

#define BAUDRATE 9600

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

volatile unsigned int ms;

volatile unsigned int s;

volatile unsigned int min;

volatile unsigned int h;

void USART_init(void);

unsigned char USART_receive(void);

void USART_send( unsigned char data);

void USART_putstring(char* StringPtr);

void TIMER_init(void);

char String[] = "Hello world\n\r";

int main(void)

{

  USART_init();

  TIMER_init();

  while(1)

  {

    if(s%2 == 0)

    {      

      USART_putstring(String);

    }

  }

}

void USART_init(void)

{

  UBRR0H = (uint8_t)(BAUD_PRESCALLER>>8);

  UBRR0L = (uint8_t)(BAUD_PRESCALLER);

  UCSR0B = (1<<RXEN0)|(1<<TXEN0);

  UCSR0C = (3<<UCSZ00);

}

unsigned char USART_receive(void)

{

  while(!(UCSR0A & (1<<RXC0)));

  return UDR0;

}

void USART_send( unsigned char data)

{

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

  UDR0 = data;

}

void USART_putstring(char* StringPtr)

{

  while(*StringPtr != 0x00)

  {

    USART_send(*StringPtr);

    StringPtr++;

  }

}

void TIMER_init(void)

{

  TCCR0A = (1<<WGM01);

  TCCR0B |= (1<<CS01);

  OCR0A = 125-1;

  TIMSK0 |= (1<<OCIE0A);

  sei();

}

ISR (TIMER0_COMPA_vect)

{

  ms++;

  if(ms == 1000)

  {

    s++;

    ms = 0;

    if(s == 60)

    {

      min++;

      s = 0;

    }

    if(min == 60)

    {

      h++;

      min = 0;

    }

    if(h == 24) h = 0;

  }

}