Hast du dir die Dokumentation der avr-libc zu dem Thema angesehen? (avr/wdt.h)

Code:
    Note that for newer devices (ATmega88 and newer, effectively any
    AVR that has the option to also generate interrupts), the watchdog
    timer remains active even after a system reset (except a power-on
    condition), using the fastest prescaler value (approximately 15
    ms).  It is therefore required to turn off the watchdog early
    during program startup, the datasheet recommends a sequence like
    the following:

    \code
    #include <stdint.h>
    #include <avr/wdt.h>

    uint8_t mcusr_mirror __attribute__ ((section (".noinit")));

    void get_mcusr(void) \
      __attribute__((naked)) \
      __attribute__((section(".init3")));
    void get_mcusr(void)
    {
      mcusr_mirror = MCUSR;
      MCUSR = 0;
      wdt_disable();
    }
mfG
Markus