STM32L100 RTC lässt sich nicht einrichten

int main(void)

{

    init_usart(&debug_uart);

    SysTick_Config(SystemCoreClock / 1000);

    if(time_rtc_init() == ERROR)

    {

        DEBUG_PUTS("RTC Error\n");

    }

    while(1);

}

ErrorStatus time_rtc_init(void)

{

    RCC_APB1PeriphClockCmd(RCC_APB1Periph_PWR, ENABLE);

    PWR_RTCAccessCmd(ENABLE);

    RCC_RTCResetCmd(ENABLE);

    RCC_RTCResetCmd(DISABLE);

                                        // See AN 3371 (Doc ID 018624) Rev 4, p. 7

    rtc_write_enable();                 // (1) Write 0xCA, 0x53 into the RTC_WPR register

    RTC->ISR |= RTC_ISR_INIT;           // (2) Set INIT bit to '1' in RTC_ISR register

    tick1ms = 0; //wird im SysTick hochgezaehlt

    while(!(RTC->ISR & RTC_ISR_INITF))  // (3) Poll INITF bit of in RTC_ISR until it is set

    {

        if(tick1ms > 1000) return ERROR;

    }

    #if defined(RTC_HSE) && (RTC_HSE)

        RCC_RTCCLKConfig(RCC_RTCCLKSource_HSE_Div16);   //Input = HSE_VALUE/16  -> 1 MHz

        RTC->PRER = (uint32_t)(7999);   // (4) Write first the synchronous value and then write the asynchronous

        RTC->PRER |= (uint32_t)(124 << 16);

    #else

        RCC_LSEConfig(RCC_LSE_ON);

        tick1ms = 0;

        while (RCC_GetFlagStatus(RCC_FLAG_LSERDY) == RESET)

        {

            if(tick1ms > 1000) return ERROR;

        }

        RCC_RTCCLKConfig(RCC_RTCCLKSource_LSE);

        RTC->PRER = (uint32_t)(255);    // (4) Write first the synchronous value and then write the asynchronous

        RTC->PRER |= (uint32_t)(127 << 16);

    #endif

    timestamp_t default_time = { 2015, 01, 01, 00, 00, 00, WDAY_SATURDAY};  //01.01.2000, 00:00:00

    rtc_store_time_int(&default_time);  // (5) Set RTC_TR and RTC_DR registers

    RTC->CR &= ~RTC_CR_FMT; //24h       // (6) Set FMT bit in RTC_CR register

    RTC->ISR &= ~RTC_ISR_INIT;          // (7) Clear the INIT bit in the RTC_ISR register

    rtc_write_protect();                // (8) Write "0xFF" into the RTC_WPR register

    return SUCCESS;

}

void rtc_store_time_int(const timestamp_t* timestamp)

{

    RTC->DR =       byte_to_bcd(timestamp->year - 2000) << 16

                |   timestamp->wday << 13                   //always < 10, so BCD == BIN

                |   byte_to_bcd(timestamp->month) << 8

                |   byte_to_bcd(timestamp->day);

    RTC->TR =       byte_to_bcd(timestamp->hour) << 16

                |   byte_to_bcd(timestamp->minute) << 8

                |   byte_to_bcd(timestamp->second);

}