STM32 RTC Write Protection

  RCC_APB1PeriphClockCmd(RCC_APB1Periph_PWR | RCC_APB1Periph_BKP, ENABLE);

  /* Allow access to BKP Domain */

  PWR_BackupAccessCmd(ENABLE);

  /* Reset Backup Domain */

  BKP_DeInit();

  /* Enable LSE */

  RCC_LSEConfig(RCC_LSE_ON);

  /* Wait till LSE is ready */

  while (RCC_GetFlagStatus(RCC_FLAG_LSERDY) == RESET)

  {}

  /* Select LSE as RTC Clock Source */

  RCC_RTCCLKConfig(RCC_RTCCLKSource_LSE);

  /* Enable RTC Clock */

  RCC_RTCCLKCmd(ENABLE);

  /* Wait for RTC registers synchronization */

  RTC_WaitForSynchro();

  /* Wait until last write operation on RTC registers has finished */

  RTC_WaitForLastTask();

  /* Enable the RTC Second */

  RTC_ITConfig(RTC_IT_SEC, ENABLE);

  /* Wait until last write operation on RTC registers has finished */

  RTC_WaitForLastTask();

  /* Set RTC prescaler: set RTC period to 1sec */

  RTC_SetPrescaler(32767); /* RTC period = RTCCLK/RTC_PR = (32.768 KHz)/(32767+1) */

  /* Wait until last write operation on RTC registers has finished */

  RTC_WaitForLastTask();

void rtc_set_alarm(uint32_t data)

{

while((RTC->CRL & RTC_FLAG_RSF) == 0);   // wait for sync

RTC->CRL |= RTC_CRL_CNF; //enter configuration mode

RTC->ALRH = data>>16;

RTC->ALRL = data&0xFFFF; 

RTC->CRL &= ~RTC_CRL_CNF; //exit configuration mode

while((RTC->CRL & RTC_FLAG_RTOFF) == 0)   // wait for last task

{}

}