急，最近小弟在做STM32F4, 想要通过RTC实现500ms的定时器， 现目前我采用的是官方提供的demo代码，但未能实现定时，还望大侠指点啊！


/**
  * @brief  Configures the RTC peripheral and select the clock source.
  * @param  None
  * @retval None
  */
static void RTC_Config(void)
{
  /* Enable the PWR clock */
  RCC_APB1PeriphClockCmd(RCC_APB1Periph_PWR, ENABLE);
  /* Allow access to RTC */
  PWR_BackupAccessCmd(ENABLE);
  /* Reset RTC Domain */
  RCC_BackupResetCmd(ENABLE);
  RCC_BackupResetCmd(DISABLE);
  /* Enable the LSE OSC */
  RCC_LSEConfig(RCC_LSE_ON);
  /* Wait till LSE is ready */  
  while(RCC_GetFlagStatus(RCC_FLAG_LSERDY) == RESET)
  {
  }
  /* Select the RTC Clock Source */
  RCC_RTCCLKConfig(RCC_RTCCLKSource_LSE);
  /* Configure the RTC data register and RTC prescaler */
  /* ck_spre(1Hz) = RTCCLK(LSI) /(AsynchPrediv + 1)*(SynchPrediv + 1)*/
  RTC_InitStructure.RTC_AsynchPrediv = 0x7F;
  RTC_InitStructure.RTC_SynchPrediv  = 0xFF;
  RTC_InitStructure.RTC_HourFormat   = RTC_HourFormat_24;
  RTC_Init(&RTC_InitStructure);
  
  /* Set the time to 00h 00mn 00s AM */
  RTC_TimeStruct.RTC_H12     = RTC_H12_AM;
  RTC_TimeStruct.RTC_Hours   = 0;
  RTC_TimeStruct.RTC_Minutes = 0;
  RTC_TimeStruct.RTC_Seconds = 0;  
  RTC_SetTime(RTC_Format_BCD, &RTC_TimeStruct);
}
/**
  * @brief  Configures the RTC Alarm.
  * @param  None
  * @retval None
  */
static void RTC_AlarmConfig(void)
{
  EXTI_InitTypeDef EXTI_InitStructure;
  RTC_AlarmTypeDef RTC_AlarmStructure;
  NVIC_InitTypeDef NVIC_InitStructure;
  
  /* EXTI configuration */
  EXTI_ClearITPendingBit(EXTI_Line17);
  EXTI_InitStructure.EXTI_Line = EXTI_Line17;
  EXTI_InitStructure.EXTI_Mode = EXTI_Mode_Interrupt;
  EXTI_InitStructure.EXTI_Trigger = EXTI_Trigger_Rising;
  EXTI_InitStructure.EXTI_LineCmd = ENABLE;
  EXTI_Init(&EXTI_InitStructure);
  
  /* Enable the RTC Alarm Interrupt */
  NVIC_InitStructure.NVIC_IRQChannel = RTC_Alarm_IRQn;
  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
  NVIC_Init(&NVIC_InitStructure);

  /* Set the alarm A Masks */
  RTC_AlarmStructure.RTC_AlarmMask = RTC_AlarmMask_All;
  RTC_AlarmStructure.RTC_AlarmDateWeekDaySel = RTC_AlarmDateWeekDaySel_Date;
  RTC_AlarmStructure.RTC_AlarmDateWeekDay = RTC_Weekday_Monday;
  RTC_AlarmStructure.RTC_AlarmTime.RTC_Hours = 0x0;
  RTC_AlarmStructure.RTC_AlarmTime.RTC_Minutes =0x0;
  RTC_AlarmStructure.RTC_AlarmTime.RTC_Seconds =0x0;
  RTC_SetAlarm(RTC_Format_BCD, RTC_Alarm_A, &RTC_AlarmStructure);
  
  /* Set alarm A sub seconds and enable SubSec Alarm : generate 8 interrupts per Second */
  RTC_AlarmSubSecondConfig(RTC_Alarm_A, 0xFF, RTC_AlarmSubSecondMask_SS14_5);
  /* Enable alarm A interrupt */
  RTC_ITConfig(RTC_IT_ALRA, ENABLE);
}

void RTC_Alarm_IRQHandler(void)
{
        if(RTC_GetITStatus(RTC_IT_ALRA) != RESET)   /* Check on the Alarm A flag and on the number of interrupts per Second (60*8) */
        {
             /* Clear RTC AlarmA Flags */
            RTC_ClearITPendingBit(RTC_IT_ALRA);
        }

        EXTI_ClearITPendingBit(EXTI_Line17);  /* Clear the EXTI line 17 */
  }

}