STM32L151 RTC寄存器不能写入

使用STM32L151进入待机模式，想利用RTC进行唤醒，根据官方示例编写程序，程序如下
void RTC_Configuration(void)
{
  RTC_InitTypeDef  RTC_InitStructure;
  RTC_TimeTypeDef  RTC_TimeStructure;
  EXTI_InitTypeDef EXTI_InitStructure;
  NVIC_InitTypeDef NVIC_InitStructure;        
        
//  RTC_DeInit();
//        PWR_DeInit();
//         RTC_WakeUpCmd(DISABLE);
/* Enable the PWR clock */
  RCC_APB1PeriphClockCmd(RCC_APB1Periph_PWR, ENABLE);

  /* Allow access to RTC */
  PWR_RTCAccessCmd(ENABLE);

  /* Enable the LSI OSC */
  RCC_LSICmd(ENABLE);

  /* Wait till LSI is ready */  
  while(RCC_GetFlagStatus(RCC_FLAG_LSIRDY) == RESET);

  /* Select the RTC Clock Source */
  RCC_RTCCLKConfig(RCC_RTCCLKSource_LSI);

  /* Enable the RTC Clock */
  RCC_RTCCLKCmd(ENABLE);

  /* Wait for RTC APB registers synchronisation */
  RTC_WaitForSynchro();

  /* Calendar Configuration */
  RTC_InitStructure.RTC_AsynchPrediv = 0x7F;
  RTC_InitStructure.RTC_SynchPrediv        =  0x120; /* (37KHz / 128) - 1 = 0x120*/
  RTC_InitStructure.RTC_HourFormat = RTC_HourFormat_24;
  RTC_Init(&RTC_InitStructure);  

  /* EXTI configuration *******************************************************/
  EXTI_ClearITPendingBit(EXTI_Line20);
  EXTI_InitStructure.EXTI_Line = EXTI_Line20;
  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 Wakeup Interrupt */
  NVIC_InitStructure.NVIC_IRQChannel = RTC_WKUP_IRQn;
  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
  NVIC_Init(&NVIC_InitStructure);  

  /* Configure the RTC WakeUp Clock source: CK_SPRE (1Hz) */
  RTC_WakeUpClockConfig(RTC_WakeUpClock_CK_SPRE_16bits);
        
  RTC_SetWakeUpCounter(0x05);

  /* Enable the RTC Wakeup Interrupt */
  RTC_ITConfig(RTC_IT_WUT, ENABLE);

  /* Enable Wakeup Counter */
  RTC_WakeUpCmd(ENABLE);
}

发现单片机进入了待机模式后无法唤醒。刚开始一直以为程序有问题，看了好多相关资料后没有找到问题。采用单步调试发现RTC的寄存器输入无法写入。调试情况见 调试截图。各位大神看看，这个到底是程序有问题呢还是什么原因。