GPIO_InitTypeDef GPIO_InitStructure;
  NVIC_InitTypeDef  NVIC_InitStructure;
  EXTI_InitTypeDef  EXTI_InitStructure;
  
  /* Allow access to RTC */
  PWR_RTCAccessCmd(ENABLE);

  /*!< LSI Enable */
  RCC_LSICmd(ENABLE);

  /*!< Wait till LSI is ready */
  while (RCC_GetFlagStatus(RCC_FLAG_LSIRDY) == RESET)
  {}

  /*!< RTC Clock Source Selection */
  RCC_RTCCLKConfig(RCC_RTCCLKSource_LSI);

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

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

  /* Configure all GPIO as analog to reduce current consumption on non used IOs */
  /* Enable GPIOs clock */
  RCC_AHBPeriphClockCmd(RCC_AHBPeriph_GPIOA | RCC_AHBPeriph_GPIOB | RCC_AHBPeriph_GPIOC |
                        RCC_AHBPeriph_GPIOD | RCC_AHBPeriph_GPIOE | RCC_AHBPeriph_GPIOH , ENABLE);

  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AN;
  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_40MHz;
  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_All;
  GPIO_Init(GPIOC, &GPIO_InitStructure);
  GPIO_Init(GPIOD, &GPIO_InitStructure);
  GPIO_Init(GPIOE, &GPIO_InitStructure);
  GPIO_Init(GPIOH, &GPIO_InitStructure);
  GPIO_Init(GPIOA, &GPIO_InitStructure); 
  GPIO_Init(GPIOB, &GPIO_InitStructure);

  /* Disable GPIOs clock */
  RCC_AHBPeriphClockCmd(RCC_AHBPeriph_GPIOA | RCC_AHBPeriph_GPIOB | RCC_AHBPeriph_GPIOC |
                        RCC_AHBPeriph_GPIOD | RCC_AHBPeriph_GPIOE | RCC_AHBPeriph_GPIOH , DISABLE);


    
  /* 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);
  

 /* Enable Ultra low power mode */
  PWR_UltraLowPowerCmd(ENABLE);


  /* RTC Wakeup Interrupt Generation: Clock Source: RTCCLK_Div16, Wakeup Time Base: ~4s 
     RTC Clock Source LSI ~37KHz  
  */
  RTC_WakeUpClockConfig(RTC_WakeUpClock_RTCCLK_Div16);
  RTC_SetWakeUpCounter(0x2421);

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


  /* Enable Wakeup Counter */
  RTC_WakeUpCmd(ENABLE);
    
  /* Enter Stop Mode */
  PWR_EnterSTOPMode(PWR_Regulator_LowPower, PWR_STOPEntry_WFI);