STM32 - RTC time, date and Backup register BKPxR aren't saved after Power off

[freecoder]

Hi,

We're faced to an issue in the RTC module. RTC registers: time, date and backup register aren't saved after power off ( we measured the VBAT supply is about 3v given by the supercapa), we used a magic number to ensure that data aren't reinitialized at each software startup:

/** Initialize RTC Only
  */
  hrtc.Instance = RTC;
  hrtc.Init.HourFormat = RTC_HOURFORMAT_24;
  hrtc.Init.AsynchPrediv = CFG_RTC_ASYNCH_PRESCALER;
  hrtc.Init.SynchPrediv = CFG_RTC_SYNCH_PRESCALER;
  hrtc.Init.OutPut = RTC_OUTPUT_DISABLE;
  hrtc.Init.OutPutPolarity = RTC_OUTPUT_POLARITY_HIGH;
  hrtc.Init.OutPutRemap = RTC_OUTPUT_REMAP_NONE;
  if (HAL_RTC_Init(&hrtc) != HAL_OK)
  {
    Error_Handler();
  }

  /* USER CODE BEGIN Check_RTC_BKUP */
   if(HAL_RTCEx_BKUPRead(&hrtc, RTC_BKP_DR0) == 0x55AA)
   {
      // Time and Date are OK
      return;
   }
  /* USER CODE END Check_RTC_BKUP */

  /** Initialize RTC and set the Time and Date
  */
  sTime.Hours = 18;
  sTime.Minutes = 4;
  sTime.Seconds = 0;
  sTime.SubSeconds = 0x0;
  sTime.DayLightSaving = RTC_DAYLIGHTSAVING_NONE;
  sTime.StoreOperation = RTC_STOREOPERATION_RESET;
  if (HAL_RTC_SetTime(&hrtc, &sTime, RTC_FORMAT_BIN) != HAL_OK)
  {
    Error_Handler();
  }
  sDate.WeekDay = RTC_WEEKDAY_THURSDAY;
  sDate.Month = RTC_MONTH_JUNE;
  sDate.Date = 20;
  sDate.Year = 24;

  if (HAL_RTC_SetDate(&hrtc, &sDate, RTC_FORMAT_BIN) != HAL_OK)
  {
    Error_Handler();
  }
  /* USER CODE BEGIN RTC_Init 2 */

  HAL_RTCEx_BKUPWrite(&hrtc, RTC_BKP_DR0, 0x55AA);

Can you please give us some suggestions.

Thank you by advance,

Best regards,

[Sarra.S · Accepted solution]

Hello @freecoder, 

This is probably a hardware issue...

If VBAT drops below 1.55V, the backup domain won't function correctly, so the RTC register content will be lost;  are you sure the Vbat level is stable? can you share how VDDA/VREF+ is exactly connected?

[freecoder]

Hi @Sarra.S ,

Thank you for your response.

Best regards,