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STM32 RTC loses one second after each reset

Question asked by Rob on May 23, 2017
Latest reply on Mar 16, 2018 by Imen D

I have a custom board based on the STM32F407 device. The board is running well except for one nagging detail: the RTC loses one second each time I power the board on/off


My device uses a low frequency external crystal and a backup coincell battery. The crystal is the CM7V-T1A 32.768kHz from Micro Crystal and it runs fine and keeps accurate time from either the coincell or main battery power. But again, the RTC looses approximately 1 second every time the board is powered on from the main external batteries. If the user powers the board on and off ten times (while keeping the backup coincell connected of course), then approximately 10 seconds of time is lost on the RTC.


I see many postings on this forum of people reporting loss of time when coming out of sleep modes. However my problem happens irregardless of whether the processor ever enters sleep mode. I do check the Wakeup Flag and the Standby Flag on power up though. Below is the first lines of code from my main() function:


int main(void)
/* Configure the system clock */

/* Initialize all configured peripherals */




/* Check the status of standby flag SB*/

      /*Resuming from standby routine */


And here is the code that initializes the RTC:


void RTC_Init(void)
   RTC_TimeTypeDef sTime;
   RTC_DateTypeDef sDate;

   /**Initialize RTC and set the Time and Date
   hrtc.Instance = RTC;
   hrtc.Init.HourFormat = RTC_HOURFORMAT_24;
   hrtc.Init.AsynchPrediv = 127;
   hrtc.Init.SynchPrediv = 255;
   hrtc.Init.OutPut = RTC_OUTPUT_DISABLE;
   hrtc.Init.OutPutPolarity = RTC_OUTPUT_POLARITY_HIGH;
   hrtc.Init.OutPutType = RTC_OUTPUT_TYPE_OPENDRAIN;

   /* Read the Back Up Register 0 Data */
   if(HAL_RTCEx_BKUPRead(&hrtc, RTC_BKP_DR0) != 0x32F2)
      /* Configure RTC Calendar */
      sTime.Hours = 4;
      sTime.Minutes = 0;
      sTime.Seconds = 0;
      sTime.SubSeconds = 0;
      sTime.TimeFormat = RTC_HOURFORMAT12_AM;
      sTime.DayLightSaving = RTC_DAYLIGHTSAVING_NONE;
      sTime.StoreOperation = RTC_STOREOPERATION_RESET;
      HAL_RTC_SetTime(&hrtc, &sTime, FORMAT_BCD);

      sDate.WeekDay = RTC_WEEKDAY_MONDAY;
      sDate.Month = RTC_MONTH_JANUARY;
      sDate.Date = 1;
      sDate.Year = 0;

      HAL_RTC_SetDate(&hrtc, &sDate, FORMAT_BCD);

      /*##-3- Writes a data in a RTC Backup data Register0 #####################*/