RTC time greater than 24 hours

[AlfRomeo]

Hello everyone, I am using an internal RTC on STM32L431. After running for 23:59:59 seconds, the time changes to 24:00:00 instead of 00:00:00, and the week day has not been incremented. Why is this?

[Sarra.S]

Hello @AlfRomeo, 

Is the RTC correctly configured to use the 24-hour format? 

[AlfRomeo]

The format of the hour is still 24 hours, and I haven't changed it

[Andrew Neil]

---

@AlfRomeo wrote:

The format of the hour is still 24 hours, and I haven't changed it

---

Did you follow @waclawek.jan's link - to details about the issues with uninitialised or incompletely-initialised config structures?

[waclawek.jan]

As @Sarra.S said above, you probably have set the RTC inadvertently to 12 hour format, probably because of uninitialized struct when calling Cube/HAL functions setting time.

JW

[Uwe Bonnes]

Does the RTC continue with 24:01 ? Otherwise, where is the problem?

[Andrew Neil]

---

@Uwe Bonnes wrote:

Otherwise, where is the problem?

---

24:00 is never a valid time - it should go from 23:59:59.999 to 00:00

[AlfRomeo]

I don't know why, but after resetting after a power outage, it improved again. The code did not make any changes, except for performing modulo operations after hour, minute, and second when obtaining the clock. Nothing else has changed. At first, after changing this setting, the problem still persisted and there was no power outage. However, after the power outage, the problem disappeared and I felt very confused

[Gaurangthaker]

We are using the STM32U5 MCU and its internal RTC in our project.
We are facing an issue where the RTC time sometimes goes beyond 24 hours — for example, we observed a timestamp showing 37 hours.

The RTC is configured in 24-hour format, and we have initialized the RTC and its time format correctly as per the reference manual and HAL documentation.

The main challenge is that this does not happen consistently. We see such out-of-range values only randomly, which makes it difficult to identify the root cause.

Has anyone faced a similar issue with the STM32U5 RTC? Could you please suggest possible causes or solutions, or any checks we should perform to debug this behavior?

[AlfRomeo]

Because it's been so long, I've forgotten how I set it up back then. The problem seems to stem from the difference in settings between the `RTC_FORMAT_BIN` and `RTC_FORMAT_BCD` parameters, which led to different results. Also, the modulo operation is very important. I'm providing the code for your reference; the code might be incorrect, but it's for informational purposes only.

void MX_RTC_Init(void)
{

 /* USER CODE BEGIN RTC_Init 0 */

 /* USER CODE END RTC_Init 0 */

 RTC_TimeTypeDef sTime = {0};
 RTC_DateTypeDef sDate = {0};
 RTC_AlarmTypeDef sAlarm = {0};

 /* USER CODE BEGIN RTC_Init 1 */

 /* USER CODE END RTC_Init 1 */

 /** Initialize RTC Only
 */
 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.OutPutRemap = RTC_OUTPUT_REMAP_NONE;
 hrtc.Init.OutPutPolarity = RTC_OUTPUT_POLARITY_HIGH;
 hrtc.Init.OutPutType = RTC_OUTPUT_TYPE_OPENDRAIN;
 if (HAL_RTC_Init(&hrtc) != HAL_OK)
 {
 Error_Handler();
 }

 /* USER CODE BEGIN Check_RTC_BKUP */
 
 /* USER CODE END Check_RTC_BKUP */

 /** Initialize RTC and set the Time and Date
 */
 sTime.Hours = 8;
 sTime.Minutes = 5;
 sTime.Seconds = 0;
 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_MONDAY;
 sDate.Month = RTC_MONTH_AUGUST;
 sDate.Date = 2;
 sDate.Year = 24;

 if (HAL_RTC_SetDate(&hrtc, &sDate, RTC_FORMAT_BIN) != HAL_OK)
 {
 Error_Handler();
 }

 /** Enable the Alarm A
 */
 sAlarm.AlarmTime.Hours = 8;
 sAlarm.AlarmTime.Minutes = 5;
 sAlarm.AlarmTime.Seconds = 1;
 sAlarm.AlarmTime.SubSeconds = 0;
 sAlarm.AlarmTime.DayLightSaving = RTC_DAYLIGHTSAVING_NONE;
 sAlarm.AlarmTime.StoreOperation = RTC_STOREOPERATION_RESET;
 sAlarm.AlarmMask = RTC_ALARMMASK_DATEWEEKDAY|RTC_ALARMMASK_HOURS
 |RTC_ALARMMASK_MINUTES;
 sAlarm.AlarmSubSecondMask = RTC_ALARMSUBSECONDMASK_ALL;
 sAlarm.AlarmDateWeekDaySel = RTC_ALARMDATEWEEKDAYSEL_DATE;
 sAlarm.AlarmDateWeekDay = 1;
 sAlarm.Alarm = RTC_ALARM_A;
 if (HAL_RTC_SetAlarm_IT(&hrtc, &sAlarm, RTC_FORMAT_BIN) != HAL_OK)
 {
 Error_Handler();
 }
 /* USER CODE BEGIN RTC_Init 2 */
 __HAL_RTC_WAKEUPTIMER_CLEAR_FLAG(&hrtc, RTC_FLAG_WUTF);
 if (HAL_RTCEx_SetWakeUpTimer_IT(&hrtc, 0, RTC_WAKEUPCLOCK_CK_SPRE_16BITS) != HAL_OK)
 {
 Error_Handler();
 }
 myRun.clock.sdatestructureget = sDate;
 myRun.clock.stimestructureget = sTime;
 /* USER CODE END RTC_Init 2 */

}

void getTimeLocal(void)
{
 HAL_RTC_GetTime(&hrtc, &GetTime, RTC_FORMAT_BIN);
 HAL_RTC_GetDate(&hrtc, &GetData, RTC_FORMAT_BIN);
}

static void dispCalender(RTC_DateTypeDef GetData, RTC_TimeTypeDef GetTime)
{
 getTimeLocal();
 // Displayed on the LCD screen.
 _intDisplayFix((int)(GetData.Year % 100), 17, 2, false); // year
 PutCharDot(18);
 _intDisplayFix(GetData.Month, 19, 2, false); // month
 _intDisplayFix(GetData.Date, 21, 2, false); // day
 _intDisplayFix(GetTime.Hours, 25, 2, false); // hour
 PutCharDot(26);
 _intDisplayFix(GetTime.Minutes, 27, 2, false); // min
 _intDisplayFix(GetTime.Seconds, 3, 2, false); // sec
}

void rtc_getCalendarTime(RTC_C_Calendar *rtc)
{
 RTC_DateTypeDef LocalData;
 RTC_TimeTypeDef LocalTime;
 HAL_RTC_GetTime(&hrtc, &LocalTime, RTC_FORMAT_BIN);
 HAL_RTC_GetDate(&hrtc, &LocalData, RTC_FORMAT_BIN);
 rtc->stimestructureget.Seconds = LocalTime.Seconds % 60;
 rtc->stimestructureget.Minutes = LocalTime.Minutes % 60;
 rtc->stimestructureget.Hours = LocalTime.Hours % 24;
 rtc->sdatestructureget.Date = LocalData.Date;
 rtc->sdatestructureget.Month = LocalData.Month;
 rtc->sdatestructureget.Year = LocalData.Year;
 rtc->sdatestructureget.WeekDay = LocalData.WeekDay;

 return;
}
/* RTC second interrupt */
void HAL_RTCEx_WakeUpTimerEventCallback(RTC_HandleTypeDef *hrtc)
{
 static int sec_cnt = 0;
 if (hrtc->State == HAL_RTC_STATE_READY)
 rtc_getCalendarTime(&myRun.clock);
 triggerLcdUpdate(1); // Trigger LCD refresh
}

My program's function is to display the current time on the LCD screen in real time every second, and the `dispCalender()` function should be called once every second. For your reference only!