Is there a way to maintain RTC ticking without any shifts while performing a power reset?

> Is there a way to maintain RTC ticking without any shifts while performing a power reset?

Yes, don't call the Cube/HAL init routine.

https://community.st.com/s/question/0D50X00009XkgBWSAZ/stm32-rtc-loses-one-second-after-each-reset and maybe a dozen of other threads here.

JW

Here's my code 

void MX_RTC_Init(void){
 
 RCC_PeriphCLKInitTypeDef PeriphClkInitStruct;
 RCC_OscInitTypeDef RCC_OscInitStruct;
 
 PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_RTC;
 PeriphClkInitStruct.RTCClockSelection = RCC_RTCCLKSOURCE_LSE;
 HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct);
 
 hrtc.Instance = RTC;
 hrtc.Init.HourFormat = RTC_HOURFORMAT_24;
 hrtc.Init.AsynchPrediv = RTC_ASYNCH_DIV - 1;
 hrtc.Init.SynchPrediv = RTC_SYNCH_DIV - 1;
 hrtc.Init.OutPut = RTC_OUTPUT_DISABLE;
 
 if(HAL_RTCEx_BKUPRead(&hrtc, RTC_BKP_DR0) != RTC_SECRET_DATA){/*if RTC 
 was not previously iinitialized*/
 RCC_OscInitStruct.LSEState = RCC_LSE_ON;
 RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_LSE;
 HAL_RCC_OscConfig(&RCC_OscInitStruct);
 HAL_RTC_Init(&hrtc) ;
 rtc_date_time_config_frst_time();
 }else{/*if RTC was initialized, we only need to call HAL_RTC_MspInit*/
 HAL_RTC_MspInit(&hrtc);
 }
 rtc_alarm_a_set_it();
}
 
void HAL_RTC_MspInit(RTC_HandleTypeDef* hrtc)
{
 __HAL_RCC_RTC_ENABLE();
 HAL_NVIC_SetPriority(RTC_Alarm_IRQn, 1, RTC_ALARM_IRQ_PRIO);
 HAL_NVIC_EnableIRQ(RTC_Alarm_IRQn);
}
 
void rtc_alarm_a_set_it(void){
 RTC_AlarmTypeDef sAlarm;
 /**Enable the Alarm A 
 */
 sAlarm.AlarmTime.Hours = 0;
 sAlarm.AlarmTime.Minutes = 0;
 sAlarm.AlarmTime.Seconds = 0;
 sAlarm.AlarmTime.SubSeconds = 0;
 sAlarm.AlarmTime.TimeFormat = RTC_HOURFORMAT_24;
 sAlarm.AlarmTime.DayLightSaving = RTC_DAYLIGHTSAVING_NONE;
 sAlarm.AlarmTime.StoreOperation = RTC_STOREOPERATION_RESET;
 sAlarm.AlarmMask = RTC_ALARMMASK_ALL;
 sAlarm.AlarmSubSecondMask = RTC_ALARMSUBSECONDMASK_None;
 sAlarm.AlarmDateWeekDaySel = RTC_ALARMDATEWEEKDAYSEL_DATE;
 sAlarm.AlarmDateWeekDay = 1;
 sAlarm.Alarm = RTC_ALARM_A;
 HAL_RTC_SetAlarm_IT(&hrtc, &sAlarm, FORMAT_BIN);
}

CubeMX puts the RTC initialization there for every reset.

You need to make the change every time you generate code with CubeMX.

But if you have the backup battery, and you edit the code CubeMX generates, the RTC is reset only when you program the backup domain reset.

> if(HAL_RTCEx_BKUPRead(&hrtc, RTC_BKP_DR0) != RTC_SECRET_DATA){/*if RTC

So, did you verify that this expression evaluates always false?

JW

Also I found out that after power reset "RTC_PRER" register (the one that contains prescaler values) resets to its default value. That event takes place before executing anything in "main()". I placed a loop at the begining to ckeck that. This loop can be skipped only in debug mode, so when I enter debug mode after performing a power reset I know that nothing was previously executed in "main()".

The startup code (start.S). You can also put a breakpoint there. You don't have to wait for main.

 Usually, though, there is just the reset vector entry that has the address of _start, and the code under that label only branches to main.

The rtc_date_time_config_frst_time() puts the RTC_SECRET_DATA to the backup register 0?

That's the way I've been doing too.

MCU starts in a Reset_Handler, which in my case is this:

THUMB
 PUBWEAK Reset_Handler
 SECTION .text:CODE:REORDER:NOROOT(2)
Reset_Handler
 
 LDR R0, =SystemInit
 BLX R0
 LDR R0, =__iar_program_start
 BX R0

"SystemItinit" contains this code:

void SystemInit(void)
{
 /* FPU settings ------------------------------------------------------------*/
 #if (__FPU_PRESENT == 1) && (__FPU_USED == 1)
 SCB->CPACR |= ((3UL << 10*2)|(3UL << 11*2)); /* set CP10 and CP11 Full Access */
 #endif
 /* Reset the RCC clock configuration to the default reset state ------------*/
 /* Set HSION bit */
 RCC->CR |= (uint32_t)0x00000001;
 
 /* Reset CFGR register */
 RCC->CFGR = 0x00000000;
 
 /* Reset HSEON, CSSON and PLLON bits */
 RCC->CR &= (uint32_t)0xFEF6FFFF;
 
 /* Reset PLLCFGR register */
 RCC->PLLCFGR = 0x24003010;
 
 /* Reset HSEBYP bit */
 RCC->CR &= (uint32_t)0xFFFBFFFF;
 
 /* Disable all interrupts */
 RCC->CIR = 0x00000000;
 
 /* Configure the Vector Table location add offset address ------------------*/
 SCB->VTOR = FLASH_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal FLASH */
}

As you see, nothing here appeals to RTC or LSE.

Oh, I found the problem.

I began working on this project when a bunch of coding had allready been done, so it was not me who made the start up routine.

There is a "Start up" project that is downloaded at 0 flash address and it jumps to another address where another project (the one I'm working in) is located.

It turned out that there is a useless RTC initialization (RTC is not being used there).

Nevertheless it was quite silly of me not to think about that possibility.

@turboscrew​ and @Community member​ really appreciate your help!