RTC resets on power-cycle despite battery

> Could it be that the standard MX_RTC_Init(); function generated by CubeMX also resets the RTC time and date?

Maybe. What's in it?

> If so, how would I fix that?

Don't use it.

JW

Well, it's basically this:

 if(hrtc->State == HAL_RTC_STATE_RESET)
 {
 /* Allocate lock resource and initialize it */
 hrtc->Lock = HAL_UNLOCKED;
 
 /* Initialize RTC MSP */
 HAL_RTC_MspInit(hrtc);
 }
 
 /* Set RTC state */
 hrtc->State = HAL_RTC_STATE_BUSY;
 
 /* Disable the write protection for RTC registers */
 __HAL_RTC_WRITEPROTECTION_DISABLE(hrtc);
 
 /* Set Initialization mode */
 if(RTC_EnterInitMode(hrtc) != HAL_OK)
 {
 /* Enable the write protection for RTC registers */
 __HAL_RTC_WRITEPROTECTION_ENABLE(hrtc);
 
 /* Set RTC state */
 hrtc->State = HAL_RTC_STATE_ERROR;
 
 return HAL_ERROR;
 }
 else
 {
 /* Clear RTC_CR FMT, OSEL and POL Bits */
 hrtc->Instance->CR &= ((uint32_t)~(RTC_CR_FMT | RTC_CR_OSEL | RTC_CR_POL));
 /* Set RTC_CR register */
 hrtc->Instance->CR |= (uint32_t)(hrtc->Init.HourFormat | hrtc->Init.OutPut | hrtc->Init.OutPutPolarity);
 
 /* Configure the RTC PRER */
 hrtc->Instance->PRER = (uint32_t)(hrtc->Init.SynchPrediv);
 hrtc->Instance->PRER |= (uint32_t)(hrtc->Init.AsynchPrediv << 16);
 
 /* Exit Initialization mode */
 hrtc->Instance->ISR &= (uint32_t)~RTC_ISR_INIT;
 
 hrtc->Instance->OR &= (uint32_t)~RTC_OR_ALARMTYPE;
 hrtc->Instance->OR |= (uint32_t)(hrtc->Init.OutPutType);
 
 /* Enable the write protection for RTC registers */
 __HAL_RTC_WRITEPROTECTION_ENABLE(hrtc);
 
 /* Set RTC state */
 hrtc->State = HAL_RTC_STATE_READY;
 
 return HAL_OK;
 }

I'd guess that HAL_RTC_MspInit(hrtc); needs to be called at least once, but maybe RTC_EnterInitMode() shouldn't? But it seems it's called everytime.

typedef enum
{
 HAL_RTC_STATE_RESET = 0x00U, /*!< RTC not yet initialized or disabled */
 HAL_RTC_STATE_READY = 0x01U, /*!< RTC initialized and ready for use */
 HAL_RTC_STATE_BUSY = 0x02U, /*!< RTC process is ongoing */
 HAL_RTC_STATE_TIMEOUT = 0x03U, /*!< RTC timeout state */
 HAL_RTC_STATE_ERROR = 0x04U /*!< RTC error state */
} HAL_RTCStateTypeDef;

You'd think that hrtc->State = HAL_RTC_STATE_BUSY survives power off, but maybe it doesn't? The init code looks like it does handle power downs with batteries.

Alright, I ended up editing MX_RTC_Init():

void MX_RTC_Init(void)
{
 
 /** 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.OutPutPolarity = RTC_OUTPUT_POLARITY_HIGH;
 hrtc.Init.OutPutType = RTC_OUTPUT_TYPE_OPENDRAIN;
 
// if (HAL_RTC_Init(&hrtc) != HAL_OK)
// {
// Error_Handler();
// }
 
 hrtc.Lock = HAL_UNLOCKED;
 HAL_RTC_MspInit(&hrtc);
}

Not sure if that HAL_UNLOCKED is necessary, but it was in the HAL_RTC_Init() function. And now it works! (Strange, through, that CubeMX wouldn't handle this ...)

https://community.st.com/s/question/0D53W00000EawLOSAZ/rtc-calendar-resets-in-cubemx-generated-init-function

I don't Cube.

JW

Wait, I don't relate to the linked post. My issue is RTC reset on power cycle, but NOT on soft reset. Also, my MX_RTC_Init() definitely doesn't contain code to reset the RTC. As shown above, the only function called is HAL_RTC_Init (which calls RTC_EnterInitMode()), whose implementation merely sets some registers and state.

HAL_StatusTypeDef RTC_EnterInitMode(RTC_HandleTypeDef* hrtc)
{
 uint32_t tickstart = 0;
 
 /* Check if the Initialization mode is set */
 if((hrtc->Instance->ISR & RTC_ISR_INITF) == (uint32_t)RESET)
 {
 /* Set the Initialization mode */
 hrtc->Instance->ISR = (uint32_t)RTC_INIT_MASK;
 
 /* Get tick */
 tickstart = HAL_GetTick();
 
 /* Wait till RTC is in INIT state and if Time out is reached exit */
 while((hrtc->Instance->ISR & RTC_ISR_INITF) == (uint32_t)RESET)
 {
 if((HAL_GetTick() - tickstart ) > RTC_TIMEOUT_VALUE)
 {
 return HAL_TIMEOUT;
 }
 }
 }
 
 return HAL_OK;
}

Still, commenting out this function solved my reset problems on power cycle. How does this actually work? MX_RTC_Init() sets all bits in ISR to one, and waits until the INITF flag is set. But where does the actual RTC reset happen?

Again, you'd want a reset if the battery is missing, and not if it is there.

> My issue is RTC reset on power cycle

How do you know? What are exactly the symptoms? How do you set up the date/time registers, and how do you check them after the reset, exactly?

Calling HAL_RTC_Init () unconditionally is a long-existing known flaw, as entering the INIT mode clears the subsecond counter; search here for "in average half second lost upon each reset".

JW