RTC too fast

[imperator-maximus]

Hi, I tried to implement RTC (LSI) but I am always getting too fast speed. So the result of a second=LL_RTC_TIME_GetSecond(RTC); is always too fast - around 20% which is out of any specs. I tried:

• using existing SystemClock_Config
• SystemClock_Config from CubeMX in LL or HAL mode LL version is pretty much the same - just different freq
• played around with different freq values (default in firmware: LL_Init1msTick(48000000); LL_SetSystemCoreClock(48000000); )

It does not matter how much I change anything - speed is always the same.

my MX_RTC_Init btw:

void MX_RTC_Init(void)
{

  /* USER CODE BEGIN RTC_Init 0 */

  /* USER CODE END RTC_Init 0 */

  LL_RTC_InitTypeDef RTC_InitStruct = {0};

  /* Peripheral clock enable */
  LL_RCC_EnableRTC();

  /* USER CODE BEGIN RTC_Init 1 */

  /* USER CODE END RTC_Init 1 */
  /** Initialize RTC and set the Time and Date
  */
  RTC_InitStruct.HourFormat = LL_RTC_HOURFORMAT_24HOUR;
  RTC_InitStruct.AsynchPrescaler = 127;
  RTC_InitStruct.SynchPrescaler = 255;
  LL_RTC_Init(RTC, &RTC_InitStruct);
  LL_RTC_SetAsynchPrescaler(RTC, 127);
  LL_RTC_SetSynchPrescaler(RTC, 255);
  /* USER CODE BEGIN RTC_Init 2 */
  LL_RTC_DisableWriteProtection(RTC);
  LL_RTC_TIME_SetHour(RTC,__LL_RTC_CONVERT_BIN2BCD (18));
    LL_RTC_TIME_SetMinute(RTC,__LL_RTC_CONVERT_BIN2BCD(30));
    LL_RTC_TIME_SetSecond(RTC,__LL_RTC_CONVERT_BIN2BCD (0));
  LL_RTC_EnableWriteProtection(RTC);
  /* USER CODE END RTC_Init 2 */
}

Thank you!

[imperator-maximus]

solution: 125-1, 320-1 as prescaler values