LSERDY is not ready when boot after power off in VBAT mode.

I had use VBat with external low frequency OSC for RTC which it's can run fine even reset has occurred.

But it's has problem when it was boot from power off with VBat.

it's going to TimeOut error while checking LSE state in RCC initial.

/* Check the LSE State */
 if (RCC_OscInitStruct->LSEState != RCC_LSE_OFF)
 {
 /* Get Start Tick*/
 tickstart = HAL_GetTick();
 
 /* Wait till LSE is ready */
 while (READ_BIT(RCC->BDCR, RCC_BDCR_LSERDY) == 0U)
 {
 if ((HAL_GetTick() - tickstart) > RCC_LSE_TIMEOUT_VALUE)
 {
 return HAL_TIMEOUT;
 }
 }
 }

the RCC_OscInitStruct->LSEState is set so it go to waiting RCC_BDCR_LSERDY was set but RCC_BDCR_LSERDY is not set even I try to extent RCC_LSE_TIMEOUT_VALUE, it's not matter.

only way I can do is power off it and remove a battery for a while.

And add a battery again then I take power on for it.

the system clock will go fine again.

I have no idea why LSE clock boot fail in VBat Mode but It boot fine without VBat Mode

this is my RCC config which it has nothing special.

the Error is in HAL_RCC_OscConfig(&RCC_OscInitStruct), that 's too early.

/**
 * @brief System Clock Configuration
 * @retval None
 */
void SystemClock_Config(void)
{
 RCC_OscInitTypeDef RCC_OscInitStruct = {0};
 RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};
 RCC_CRSInitTypeDef pInit = {0};
 
 /** Configure the main internal regulator output voltage
 */
 HAL_PWREx_ControlVoltageScaling(PWR_REGULATOR_VOLTAGE_SCALE1);
 
 /** Configure LSE Drive Capability
 */
 HAL_PWR_EnableBkUpAccess();
 __HAL_RCC_LSEDRIVE_CONFIG(RCC_LSEDRIVE_HIGH);
 
 /** Initializes the RCC Oscillators according to the specified parameters
 * in the RCC_OscInitTypeDef structure.
 */
 RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI|RCC_OSCILLATORTYPE_LSE
 |RCC_OSCILLATORTYPE_HSI48;
 RCC_OscInitStruct.LSEState = RCC_LSE_ON;
 RCC_OscInitStruct.HSIState = RCC_HSI_ON;
 RCC_OscInitStruct.HSI48State = RCC_HSI48_ON;
 RCC_OscInitStruct.HSIDiv = RCC_HSI_DIV1;
 RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
 RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
 RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSI;
 RCC_OscInitStruct.PLL.PLLM = RCC_PLLM_DIV1;
 RCC_OscInitStruct.PLL.PLLN = 8;
 RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV2;
 RCC_OscInitStruct.PLL.PLLQ = RCC_PLLQ_DIV2;
 RCC_OscInitStruct.PLL.PLLR = RCC_PLLR_DIV2;
 if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
 {
 Error_Handler();
 }
 
 /** Initializes the CPU, AHB and APB buses clocks
 */
 RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
 |RCC_CLOCKTYPE_PCLK1;
 RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
 RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
 RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;
 
 if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_2) != HAL_OK)
 {
 Error_Handler();
 }
 
 /** Enables the Clock Security System
 */
 HAL_RCC_EnableLSECSS();
 
 /** Enable the SYSCFG APB clock
 */
 __HAL_RCC_CRS_CLK_ENABLE();
 
 /** Configures CRS
 */
 pInit.Prescaler = RCC_CRS_SYNC_DIV1;
 pInit.Source = RCC_CRS_SYNC_SOURCE_LSE;
 pInit.Polarity = RCC_CRS_SYNC_POLARITY_RISING;
 pInit.ReloadValue = __HAL_RCC_CRS_RELOADVALUE_CALCULATE(48000000,32768);
 pInit.ErrorLimitValue = 34;
 pInit.HSI48CalibrationValue = 32;
 
 HAL_RCCEx_CRSConfig(&pInit);
}

Thank You