RTC Initialization Fails Using External LSE on STM32F407G-DISC1

[Noushadalik]

We are testing the STM32F407G-DISC1 development board and trying to use an external 32.768 kHz LSE oscillator for RTC clock source. Since the onboard crystal footprint was empty, we manually soldered the following components on the LSE pins (PC14 and PC15):

• Crystal oscillator: ABS25-32.768KHZ-4-T from Abracon
  (Product link: https://robu.in/product/abs25-32-768khz-4-t-abracon/#tab-specification)

• Load capacitors: CC0603CRNPO9BN6R8 (Recommended value per crystal datasheet)

Despite carefully soldering the components and verifying the connections, RTC initialization (HAL_RTC_Init) always fails with error code 1 (HAL_ERROR).

I have attached my main.c source code here for your reference

/* USER CODE BEGIN Header */
/**
  ******************************************************************************
  * @file           : main.c
  * @brief          : Main program body
  ******************************************************************************
  * @attention
  *
  * Copyright (c) 2025 STMicroelectronics.
  * All rights reserved.
  *
  * This software is licensed under terms that can be found in the LICENSE file
  * in the root directory of this software component.
  * If no LICENSE file comes with it, it is provided AS-IS.
  *
  ******************************************************************************
  */
/* USER CODE END Header */
/* Includes ------------------------------------------------------------------*/
#include "main.h"
#include "usb_host.h"
#include "stdio.h"

/* Private includes ----------------------------------------------------------*/
/* USER CODE BEGIN Includes */

/* USER CODE END Includes */

/* Private typedef -----------------------------------------------------------*/
/* USER CODE BEGIN PTD */

/* USER CODE END PTD */

/* Private define ------------------------------------------------------------*/
/* USER CODE BEGIN PD */

/* USER CODE END PD */

/* Private macro -------------------------------------------------------------*/
/* USER CODE BEGIN PM */

/* USER CODE END PM */

/* Private variables ---------------------------------------------------------*/
I2C_HandleTypeDef hi2c1;
I2S_HandleTypeDef hi2s3;
RTC_HandleTypeDef hrtc;
SPI_HandleTypeDef hspi1;
UART_HandleTypeDef huart2;

/* USER CODE BEGIN PV */

/* USER CODE END PV */

/* Private function prototypes -----------------------------------------------*/
void SystemClock_Config(void);
static void MX_GPIO_Init(void);
static void MX_I2C1_Init(void);
static void MX_I2S3_Init(void);
static void MX_SPI1_Init(void);
static void MX_RTC_Init(void);
static void MX_USART2_UART_Init(void);
void MX_USB_HOST_Process(void);

/* USER CODE BEGIN PFP */

/* USER CODE END PFP */

/* Private user code ---------------------------------------------------------*/
/* USER CODE BEGIN 0 */
/* Retarget printf to UART ---------------------------------------------------*/
int __io_putchar(int ch)
{
    HAL_UART_Transmit(&huart2, (uint8_t *)&ch, 1, HAL_MAX_DELAY);
    return ch;
}
/* USER CODE END 0 */

/**
  * @brief  The application entry point.
  * @retval int
  */
int main(void)
{
  /* USER CODE BEGIN 1 */

  /* USER CODE END 1 */

  /* MCU Configuration--------------------------------------------------------*/

  HAL_Init();

  SystemClock_Config();

  MX_GPIO_Init();
  MX_I2C1_Init();
  MX_I2S3_Init();
  MX_SPI1_Init();
  MX_USB_HOST_Init();
  MX_USART2_UART_Init();
  MX_RTC_Init();

  /* USER CODE BEGIN 2 */
  RTC_TimeTypeDef sTime;
  RTC_DateTypeDef sDate;
  /* USER CODE END 2 */

  /* Infinite loop */
  while (1)
  {
    HAL_RTC_GetTime(&hrtc, &sTime, RTC_FORMAT_BIN);
    HAL_RTC_GetDate(&hrtc, &sDate, RTC_FORMAT_BIN);

    printf("Time: %02d:%02d:%02d  Date: %02d-%02d-20%02d\r\n",
           sTime.Hours, sTime.Minutes, sTime.Seconds,
           sDate.Date, sDate.Month, sDate.Year);
    HAL_Delay(1000);

    MX_USB_HOST_Process();
  }
}

/**
  * @brief System Clock Configuration
  * @retval None
  */
void SystemClock_Config(void)
{
  RCC_OscInitTypeDef RCC_OscInitStruct = {0};
  RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};

  __HAL_RCC_PWR_CLK_ENABLE();
  __HAL_PWR_VOLTAGESCALING_CONFIG(PWR_REGULATOR_VOLTAGE_SCALE1);

  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSE|RCC_OSCILLATORTYPE_LSE;
  RCC_OscInitStruct.HSEState = RCC_HSE_ON;
  RCC_OscInitStruct.LSEState = RCC_LSE_ON;
  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
  RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSE;
  RCC_OscInitStruct.PLL.PLLM = 8;
  RCC_OscInitStruct.PLL.PLLN = 336;
  RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV2;
  RCC_OscInitStruct.PLL.PLLQ = 7;
  if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
  {
      printf("ERROR: Oscillator config failed!\r\n");
      Error_Handler();
  }

  while (__HAL_RCC_GET_FLAG(RCC_FLAG_LSERDY) == RESET)
  {
      HAL_Delay(1);
  }
  printf("LSE is running.\r\n");

  RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
                               |RCC_CLOCKTYPE_PCLK1|RCC_CLOCKTYPE_PCLK2;
  RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
  RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
  RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV4;
  RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV2;

  if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_5) != HAL_OK)
  {
      printf("ERROR: Clock config failed!\r\n");
      Error_Handler();
  }

  RCC_PeriphCLKInitTypeDef PeriphClkInitStruct = {0};
  PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_RTC;
  PeriphClkInitStruct.RTCClockSelection = RCC_RTCCLKSOURCE_LSE;
  if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct) != HAL_OK)
  {
      printf("ERROR: Failed to set RTC clock source to LSE!\r\n");
      Error_Handler();
  }
}

/**
  * @brief RTC Initialization Function
  * @retval None
  */
static void MX_RTC_Init(void)
{
    RTC_TimeTypeDef sTime = {0};
    RTC_DateTypeDef sDate = {0};

    __HAL_RCC_PWR_CLK_ENABLE();
    HAL_PWR_EnableBkUpAccess();

    hrtc.Instance = RTC;
    hrtc.Init.HourFormat = RTC_HOURFORMAT_24;
    hrtc.Init.AsynchPrediv = 127;
    hrtc.Init.SynchPrediv = 255; // 32768 Hz / (127+1) / (255+1) = 1 Hz
    hrtc.Init.OutPut = RTC_OUTPUT_DISABLE;
    hrtc.Init.OutPutPolarity = RTC_OUTPUT_POLARITY_HIGH;
    hrtc.Init.OutPutType = RTC_OUTPUT_TYPE_OPENDRAIN;

    HAL_StatusTypeDef status = HAL_RTC_Init(&hrtc);
    if (status != HAL_OK)
    {
        printf("ERROR: HAL_RTC_Init failed with error code: %d\r\n", status);
        Error_Handler();
    }
    else
    {
        printf("HAL_RTC_Init succeeded.\r\n");
    }

    if (HAL_RTCEx_BKUPRead(&hrtc, RTC_BKP_DR0) != 0x32F2)
    {
        printf("RTC not configured, setting default time and date...\r\n");

        sTime.Hours = 12;
        sTime.Minutes = 0;
        sTime.Seconds = 0;
        sTime.DayLightSaving = RTC_DAYLIGHTSAVING_NONE;
        sTime.StoreOperation = RTC_STOREOPERATION_RESET;
        if (HAL_RTC_SetTime(&hrtc, &sTime, RTC_FORMAT_BIN) != HAL_OK)
        {
            printf("ERROR: HAL_RTC_SetTime failed!\r\n");
            Error_Handler();
        }

        sDate.WeekDay = RTC_WEEKDAY_TUESDAY;
        sDate.Month = RTC_MONTH_SEPTEMBER;
        sDate.Date = 2;
        sDate.Year = 25;
        if (HAL_RTC_SetDate(&hrtc, &sDate, RTC_FORMAT_BIN) != HAL_OK)
        {
            printf("ERROR: HAL_RTC_SetDate failed!\r\n");
            Error_Handler();
        }

        HAL_RTCEx_BKUPWrite(&hrtc, RTC_BKP_DR0, 0x32F2);
        printf("RTC time and date set, backup register written.\r\n");
    }
    else
    {
        printf("RTC already configured, skipping time/date set.\r\n");
    }
}

/**
  * @brief I2C1 Initialization Function
  * @retval None
  */
static void MX_I2C1_Init(void)
{
  hi2c1.Instance = I2C1;
  hi2c1.Init.ClockSpeed = 100000;
  hi2c1.Init.DutyCycle = I2C_DUTYCYCLE_2;
  hi2c1.Init.OwnAddress1 = 0;
  hi2c1.Init.AddressingMode = I2C_ADDRESSINGMODE_7BIT;
  hi2c1.Init.DualAddressMode = I2C_DUALADDRESS_DISABLE;
  hi2c1.Init.OwnAddress2 = 0;
  hi2c1.Init.GeneralCallMode = I2C_GENERALCALL_DISABLE;
  hi2c1.Init.NoStretchMode = I2C_NOSTRETCH_DISABLE;
  if (HAL_I2C_Init(&hi2c1) != HAL_OK)
  {
    Error_Handler();
  }
}

/**
  * @brief I2S3 Initialization Function
  * @retval None
  */
static void MX_I2S3_Init(void)
{
  hi2s3.Instance = SPI3;
  hi2s3.Init.Mode = I2S_MODE_MASTER_TX;
  hi2s3.Init.Standard = I2S_STANDARD_PHILIPS;
  hi2s3.Init.DataFormat = I2S_DATAFORMAT_16B;
  hi2s3.Init.MCLKOutput = I2S_MCLKOUTPUT_ENABLE;
  hi2s3.Init.AudioFreq = I2S_AUDIOFREQ_96K;
  hi2s3.Init.CPOL = I2S_CPOL_LOW;
  hi2s3.Init.ClockSource = I2S_CLOCK_PLL;
  hi2s3.Init.FullDuplexMode = I2S_FULLDUPLEXMODE_DISABLE;
  if (HAL_I2S_Init(&hi2s3) != HAL_OK)
  {
    Error_Handler();
  }
}

/**
  * @brief SPI1 Initialization Function
  * @retval None
  */
static void MX_SPI1_Init(void)
{
  hspi1.Instance = SPI1;
  hspi1.Init.Mode = SPI_MODE_MASTER;
  hspi1.Init.Direction = SPI_DIRECTION_2LINES;
  hspi1.Init.DataSize = SPI_DATASIZE_8BIT;
  hspi1.Init.CLKPolarity = SPI_POLARITY_LOW;
  hspi1.Init.CLKPhase = SPI_PHASE_1EDGE;
  hspi1.Init.NSS = SPI_NSS_SOFT;
  hspi1.Init.BaudRatePrescaler = SPI_BAUDRATEPRESCALER_2;
  hspi1.Init.FirstBit = SPI_FIRSTBIT_MSB;
  hspi1.Init.TIMode = SPI_TIMODE_DISABLE;
  hspi1.Init.CRCCalculation = SPI_CRCCALCULATION_DISABLE;
  hspi1.Init.CRCPolynomial = 10;
  if (HAL_SPI_Init(&hspi1) != HAL_OK)
  {
    Error_Handler();
  }
}

/**
  * @brief USART2 Initialization Function
  * @retval None
  */
static void MX_USART2_UART_Init(void)
{
  huart2.Instance = USART2;
  huart2.Init.BaudRate = 115200;
  huart2.Init.WordLength = UART_WORDLENGTH_8B;
  huart2.Init.StopBits = UART_STOPBITS_1;
  huart2.Init.Parity = UART_PARITY_NONE;
  huart2.Init.Mode = UART_MODE_TX_RX;
  huart2.Init.HwFlowCtl = UART_HWCONTROL_NONE;
  huart2.Init.OverSampling = UART_OVERSAMPLING_16;
  if (HAL_UART_Init(&huart2) != HAL_OK)
  {
    Error_Handler();
  }
}

/**
  * @brief GPIO Initialization Function
  * @retval None
  */
static void MX_GPIO_Init(void)
{
  GPIO_InitTypeDef GPIO_InitStruct = {0};

  __HAL_RCC_GPIOE_CLK_ENABLE();
  __HAL_RCC_GPIOC_CLK_ENABLE();
  __HAL_RCC_GPIOH_CLK_ENABLE();
  __HAL_RCC_GPIOA_CLK_ENABLE();
  __HAL_RCC_GPIOB_CLK_ENABLE();
  __HAL_RCC_GPIOD_CLK_ENABLE();

  HAL_GPIO_WritePin(CS_I2C_SPI_GPIO_Port, CS_I2C_SPI_Pin, GPIO_PIN_RESET);
  HAL_GPIO_WritePin(OTG_FS_PowerSwitchOn_GPIO_Port, OTG_FS_PowerSwitchOn_Pin, GPIO_PIN_SET);
  HAL_GPIO_WritePin(GPIOD, LD4_Pin|LD3_Pin|LD5_Pin|LD6_Pin|Audio_RST_Pin, GPIO_PIN_RESET);

  GPIO_InitStruct.Pin = CS_I2C_SPI_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(CS_I2C_SPI_GPIO_Port, &GPIO_InitStruct);

  GPIO_InitStruct.Pin = OTG_FS_PowerSwitchOn_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(OTG_FS_PowerSwitchOn_GPIO_Port, &GPIO_InitStruct);

  GPIO_InitStruct.Pin = PDM_OUT_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF5_SPI2;
  HAL_GPIO_Init(PDM_OUT_GPIO_Port, &GPIO_InitStruct);

  GPIO_InitStruct.Pin = B1_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_EVT_RISING;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  HAL_GPIO_Init(B1_GPIO_Port, &GPIO_InitStruct);

  GPIO_InitStruct.Pin = BOOT1_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_INPUT;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  HAL_GPIO_Init(BOOT1_GPIO_Port, &GPIO_InitStruct);

  GPIO_InitStruct.Pin = CLK_IN_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF5_SPI2;
  HAL_GPIO_Init(CLK_IN_GPIO_Port, &GPIO_InitStruct);

  GPIO_InitStruct.Pin = LD4_Pin|LD3_Pin|LD5_Pin|LD6_Pin|Audio_RST_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(GPIOD, &GPIO_InitStruct);

  GPIO_InitStruct.Pin = OTG_FS_OverCurrent_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_INPUT;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  HAL_GPIO_Init(OTG_FS_OverCurrent_GPIO_Port, &GPIO_InitStruct);

  GPIO_InitStruct.Pin = MEMS_INT2_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_EVT_RISING;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  HAL_GPIO_Init(MEMS_INT2_GPIO_Port, &GPIO_InitStruct);
}

/**
  * @brief  This function is executed in case of error occurrence.
  * @retval None
  */
void Error_Handler(void)
{
  __disable_irq();
  while (1)
  {
  }
}

#ifdef  USE_FULL_ASSERT
/**
  * @brief  Reports the name of the source file and the source line number
  *         where the assert_param error has occurred.
  *   file: pointer to the source file name
  *   line: assert_param error line source number
  * @retval None
  */
void assert_failed(uint8_t *file, uint32_t line)
{
  /* User can add implementation for assertion errors */
}
#endif /* USE_FULL_ASSERT */

[Peter BENSCH]

On the  STM32F407G-DISC1, not only must the crystal and load capacitors be soldered on, but also the two jumpers R22 and R21, which establish the connection to PC14/PC15.

In this context, it would also be advisable to remove SB15 and SB16 so that PC14/PC15 are separated from header P2 and the LSE cannot be disrupted.

Regards
/Peter

[Noushadalik]

Hello,

Thank you for the suggestion. We have already shorted R22 and R21; sorry for missing to mention that earlier. Regarding SB15 and SB16, is it necessary to remove them to prevent disruption of PC14/PC15 in our case? If they are not removed, will that affect RTC initialization?

We are currently facing RTC initialization failure. Could you please review the code we shared above and advise if any alterations are required?

[Peter BENSCH]

If SB15 and SB16 are not removed, you will have all the tracks to the pins on P2 acting as an antenna for interference, as well as the risk of signals being accidentally coupled in there. If you are using the LSE with crystal, I would definitely remove these solder bridges.

I don't see any problem in the code. LSE_STARTUP_TIMEOUT is not included in main.c and is therefore not visible here (but it is in stm32f4xx_hal_conf.h). I assume that the default value of 5000 is applies?

But there is one potential problem: you mentioned that the crystal data sheet recommends load capacitors CC0603CRNPO9BN6R8 – which is not correct, not even the 6.8pF. Your order number does not contain any load capacitance, so the default value of 12.5pF applies. This, in turn, is not the value of each of the two capacitors, but their series connection, together with the pin and board capacitance (see also this Knowledge Base article How to select a compatible crystal and load capacitors). The capacitors should therefore each have approx. CL1 = CL2 = 2*(12.5-3) = 19pF.

Regards
/Peter