STM32F103C8T6 Blue Pill : RTC doesn't run when VBat is only supplied

[AbhijithKannan]

I have an STM32F103C8T6 Blue Pill board, and I'm trying to get the RTC to function correctly when Vdd is removed. However, when I disconnect Vdd, the RTC doesn't operate as expected. Sometimes, when Vdd is supplied, the RTC starts counting from the time I disconnected Vdd, or it begins from an entirely different time. Additionally, I'm observing a voltage of 2.2V on Vdd when only Vbat is connected. Interestingly, the contents of the RTC backup registers are saved when only Vbat is supplied but the RTC doesn't work.

/* USER CODE BEGIN Header */
/**
 ******************************************************************************
 * @file           : main.c
 * @brief          : Main program body
 ******************************************************************************
 * @attention
 *
 * Copyright (c) 2024 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 this software, it is provided AS-IS.
 *
 ******************************************************************************
 */
/* USER CODE END Header */
/* Includes ------------------------------------------------------------------*/
#include "main.h"

/* Private includes ----------------------------------------------------------*/
/* USER CODE BEGIN Includes */
#include <stdio.h>
#include <string.h>
/* USER CODE END Includes */

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

/* USER CODE END PTD */

/* Private define ------------------------------------------------------------*/
/* USER CODE BEGIN PD */
RTC_TimeTypeDef Time = {0};
RTC_DateTypeDef Date = {0};

/* USER CODE END PD */

/* Private macro -------------------------------------------------------------*/
/* USER CODE BEGIN PM */
int _write(int file, char *ptr, int len)
{
	int DataIdx;
	for (DataIdx = 0; DataIdx < len; DataIdx++)
	{
		ITM_SendChar(*ptr++);
	}
	return len;
}
/* USER CODE END PM */

/* Private variables ---------------------------------------------------------*/
RTC_HandleTypeDef hrtc;

UART_HandleTypeDef huart1;

/* USER CODE BEGIN PV */

/* USER CODE END PV */

/* Private function prototypes -----------------------------------------------*/
void SystemClock_Config(void);
static void MX_GPIO_Init(void);
static void MX_RTC_Init(void);
static void MX_USART1_UART_Init(void);
/* USER CODE BEGIN PFP */

/* USER CODE END PFP */

/* Private user code ---------------------------------------------------------*/
/* USER CODE BEGIN 0 */
static void SetTime()
{
	RTC_TimeTypeDef sTime = {0};
	RTC_DateTypeDef DateToUpdate = {0};
	sTime.Hours = 0x23;
	sTime.Minutes = 0x59;
	sTime.Seconds = 0x50;

	if (HAL_RTC_SetTime(&hrtc, &sTime, RTC_FORMAT_BCD) != HAL_OK)
	{
		Error_Handler();
	}
	DateToUpdate.WeekDay = RTC_WEEKDAY_WEDNESDAY;
	DateToUpdate.Month = RTC_MONTH_OCTOBER;
	DateToUpdate.Date = 0x2;
	DateToUpdate.Year = 0x24;

	if (HAL_RTC_SetDate(&hrtc, &DateToUpdate, RTC_FORMAT_BCD) != HAL_OK)
	{
		Error_Handler();
	}
	
}

/* USER CODE END 0 */

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

	/* USER CODE END 1 */

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

	/* Reset of all peripherals, Initializes the Flash interface and the Systick. */
	HAL_Init();

	/* USER CODE BEGIN Init */

	/* USER CODE END Init */

	/* Configure the system clock */
	SystemClock_Config();

	/* USER CODE BEGIN SysInit */
	__HAL_RCC_BKP_CLK_ENABLE();
	/* USER CODE END SysInit */

	/* Initialize all configured peripherals */
	MX_GPIO_Init();
	MX_RTC_Init();
	MX_USART1_UART_Init();
	/* USER CODE BEGIN 2 */

	/* USER CODE END 2 */

	/* Infinite loop */
	/* USER CODE BEGIN WHILE */
	// SET_BIT(RCC->APB1ENR, RCC_APB1ENR_PWREN);

	char buff[200] = {0};
	while (1)
	{
		HAL_RTC_GetTime(&hrtc, &Time, RTC_FORMAT_BIN);
		HAL_RTC_GetDate(&hrtc, &Date, RTC_FORMAT_BIN);
		snprintf(buff, 200, "%02d/%02d/%02d %02d:%02d:%02d\r\n", Date.Date,
				 Date.Month, Date.Year, Time.Hours, Time.Minutes, Time.Seconds);
		HAL_UART_Transmit(&huart1, buff, strlen(buff), 100);
		HAL_UART_Receive(&huart1, buff, 1, 1000);
		if (buff[0] == 'A')
		{
			SetTime();
		}
		else if (buff[0] == 'B')
		{
			HAL_RTC_GetDate(&hrtc, &Date, RTC_FORMAT_BCD);
		
			HAL_RTCEx_BKUPWrite(&hrtc, RTC_BKP_DR2, Date.WeekDay);
			HAL_RTCEx_BKUPWrite(&hrtc, RTC_BKP_DR3, Date.Month);
			HAL_RTCEx_BKUPWrite(&hrtc, RTC_BKP_DR4, Date.Date);
			HAL_RTCEx_BKUPWrite(&hrtc, RTC_BKP_DR5, Date.Year);
			
			HAL_Delay(100);
			HAL_PWR_EnterSTANDBYMode();
		}
		//		HAL_Delay(1000);
		/* USER CODE END WHILE */

		/* USER CODE BEGIN 3 */
	}
	/* USER CODE END 3 */
}

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

	/** Initializes the RCC Oscillators according to the specified parameters
	 * in the RCC_OscInitTypeDef structure.
	 */
	RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI | RCC_OSCILLATORTYPE_LSE;
	RCC_OscInitStruct.LSEState = RCC_LSE_ON;
	RCC_OscInitStruct.HSIState = RCC_HSI_ON;
	RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
	RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
	RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSI_DIV2;
	RCC_OscInitStruct.PLL.PLLMUL = RCC_PLL_MUL16;
	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_CLOCKTYPE_PCLK2;
	RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
	RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
	RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV2;
	RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;

	if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_2) != HAL_OK)
	{
		Error_Handler();
	}
	PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_RTC;
	PeriphClkInit.RTCClockSelection = RCC_RTCCLKSOURCE_LSE;
	if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInit) != HAL_OK)
	{
		Error_Handler();
	}
}

/**
 * @brief RTC Initialization Function
 * @PAram None
 * @retval None
 */
static void MX_RTC_Init(void)
{

	/* USER CODE BEGIN RTC_Init 0 */
	// #ifdef RTC_SET_VALUES
	/* USER CODE END RTC_Init 0 */

	RTC_TimeTypeDef sTime = {0};
	RTC_DateTypeDef DateToUpdate = {0};

	/* USER CODE BEGIN RTC_Init 1 */

	/* USER CODE END RTC_Init 1 */

	/** Initialize RTC Only
	 */
	hrtc.Instance = RTC;
	hrtc.Init.AsynchPrediv = RTC_AUTO_1_SECOND;
	hrtc.Init.OutPut = RTC_OUTPUTSOURCE_ALARM;
	if (HAL_RTC_Init(&hrtc) != HAL_OK)
	{
		Error_Handler();
	}

	/* USER CODE BEGIN Check_RTC_BKUP */
	//	HAL_RTCEx_BKUPRead(&hrtc, RTC_BKP_DR1);
	//	HAL_RTC_WaitForSynchro(&hrtc);
	// #ifdef RTC_SET_VALUES
	if (HAL_RTCEx_BKUPRead(&hrtc, RTC_BKP_DR1) != 0x32F2)
	{
		if (HAL_RTC_WaitForSynchro(&hrtc) != HAL_OK)
		{
			Error_Handler();
		}
		/* USER CODE END Check_RTC_BKUP */

		/** Initialize RTC and set the Time and Date
		 */
		sTime.Hours = 0x23;
		sTime.Minutes = 0x59;
		sTime.Seconds = 0x50;

		if (HAL_RTC_SetTime(&hrtc, &sTime, RTC_FORMAT_BCD) != HAL_OK)
		{
			Error_Handler();
		}
		DateToUpdate.WeekDay = RTC_WEEKDAY_WEDNESDAY;
		DateToUpdate.Month = RTC_MONTH_OCTOBER;
		DateToUpdate.Date = 0x2;
		DateToUpdate.Year = 0x24;

		if (HAL_RTC_SetDate(&hrtc, &DateToUpdate, RTC_FORMAT_BCD) != HAL_OK)
		{
			Error_Handler();
		}
		/* USER CODE BEGIN RTC_Init 2 */
		// #endif
	
		HAL_RTCEx_BKUPWrite(&hrtc, RTC_BKP_DR1, 0x32F2);
		
	}
	else
	{
	
		DateToUpdate.WeekDay = HAL_RTCEx_BKUPRead(&hrtc, RTC_BKP_DR2);
		DateToUpdate.Month = HAL_RTCEx_BKUPRead(&hrtc, RTC_BKP_DR3);
		DateToUpdate.Date = HAL_RTCEx_BKUPRead(&hrtc, RTC_BKP_DR4);
		DateToUpdate.Year = HAL_RTCEx_BKUPRead(&hrtc, RTC_BKP_DR5);

		if (HAL_RTC_SetDate(&hrtc, &DateToUpdate, RTC_FORMAT_BCD) != HAL_OK)
		{
			Error_Handler();
		}
		
	}
	/* USER CODE END RTC_Init 2 */
}

/**
 * @brief USART1 Initialization Function
 * @PAram None
 * @retval None
 */
static void MX_USART1_UART_Init(void)
{

	/* USER CODE BEGIN USART1_Init 0 */

	/* USER CODE END USART1_Init 0 */

	/* USER CODE BEGIN USART1_Init 1 */

	/* USER CODE END USART1_Init 1 */
	huart1.Instance = USART1;
	huart1.Init.BaudRate = 9600;
	huart1.Init.WordLength = UART_WORDLENGTH_8B;
	huart1.Init.StopBits = UART_STOPBITS_1;
	huart1.Init.Parity = UART_PARITY_NONE;
	huart1.Init.Mode = UART_MODE_TX_RX;
	huart1.Init.HwFlowCtl = UART_HWCONTROL_NONE;
	huart1.Init.OverSampling = UART_OVERSAMPLING_16;
	if (HAL_UART_Init(&huart1) != HAL_OK)
	{
		Error_Handler();
	}
	/* USER CODE BEGIN USART1_Init 2 */

	/* USER CODE END USART1_Init 2 */
}

/**
 * @brief GPIO Initialization Function
 * @PAram None
 * @retval None
 */
static void MX_GPIO_Init(void)
{
	/* USER CODE BEGIN MX_GPIO_Init_1 */
	/* USER CODE END MX_GPIO_Init_1 */

	/* GPIO Ports Clock Enable */
	__HAL_RCC_GPIOC_CLK_ENABLE();
	__HAL_RCC_GPIOA_CLK_ENABLE();

	/* USER CODE BEGIN MX_GPIO_Init_2 */
	/* USER CODE END MX_GPIO_Init_2 */
}

/* USER CODE BEGIN 4 */

/* USER CODE END 4 */

/**
 * @brief  This function is executed in case of error occurrence.
 * @retval None
 */
void Error_Handler(void)
{
	/* USER CODE BEGIN Error_Handler_Debug */
	/* User can add his own implementation to report the HAL error return state */
	__disable_irq();
	while (1)
	{
	}
	/* USER CODE END Error_Handler_Debug */
}

#ifdef USE_FULL_ASSERT
/**
 * @brief  Reports the name of the source file and the source line number
 *         where the assert_param error has occurred.
 * @PAram  file: pointer to the source file name
 * @PAram  line: assert_param error line source number
 * @retval None
 */
void assert_failed(uint8_t *file, uint32_t line)
{
	/* USER CODE BEGIN 6 */
	/* User can add his own implementation to report the file name and line number,
	   ex: printf("Wrong parameters value: file %s on line %d\r\n", file, line) */
	/* USER CODE END 6 */
}
#endif /* USE_FULL_ASSERT */