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STM32有个自带RTC外设，外接32.768KHz的晶振后可得到相对精确的计时功能。
实测了一个一小时快个1秒多。

1 cubeMX设置了RTC后自动生成的初始化代码如下

static void MX_RTC_Init(void)
{/* USER CODE BEGIN RTC_Init 0 *//* USER CODE END RTC_Init 0 */RTC_TimeTypeDef sTime = {0};RTC_DateTypeDef sDate = {0};/* USER CODE BEGIN RTC_Init 1 *//* USER CODE END RTC_Init 1 *//** 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.OutPutRemap = RTC_OUTPUT_REMAP_NONE;hrtc.Init.OutPutPolarity = RTC_OUTPUT_POLARITY_HIGH;hrtc.Init.OutPutType = RTC_OUTPUT_TYPE_OPENDRAIN;hrtc.Init.OutPutPullUp = RTC_OUTPUT_PULLUP_NONE;if (HAL_RTC_Init(&hrtc) != HAL_OK){Error_Handler();}/* USER CODE BEGIN Check_RTC_BKUP *//* USER CODE END Check_RTC_BKUP *//** Initialize RTC and set the Time and Date*/sTime.Hours = 9;sTime.Minutes = 0;sTime.Seconds = 0;sTime.SubSeconds = 0;sTime.DayLightSaving = RTC_DAYLIGHTSAVING_NONE;sTime.StoreOperation = RTC_STOREOPERATION_RESET;if (HAL_RTC_SetTime(&hrtc, &sTime, RTC_FORMAT_BIN) != HAL_OK){Error_Handler();}sDate.WeekDay = RTC_WEEKDAY_MONDAY;sDate.Month = RTC_MONTH_JANUARY;sDate.Date = 1;sDate.Year = 25;if (HAL_RTC_SetDate(&hrtc, &sDate, RTC_FORMAT_BIN) != HAL_OK){Error_Handler();}/* USER CODE BEGIN RTC_Init 2 *//* USER CODE END RTC_Init 2 */}

2 修改RTC时间设置

HAL_RTC_SetTime(&hrtc, &sTime, RTC_FORMAT_BIN)；
HAL_RTC_SetDate(&hrtc, &sDate, RTC_FORMAT_BIN)；

3 读取当前日期时间

HAL_RTC_GetTime(&RTC_Handler,&RTC_TimeStruct,RTC_FORMAT_BIN);
HAL_RTC_GetDate(&RTC_Handler,&RTC_DateStruct,RTC_FORMAT_BIN);

注意先读时间，再读日期。如果顺序调过来，会很耗时（像被卡住好几秒）。