STM32L4+ UART doesn't work after entering STOP2

Question

I'm just getting into learning the low power states of STM32s and this is my first project trying to use STOP mode so please forgive any noob mistakes. This code is based very similarly of the example STM provides for STOP mode, except they don't show how to re-initialize any of the peripherals, so I'm trying to figure it out.

/* USER CODE BEGIN Header */
/**
  ******************************************************************************
  * @file           : main.c
  * @brief          : Main program body
  ******************************************************************************
  * @attention
  *
  * <h2><center>&copy; Copyright (c) 2020 STMicroelectronics.
  * All rights reserved.</center></h2>
  *
  * This software component is licensed by ST under BSD 3-Clause license,
  * the "License"; You may not use this file except in compliance with the
  * License. You may obtain a copy of the License at:
  *                        opensource.org/licenses/BSD-3-Clause
  *
  ******************************************************************************
  */
/* USER CODE END Header */

/* Includes ------------------------------------------------------------------*/
#include "main.h"

/* Private includes ----------------------------------------------------------*/
/* USER CODE BEGIN Includes */
#include <string.h>
/* 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 ---------------------------------------------------------*/
UART_HandleTypeDef hlpuart1;
UART_HandleTypeDef huart1;

RTC_HandleTypeDef hrtc;

/* 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_LPUART1_UART_Init(void);
/* USER CODE BEGIN PFP */

/* USER CODE END PFP */

/* Private user code ---------------------------------------------------------*/
/* USER CODE BEGIN 0 */
void HAL_RTC_AlarmAEventCallback(RTC_HandleTypeDef *hrtc){
  RTC_TimeTypeDef myTime = {0};
  RTC_DateTypeDef myDate = {0};

  HAL_RTC_GetTime(hrtc, &myTime, RTC_FORMAT_BIN);
  HAL_RTC_GetDate(hrtc, &myDate, RTC_FORMAT_BIN);
}
/* USER CODE END 0 */

/**
  * @brief  The application entry point.
  * @retval int
  */
int main(void)
{
  /* USER CODE BEGIN 1 */
  GPIO_InitTypeDef GPIO_InitStructure;
  /* 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_WAKEUPSTOP_CLK_CONFIG(RCC_STOP_WAKEUPCLOCK_MSI);
  /* USER CODE END SysInit */

  /* Initialize all configured peripherals */
  MX_GPIO_Init();
  MX_RTC_Init();
  MX_LPUART1_UART_Init();
  /* USER CODE BEGIN 2 */

  /* USER CODE END 2 */

  /* Infinite loop */
  /* USER CODE BEGIN WHILE */
  while (1)
  {
    //HAL_GPIO_TogglePin(LED_GREEN_GPIO_Port, LED_GREEN_Pin);
    HAL_Delay(5000);
    char* sleep_string = "going to sleep!\r\n";
    char* awake_string = "I'm awake!!\r\n";
    HAL_UART_Transmit(&hlpuart1,(uint8_t*) sleep_string ,strlen(sleep_string) , 2000);
    HAL_GPIO_WritePin(LED_GREEN_GPIO_Port, LED_GREEN_Pin, GPIO_PIN_SET);

    /* Configure all GPIO port pins in Analog Input mode (floating input trigger OFF) */
    /* Note: Debug using ST-Link is not possible during the execution of this   */
    /*       example because communication between ST-link and the device       */
    /*       under test is done through UART. All GPIO pins are disabled (set   */
    /*       to analog input mode) including  UART I/O pins.           */
    GPIO_InitStructure.Pin = GPIO_PIN_All;
    GPIO_InitStructure.Mode = GPIO_MODE_ANALOG;
    GPIO_InitStructure.Pull = GPIO_NOPULL;

    HAL_GPIO_Init(GPIOA, &GPIO_InitStructure);
    HAL_GPIO_Init(GPIOB, &GPIO_InitStructure);
    HAL_GPIO_Init(GPIOC, &GPIO_InitStructure);
    HAL_GPIO_Init(GPIOD, &GPIO_InitStructure);
    HAL_GPIO_Init(GPIOE, &GPIO_InitStructure);
    HAL_GPIO_Init(GPIOF, &GPIO_InitStructure);
    HAL_GPIO_Init(GPIOG, &GPIO_InitStructure);
    HAL_GPIO_Init(GPIOH, &GPIO_InitStructure);
    HAL_GPIO_Init(GPIOI, &GPIO_InitStructure);

    /* Disable GPIOs clock */
    __HAL_RCC_GPIOA_CLK_DISABLE();
    __HAL_RCC_GPIOB_CLK_DISABLE();
    __HAL_RCC_GPIOC_CLK_DISABLE();
    __HAL_RCC_GPIOD_CLK_DISABLE();
    __HAL_RCC_GPIOE_CLK_DISABLE();
    __HAL_RCC_GPIOF_CLK_DISABLE();
    __HAL_RCC_GPIOG_CLK_DISABLE();
    __HAL_RCC_GPIOH_CLK_DISABLE();
    __HAL_RCC_GPIOI_CLK_DISABLE();

    /* Enter STOP 2 mode */
    HAL_PWREx_EnterSTOP2Mode(PWR_STOPENTRY_WFI);

    /* ... STOP2 mode ... */

    /* Re-configure the system clock to 120 MHz based on MSI, enable and
       select PLL as system clock source (PLL is disabled in STOP mode) */
    //SYSCLKConfig_STOP();
    SystemClock_Config();

    MX_GPIO_Init();

    HAL_GPIO_WritePin(LED_GREEN_GPIO_Port, LED_GREEN_Pin, GPIO_PIN_RESET);

    HAL_UART_MspInit(&hlpuart1);

    HAL_UART_Transmit(&hlpuart1,(uint8_t*) awake_string ,strlen(awake_string) , 2000);

    /* 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};

  /** Configure the main internal regulator output voltage 
  */
  if (HAL_PWREx_ControlVoltageScaling(PWR_REGULATOR_VOLTAGE_SCALE1_BOOST) != HAL_OK)
  {
    Error_Handler();
  }
  /** Configure LSE Drive Capability 
  */
  HAL_PWR_EnableBkUpAccess();
  __HAL_RCC_LSEDRIVE_CONFIG(RCC_LSEDRIVE_LOW);
  /** Initializes the CPU, AHB and APB busses clocks 
  */
  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_LSE|RCC_OSCILLATORTYPE_MSI;
  RCC_OscInitStruct.LSEState = RCC_LSE_ON;
  RCC_OscInitStruct.MSIState = RCC_MSI_ON;
  RCC_OscInitStruct.MSICalibrationValue = 0;
  RCC_OscInitStruct.MSIClockRange = RCC_MSIRANGE_6;
  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
  RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_MSI;
  RCC_OscInitStruct.PLL.PLLM = 1;
  RCC_OscInitStruct.PLL.PLLN = 60;
  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 busses 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_DIV1;
  RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;

  if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_5) != HAL_OK)
  {
    Error_Handler();
  }
  PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_RTC|RCC_PERIPHCLK_LPUART1;
  PeriphClkInit.Lpuart1ClockSelection = RCC_LPUART1CLKSOURCE_LSE;
  PeriphClkInit.RTCClockSelection = RCC_RTCCLKSOURCE_LSE;
  if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInit) != HAL_OK)
  {
    Error_Handler();
  }
}

/**
  * @brief LPUART1 Initialization Function
  * @param None
  * @retval None
  */
static void MX_LPUART1_UART_Init(void)
{

  /* USER CODE BEGIN LPUART1_Init 0 */

  /* USER CODE END LPUART1_Init 0 */

  /* USER CODE BEGIN LPUART1_Init 1 */

  /* USER CODE END LPUART1_Init 1 */
  hlpuart1.Instance = LPUART1;
  hlpuart1.Init.BaudRate = 9600;
  hlpuart1.Init.WordLength = UART_WORDLENGTH_8B;
  hlpuart1.Init.StopBits = UART_STOPBITS_1;
  hlpuart1.Init.Parity = UART_PARITY_NONE;
  hlpuart1.Init.Mode = UART_MODE_TX_RX;
  hlpuart1.Init.HwFlowCtl = UART_HWCONTROL_NONE;
  hlpuart1.Init.OneBitSampling = UART_ONE_BIT_SAMPLE_DISABLE;
  hlpuart1.Init.ClockPrescaler = UART_PRESCALER_DIV1;
  hlpuart1.AdvancedInit.AdvFeatureInit = UART_ADVFEATURE_NO_INIT;
  hlpuart1.FifoMode = UART_FIFOMODE_DISABLE;
  if (HAL_UART_Init(&hlpuart1) != HAL_OK)
  {
    Error_Handler();
  }
  if (HAL_UARTEx_SetTxFifoThreshold(&hlpuart1, UART_TXFIFO_THRESHOLD_1_8) != HAL_OK)
  {
    Error_Handler();
  }
  if (HAL_UARTEx_SetRxFifoThreshold(&hlpuart1, UART_RXFIFO_THRESHOLD_1_8) != HAL_OK)
  {
    Error_Handler();
  }
  if (HAL_UARTEx_DisableFifoMode(&huart1) != HAL_OK)
  {
    Error_Handler();
  }
  /* USER CODE BEGIN LPUART1_Init 2 */

  /* USER CODE END LPUART1_Init 2 */

}

/**
  * @brief RTC Initialization Function
  * @param None
  * @retval None
  */
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};
  RTC_AlarmTypeDef sAlarm = {0};

  /* USER CODE BEGIN RTC_Init 1 */

  /* USER CODE END RTC_Init 1 */
  /** Initialize RTC Only 
  */
  hrtc.Instance = RTC;
  hrtc.Init.HourFormat = RTC_HOURFORMAT_12;
  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;
  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 = 2;
  sTime.Minutes = 29;
  sTime.Seconds = 0;
  sTime.TimeFormat = RTC_HOURFORMAT12_PM;
  sTime.DayLightSaving = RTC_DAYLIGHTSAVING_ADD1H;
  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_MAY;
  sDate.Date = 6;
  sDate.Year = 20;

  if (HAL_RTC_SetDate(&hrtc, &sDate, RTC_FORMAT_BIN) != HAL_OK)
  {
    Error_Handler();
  }
  /** Enable the Alarm A 
  */
  sAlarm.AlarmTime.Hours = 1;
  sAlarm.AlarmTime.Minutes = 0;
  sAlarm.AlarmTime.Seconds = 30;
  sAlarm.AlarmTime.SubSeconds = 0;
  sAlarm.AlarmTime.TimeFormat = RTC_HOURFORMAT12_PM;
  sAlarm.AlarmTime.DayLightSaving = RTC_DAYLIGHTSAVING_ADD1H;
  sAlarm.AlarmTime.StoreOperation = RTC_STOREOPERATION_RESET;
  sAlarm.AlarmMask = RTC_ALARMMASK_DATEWEEKDAY|RTC_ALARMMASK_HOURS
                              |RTC_ALARMMASK_MINUTES;
  sAlarm.AlarmSubSecondMask = RTC_ALARMSUBSECONDMASK_ALL;
  sAlarm.AlarmDateWeekDaySel = RTC_ALARMDATEWEEKDAYSEL_DATE;
  sAlarm.AlarmDateWeekDay = 1;
  sAlarm.Alarm = RTC_ALARM_A;
  if (HAL_RTC_SetAlarm_IT(&hrtc, &sAlarm, RTC_FORMAT_BIN) != HAL_OK)
  {
    Error_Handler();
  }
  /* USER CODE BEGIN RTC_Init 2 */

  /* USER CODE END RTC_Init 2 */

}

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

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

  /*Configure GPIO pin Output Level */
  HAL_GPIO_WritePin(LED_GREEN_GPIO_Port, LED_GREEN_Pin, GPIO_PIN_RESET);

  /*Configure GPIO pin : LED_GREEN_Pin */
  GPIO_InitStruct.Pin = LED_GREEN_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(LED_GREEN_GPIO_Port, &GPIO_InitStruct);

}

/* 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 */

  /* 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,
     tex: printf("Wrong parameters value: file %s on line %d\r\n", file, line) */
  /* USER CODE END 6 */
}
#endif /* USE_FULL_ASSERT */

/************************ (C) COPYRIGHT STMicroelectronics *****END OF FILE****/

All I expect it to do is go to sleep and wake up once per minute when the RTC interrupt triggers (I'm sure the interrupt works, verified with a debugger already).

When it wakes up it should:

1. turn on an LED
2. print some text over UART
3. wait 5sec
4. turn off the LED
5. print some more text
6. go back to sleep.

What actually happens is that it goes to sleep normally, then wakes up, turns on the LED, then freezes in that state (presumably waiting to print to UART). Any idea why?

A very similar issue is reported here: https://stackoverflow.com/questions/46383050/stm32l0-stop-mode-not-working-properly but the solution didn't work in my case

Thank you

Update/edit: I've managed to get a debugger connection established after the stop mode wakes back up. The issue actually doesn't seem to be with any line in particular, it now just randomly hard faults at some point after waking up. It changes depending on what I have commented in or out, but just adding in random read instructions will affect whether or not it loops to the top of the while loop and prints "I'm awake" to UART.

Answer 1

Found the problem!

What was actually happening was that my stack is stored in SRAM3 (the highest address). Due to some terrible default setting choices by ST, SRAM3 by default is not preserved in STOP2 mode, but SRAM1 and SRAM2 are.

RM0432 Rev 6 , 5.3 Low-power modes:

• Stop 0, Stop 1 and Stop 2 modes: SRAM1, SRAM2, SRAM3 and all registers content are retained. All clocks in the VCORE domain are stopped, the PLL, the MSI, the HSI16 and the HSE are disabled. The LSI and the LSE can be kept running

but then, further down

Stop 2 mode with SRAM3 content lost when the RRSTP bit is cleared in PWR_CR1 register (default setting). [emphasis mine]

Whoever wrote this manual should make the fact that SRAM3 content is not preserved by default way more clear.

Anyways, after tracking it down to stack corruption resulting in a hard fault, someone on the ST forum was able to point me in the right direction. All I had to do was set the RRSTP bit and everything worked properly.

Answer 2

In STM32L4+ Refererence manual in section "Entering Stop 2 mode" on page 211 you can find:

All the peripherals which cannot be enabled in Stop 2 mode must be either disabled by clearing the Enable bit in the peripheral itself, or put under reset state by setting the corresponding bit in the AHB1 peripheral reset register (RCC_AHB1RSTR), AHB2 peripheral reset register (RCC_AHB2RSTR), AHB3 peripheral reset register (RCC_AHB3RSTR), APB1 peripheral reset register 1 (RCC_APB1RSTR1), APB1 peripheral reset register 2 (RCC_APB1RSTR2), APB2 peripheral reset register (RCC_APB2RSTR).

So, according to manual, you have to reset or disable you peripherals (except LPTIM1, I2C3, LPUART or comparators, if you want to use them in Stop 2 mode) before entering Stop 2 mode. Try to deinitialize it before entering Stop 2 mode.