HAL_UART_Transmit_DMA not working - Transfer never starts and TxCpltCallback never called

[Andrea Verdecchia]

I'm trying to enable USART transmission (TX only for now) via DMA, but when I call HAL_UART_Transmit_DMA, the transfer doesn't seem to start at all and HAL_UART_TxCpltCallback is never called.

I've configured everything through CubeMX: enabled USART with DMA, enabled FreeRTOS, and enabled interrupts. Without DMA, transmission works fine using HAL_UART_Transmit, but when I switch to HAL_UART_Transmit_DMA, nothing happens.

I can't figure out what the problem might be. Does anyone have any suggestions?

Here's my main code:

/* USER CODE BEGIN Header */
/**
 ******************************************************************************
 * @file           : main.c
 * @brief          : Main program body
 ******************************************************************************
 * @attention
 *
 * Copyright (c) 2026 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"
#include "cmsis_os.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 huart2;
DMA_HandleTypeDef hdma_usart2_tx;

/* Definitions for idleTask */
osThreadId_t idleTaskHandle;
const osThreadAttr_t idleTask_attributes = {
  .name = "idleTask",
  .stack_size = 512 * 4,
  .priority = (osPriority_t) osPriorityNormal,
};
/* USER CODE BEGIN PV */

/* USER CODE END PV */

/* Private function prototypes -----------------------------------------------*/
void SystemClock_Config(void);
static void MX_GPIO_Init(void);
static void MX_DMA_Init(void);
static void MX_USART2_UART_Init(void);
void StartIdleTask(void *argument);

/* USER CODE BEGIN PFP */

/* USER CODE END PFP */

/* Private user code ---------------------------------------------------------*/
/* USER CODE BEGIN 0 */
volatile uint8_t txComplete = 0;

// Buffer DMA allineato a 32 byte per cache STM32H7
__attribute__((aligned(32))) uint8_t dma_tx_buffer[32];
const char *msg_str = "hallo\r\n";
uint8_t msg_len = 7;

void HAL_UART_TxCpltCallback(UART_HandleTypeDef *huart)
{
    if (huart->Instance == USART2)
    {
        // Trasmissione completata
        txComplete = 1;
        HAL_GPIO_TogglePin(GPIOE, LED_YELLOW_Pin); // Debug visivo
    }
}
/* 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 */

  /* USER CODE END SysInit */

  /* Initialize all configured peripherals */
  MX_GPIO_Init();
  MX_DMA_Init();
  MX_USART2_UART_Init();
  /* USER CODE BEGIN 2 */

  /* USER CODE END 2 */

  /* Init scheduler */
  osKernelInitialize();

  /* USER CODE BEGIN RTOS_MUTEX */
    /* add mutexes, ... */
  /* USER CODE END RTOS_MUTEX */

  /* USER CODE BEGIN RTOS_SEMAPHORES */
    /* add semaphores, ... */
  /* USER CODE END RTOS_SEMAPHORES */

  /* USER CODE BEGIN RTOS_TIMERS */
    /* start timers, add new ones, ... */
  /* USER CODE END RTOS_TIMERS */

  /* USER CODE BEGIN RTOS_QUEUES */
    /* add queues, ... */
  /* USER CODE END RTOS_QUEUES */

  /* Create the thread(s) */
  /* creation of idleTask */
  idleTaskHandle = osThreadNew(StartIdleTask, NULL, &idleTask_attributes);

  /* USER CODE BEGIN RTOS_THREADS */
    /* add threads, ... */
  /* USER CODE END RTOS_THREADS */

  /* USER CODE BEGIN RTOS_EVENTS */
    /* add events, ... */
  /* USER CODE END RTOS_EVENTS */

  /* Start scheduler */
  osKernelStart();

  /* We should never get here as control is now taken by the scheduler */

  /* Infinite loop */
  /* USER CODE BEGIN WHILE */
    while (1)
    {
    /* 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};

  /** Supply configuration update enable
  */
  HAL_PWREx_ConfigSupply(PWR_LDO_SUPPLY);

  /** Configure the main internal regulator output voltage
  */
  __HAL_PWR_VOLTAGESCALING_CONFIG(PWR_REGULATOR_VOLTAGE_SCALE0);

  while(!__HAL_PWR_GET_FLAG(PWR_FLAG_VOSRDY)) {}

  /** Initializes the RCC Oscillators according to the specified parameters
  * in the RCC_OscInitTypeDef structure.
  */
  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
  RCC_OscInitStruct.HSIState = 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 = 4;
  RCC_OscInitStruct.PLL.PLLN = 60;
  RCC_OscInitStruct.PLL.PLLP = 2;
  RCC_OscInitStruct.PLL.PLLQ = 4;
  RCC_OscInitStruct.PLL.PLLR = 2;
  RCC_OscInitStruct.PLL.PLLRGE = RCC_PLL1VCIRANGE_3;
  RCC_OscInitStruct.PLL.PLLVCOSEL = RCC_PLL1VCOWIDE;
  RCC_OscInitStruct.PLL.PLLFRACN = 0;
  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_CLOCKTYPE_D3PCLK1|RCC_CLOCKTYPE_D1PCLK1;
  RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
  RCC_ClkInitStruct.SYSCLKDivider = RCC_SYSCLK_DIV1;
  RCC_ClkInitStruct.AHBCLKDivider = RCC_HCLK_DIV2;
  RCC_ClkInitStruct.APB3CLKDivider = RCC_APB3_DIV2;
  RCC_ClkInitStruct.APB1CLKDivider = RCC_APB1_DIV2;
  RCC_ClkInitStruct.APB2CLKDivider = RCC_APB2_DIV2;
  RCC_ClkInitStruct.APB4CLKDivider = RCC_APB4_DIV2;

  if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_4) != HAL_OK)
  {
    Error_Handler();
  }
}

/**
  * @brief USART2 Initialization Function
  * @PAram None
  * @retval None
  */
static void MX_USART2_UART_Init(void)
{

  /* USER CODE BEGIN USART2_Init 0 */

  /* USER CODE END USART2_Init 0 */

  /* USER CODE BEGIN USART2_Init 1 */

  /* USER CODE END USART2_Init 1 */
  huart2.Instance = USART2;
  huart2.Init.BaudRate = 12000000;
  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_8;
  huart2.Init.OneBitSampling = UART_ONE_BIT_SAMPLE_DISABLE;
  huart2.Init.ClockPrescaler = UART_PRESCALER_DIV1;
  huart2.AdvancedInit.AdvFeatureInit = UART_ADVFEATURE_NO_INIT;
  if (HAL_UART_Init(&huart2) != HAL_OK)
  {
    Error_Handler();
  }
  if (HAL_UARTEx_SetTxFifoThreshold(&huart2, UART_TXFIFO_THRESHOLD_1_8) != HAL_OK)
  {
    Error_Handler();
  }
  if (HAL_UARTEx_SetRxFifoThreshold(&huart2, UART_RXFIFO_THRESHOLD_1_8) != HAL_OK)
  {
    Error_Handler();
  }
  if (HAL_UARTEx_DisableFifoMode(&huart2) != HAL_OK)
  {
    Error_Handler();
  }
  /* USER CODE BEGIN USART2_Init 2 */

  /* USER CODE END USART2_Init 2 */

}

/**
  * Enable DMA controller clock
  */
static void MX_DMA_Init(void)
{

  /* DMA controller clock enable */
  __HAL_RCC_DMA1_CLK_ENABLE();

  /* DMA interrupt init */
  /* DMA1_Stream0_IRQn interrupt configuration */
  HAL_NVIC_SetPriority(DMA1_Stream0_IRQn, 5, 0);
  HAL_NVIC_EnableIRQ(DMA1_Stream0_IRQn);

}

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

  /*Configure GPIO pin Output Level */
  HAL_GPIO_WritePin(GPIOB, LED_GREEN_Pin|LED_RED_Pin, GPIO_PIN_RESET);

  /*Configure GPIO pin Output Level */
  HAL_GPIO_WritePin(LED_YELLOW_GPIO_Port, LED_YELLOW_Pin, GPIO_PIN_RESET);

  /*Configure GPIO pins : LED_GREEN_Pin LED_RED_Pin */
  GPIO_InitStruct.Pin = LED_GREEN_Pin|LED_RED_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

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

  /* USER CODE BEGIN MX_GPIO_Init_2 */

  /* USER CODE END MX_GPIO_Init_2 */
}

/* USER CODE BEGIN 4 */

/* USER CODE END 4 */

/* USER CODE BEGIN Header_StartIdleTask */
/**
 * @brief  Function implementing the idleTask thread.
 * @PAram  argument: Not used
 * @retval None
 */
/* USER CODE END Header_StartIdleTask */
void StartIdleTask(void *argument)
{
  /* USER CODE BEGIN 5 */
    // Copia messaggio nel buffer DM

    /* Infinite loop */
    for (;;)
    {
        // Copia stringa nel buffer DMA
        memcpy(dma_tx_buffer, msg_str, msg_len);

        // Invia "hallo" tramite USART2 in DMA (non bloccante)
        // HAL_UART_Transmit(&huart2, dma_tx_buffer, msg_len, HAL_MAX_DELAY);
        HAL_StatusTypeDef status = HAL_UART_Transmit_DMA(&huart2, dma_tx_buffer, msg_len);

        if (status == HAL_OK)
        {
            // DMA avviato correttamente, LED rosso ON
            HAL_GPIO_WritePin(GPIOB, LED_RED_Pin, GPIO_PIN_SET);

            while (txComplete == 0)
            {
                osDelay(10);
            }

            HAL_GPIO_WritePin(GPIOB, LED_RED_Pin, GPIO_PIN_RESET);
        }
        else
        {
            // Errore DMA, LED verde lampeggia veloce
            HAL_GPIO_WritePin(GPIOB, LED_GREEN_Pin, GPIO_PIN_SET);
            osDelay(50);
            HAL_GPIO_WritePin(GPIOB, LED_GREEN_Pin, GPIO_PIN_RESET);
            osDelay(50);
        }

        txComplete = 0;

        osDelay(1700);
    }
  /* USER CODE END 5 */
}

/**
  * @brief  Period elapsed callback in non blocking mode
  * @note   This function is called  when TIM1 interrupt took place, inside
  * HAL_TIM_IRQHandler(). It makes a direct call to HAL_IncTick() to increment
  * a global variable "uwTick" used as application time base.
  * @PAram  htim : TIM handle
  * @retval None
  */
void HAL_TIM_PeriodElapsedCallback(TIM_HandleTypeDef *htim)
{
  /* USER CODE BEGIN Callback 0 */

  /* USER CODE END Callback 0 */
  if (htim->Instance == TIM1)
  {
    HAL_IncTick();
  }
  /* USER CODE BEGIN Callback 1 */

  /* USER CODE END Callback 1 */
}

/**
  * @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 */

[Vidar]

Are your DMA buffers in non-cacheble regions? Can you share your MCU, .ioc and MPU config?

[Andrea Verdecchia]

MCU: STM32H753ZI (NUCLEO-H753ZI board)

I'll attach my .ioc file.

Regarding MPU config: MPU NOT USED

[Imen.D]

Hello @Andrea Verdecchia ,

Did you try to debug, put a breakpoint and check the data in the buffer?

These articles may help you as they contain UART examples:

• STM32 UART DMA RX/TX
• Implementing UART receive and transmit functions on an STM32

[Andrea Verdecchia]

I looked at the two examples you sent me, but the first one configures everything manually in code without using CubeMX—I haven't tested it yet but I will—while the second one only configures DMA for RX and doesn't set anything I haven't already set. I double-checked my code, removed FreeRTOS in case that was the problem, and also tested sending via interrupt with HAL_UART_Transmit_IT and it works. The only thing that's not working is DMA. I'm new to the ST environment and I'd like to understand if I'm missing something in the CubeMX configuration.

[Vidar]

Okay, there is the issue: you need to use the MPU or do proper cache maintainance, I would go for the MPU.

https://community.st.com/t5/stm32-mcus-embedded-software/adc-dma-on-stm32h753zi/td-p/856589

The MPU should define a memory region which is sharable and non-cacheble, then you set via the linker file a region to a specific andress, which matches the region configured by the MPU. Then you can place your buffer there.