Skip to main content
Graduate II
November 15, 2025
Solved

Can't make printf via USART work

  • November 15, 2025
  • 1 reply
  • 179 views

I'm using an STM32U5A9J-DK and I've followed this guide step-by-step https://community.st.com/t5/stm32-mcus/how-to-redirect-the-printf-function-to-a-uart-for-debug-messages/ta-p/49865

From the docs https://www.st.com/resource/en/user_manual/um2967-discovery-kits-with-stm32u5x9nj-mcus-stmicroelectronics.pdf I'm seeing that the virtual port is set to USART1, so I've set it up with CubeMX which generated

/**
 * @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 = 115200;
 huart1.Init.WordLength = UART_WORDLENGTH_7B;
 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;
 huart1.Init.OneBitSampling = UART_ONE_BIT_SAMPLE_DISABLE;
 huart1.Init.ClockPrescaler = UART_PRESCALER_DIV1;
 huart1.AdvancedInit.AdvFeatureInit = UART_ADVFEATURE_NO_INIT;
 if (HAL_UART_Init(&huart1) != HAL_OK)
 {
 Error_Handler();
 }
 if (HAL_UARTEx_SetTxFifoThreshold(&huart1, UART_TXFIFO_THRESHOLD_1_8) != HAL_OK)
 {
 Error_Handler();
 }
 if (HAL_UARTEx_SetRxFifoThreshold(&huart1, UART_RXFIFO_THRESHOLD_1_8) != HAL_OK)
 {
 Error_Handler();
 }
 if (HAL_UARTEx_DisableFifoMode(&huart1) != HAL_OK)
 {
 Error_Handler();
 }
 /* USER CODE BEGIN USART1_Init 2 */

 /* USER CODE END USART1_Init 2 */

}

but, despite having the same parameters on my TeraTerm connected to the right virtual port

nico23_0-1763207162487.png

I can't see any text printed via printf (despite compiling and correctly debugging that the 

PUTCHAR_PROTOTYPE
{
 /* Place your implementation of fputc here */
 /* e.g. write a character to the USART1 and Loop until the end of transmission */
 HAL_UART_Transmit(&huart1, (uint8_t *)&ch, 1, 0xFFFF);

 return ch;
}

is being called correctly. What could be the issue?

    This topic has been closed for replies.
    Best answer by nico23

    Just figured out the auto-assignment of the pins was wrong https://community.st.com/t5/stm32-mcus-embedded-software/stm32u5xxx-uart-communication-problems/td-p/651344

    1 reply

    nico23AuthorAnswer
    Graduate II
    November 15, 2025