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I am using USB-TTL converter (PL-2303 XA/ HXA chip) to make serial communication between STM32L476G-Eval board and my laptop. I didn't find where to connect TX and RX wires in the eval board because Pins are smaller than my wires, So I connected them to DB9 connector directly

Tx and Rx pins

I fixed baud rate and other information to be the same in both laptop and baord and I used a cubeMx example to test communication over UART:

    /**
  ******************************************************************************
  * @file    UART/UART_HyperTerminal_DMA/Src/main.c
  * @author  MCD Application Team
  * @brief   This sample code shows how to use UART HAL API to transmit
  *          and receive a data buffer with a communication process based on
  *          DMA transfer.
  *          The communication is done with the Hyperterminal PC application.
  ******************************************************************************
  * @attention
  *
  * <h2><center>&copy; COPYRIGHT(c) 2017 STMicroelectronics</center></h2>
  *
  * Redistribution and use in source and binary forms, with or without modification,
  * are permitted provided that the following conditions are met:
  *   1. Redistributions of source code must retain the above copyright notice,
  *      this list of conditions and the following disclaimer.
  *   2. Redistributions in binary form must reproduce the above copyright notice,
  *      this list of conditions and the following disclaimer in the documentation
  *      and/or other materials provided with the distribution.
  *   3. Neither the name of STMicroelectronics nor the names of its contributors
  *      may be used to endorse or promote products derived from this software
  *      without specific prior written permission.
  *
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
  * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
  * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  ******************************************************************************
  */

/* Includes ------------------------------------------------------------------*/
#include "main.h"
#include "stdio.h"
/** @addtogroup STM32L4xx_HAL_Examples
  * @{
  */

/** @addtogroup UART_Hyperterminal_DMA
  * @{
  */

/* Private typedef -----------------------------------------------------------*/
/* Private define ------------------------------------------------------------*/
/* Private macro -------------------------------------------------------------*/
/* Private variables ---------------------------------------------------------*/
/* UART handler declaration */
UART_HandleTypeDef UartHandle;

/* Buffer used for transmission */
uint8_t aTxBuffer[] = "\n\r ****UART-Hyperterminal communication based on DMA****\n\r Enter 10 characters using keyboard :\n\r";

/* Buffer used for reception */
uint8_t aRxBuffer[RXBUFFERSIZE];

/* Private function prototypes -----------------------------------------------*/
void SystemClock_Config(void);
static void Error_Handler(void);

/* Private functions ---------------------------------------------------------*/

/**
  * @brief  Main program
  * @param  None
  * @retval None
  */
int main(void)
{
  /* STM32L4xx HAL library initialization:
       - Configure the Flash prefetch
       - Systick timer is configured by default as source of time base, but user 
         can eventually implement his proper time base source (a general purpose 
         timer for example or other time source), keeping in mind that Time base 
         duration should be kept 1ms since PPP_TIMEOUT_VALUEs are defined and 
         handled in milliseconds basis.
       - Set NVIC Group Priority to 4
       - Low Level Initialization
     */
  HAL_Init();

  /* Configure the system clock to 80 MHz */
  SystemClock_Config();

  /* Configure LED1, LED2 and LED3 */
  BSP_LED_Init(LED1);
  BSP_LED_Init(LED2);
  BSP_LED_Init(LED3);

  /*##-1- Configure the UART peripheral ######################################*/
  /* Put the USART peripheral in the Asynchronous mode (UART Mode) */
  /* UART configured as follows:
      - Word Length = 8 Bits
      - Stop Bit    = One Stop bit
      - Parity      = ODD parity
      - BaudRate    = 9600 baud
      - Hardware flow control disabled (RTS and CTS signals) */
  UartHandle.Instance        = USARTx;

  UartHandle.Init.BaudRate   = 9600;
  UartHandle.Init.WordLength = UART_WORDLENGTH_8B;
  UartHandle.Init.StopBits   = UART_STOPBITS_1;
  UartHandle.Init.Parity     = UART_PARITY_NONE;
  UartHandle.Init.HwFlowCtl  = UART_HWCONTROL_NONE;
  UartHandle.Init.Mode       = UART_MODE_TX_RX;

  if (HAL_UART_Init(&UartHandle) != HAL_OK)
  {
    /* Initialization Error */
    Error_Handler();
  }
  /*##-2- Start the transmission process #####################################*/
  /* User start transmission data through "TxBuffer" buffer */
  if (HAL_UART_Transmit_DMA(&UartHandle, (uint8_t *)aTxBuffer, TXBUFFERSIZE) != HAL_OK)
  {
    /* Transfer error in transmission process */
    Error_Handler();
  }

  /*##-3- Put UART peripheral in reception process ###########################*/
  /* Any data received will be stored in "RxBuffer" buffer : the number max of
     data received is 10 */
  if (HAL_UART_Receive_DMA(&UartHandle, (uint8_t *)aRxBuffer, RXBUFFERSIZE) != HAL_OK)
  {
    /* Transfer error in reception process */
    Error_Handler();
  }

  /*##-4- Wait for the end of the transfer ###################################*/
  /*  Before starting a new communication transfer, you need to check the current
      state of the peripheral; if it’s busy you need to wait for the end of current
      transfer before starting a new one.
      For simplicity reasons, this example is just waiting till the end of the
      transfer, but application may perform other tasks while transfer operation
      is ongoing. */
  while (HAL_UART_GetState(&UartHandle) != HAL_UART_STATE_READY)
  {
  }

  /*##-5- Send the received Buffer ###########################################*/
  if (HAL_UART_Transmit_DMA(&UartHandle, (uint8_t *)aRxBuffer, RXBUFFERSIZE) != HAL_OK)
  {
    /* Transfer error in transmission process */
    Error_Handler();
  }

  /*##-6- Wait for the end of the transfer ###################################*/
  /*  Before starting a new communication transfer, you need to check the current
      state of the peripheral; if it’s busy you need to wait for the end of current
      transfer before starting a new one.
      For simplicity reasons, this example is just waiting till the end of the
      transfer, but application may perform other tasks while transfer operation
      is ongoing. */
  while (HAL_UART_GetState(&UartHandle) != HAL_UART_STATE_READY)
  {
  }

  /* Infinite loop */
  while (1)
  {

  }
}

/**
  * @brief  System Clock Configuration
  *         The system Clock is configured as follows :
  *            System Clock source            = PLL (HSE)
  *            SYSCLK(Hz)                     = 80000000
  *            HCLK(Hz)                       = 80000000
  *            AHB Prescaler                  = 1
  *            APB1 Prescaler                 = 1
  *            APB2 Prescaler                 = 1
  *            HSE Frequency(Hz)              = 8000000
  *            PLL_M                          = 1
  *            PLL_N                          = 20
  *            PLL_P                          = 7
  *            PLL_Q                          = 4
  *            PLL_R                          = 2
  *            Flash Latency(WS)              = 4
  * @param  None
  * @retval None
  */
void SystemClock_Config(void)
{
  RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};
  RCC_OscInitTypeDef RCC_OscInitStruct = {0};

  /* Enable HSE Oscillator and activate PLL with HSE as source   */
  /* (Default MSI Oscillator enabled at system reset remains ON) */
  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSE;
  RCC_OscInitStruct.HSEState = RCC_HSE_ON;
  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
  RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSE;
  RCC_OscInitStruct.PLL.PLLM = 1;
  RCC_OscInitStruct.PLL.PLLN = 20;
  RCC_OscInitStruct.PLL.PLLR = 2;
  RCC_OscInitStruct.PLL.PLLP = 7;
  RCC_OscInitStruct.PLL.PLLQ = 4;
  if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
  {
    /* Initialization Error */
    while(1);
  }

  /* Select PLL as system clock source and configure the HCLK, PCLK1 and PCLK2
     clocks dividers */
  RCC_ClkInitStruct.ClockType = (RCC_CLOCKTYPE_SYSCLK | RCC_CLOCKTYPE_HCLK | 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_4) != HAL_OK)
  {
    /* Initialization Error */
    while(1);
  }
}

/**
  * @brief  This function is executed in case of error occurrence.
  * @param  None
  * @retval None
  */
static void Error_Handler(void)
{
  /* Turn LED3 on */
  BSP_LED_On(LED3);
  while(1)
  {
  }
}

/**
  * @brief  Tx Transfer completed callback
  * @param  huart: UART handle.
  * @note   This example shows a simple way to report end of DMA Tx transfer, and
  *         you can add your own implementation.
  * @retval None
  */
void HAL_UART_TxCpltCallback(UART_HandleTypeDef *huart)
{
  /* Turn LED1 on: Transfer in transmission process is correct */
  BSP_LED_On(LED1);
}

/**
  * @brief  Rx Transfer completed callback
  * @param  huart: UART handle
  * @note   This example shows a simple way to report end of DMA Rx transfer, and
  *         you can add your own implementation.
  * @retval None
  */
void HAL_UART_RxCpltCallback(UART_HandleTypeDef *huart)
{
  /* Turn LED2 on: Transfer in reception process is correct */
  BSP_LED_On(LED2);
}

/**
  * @brief  UART error callbacks
  * @param  huart: UART handle
  * @note   This example shows a simple way to report transfer error, and you can
  *         add your own implementation.
  * @retval None
  */
void HAL_UART_ErrorCallback(UART_HandleTypeDef *huart)
{
  /* Turn LED3 on: Transfer error in reception/transmission process */
  BSP_LED_On(LED3);
}

#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(char *file, uint32_t line)
{
  /* 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) */

  /* Infinite loop */
  while (1)
  {
  }
}
#endif

/**
  * @}
  */

/**
  * @}
  */

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

I received something like this:

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Is this because I wired directly TX and RX cables to DB9 connector?

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  • \$\begingroup\$ have you initialized the clock explicitly for USART in question? i believe its happening most probably here HAL_UART_Init(&UartHandle) \$\endgroup\$ – Abel Tom Mar 22 '18 at 13:41
  • \$\begingroup\$ @AbelTom I am using STM32CubeL4 examples. They are pre configured I guess. I've changed nothing in clock configuration \$\endgroup\$ – Pryda Mar 22 '18 at 13:44
  • \$\begingroup\$ Then i would suggest you check the voltage levels, preferably on the oscilloscope. \$\endgroup\$ – Abel Tom Mar 22 '18 at 13:48
6
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The schematic shows that there is a ST3241EBPR between the DB9 and the processor pins. The ST3241EBPR is a RS-232 transceiver, so the signal levels on the DB9 port are RS-232 levels, which are not compatible with TTL.

That is likely the first issue you have to overcome.

I haven't seen very convenient connectors for TTL directly - but there are some solder-bridges around, so maybe you could hook your TTL cable up to those (but check the voltages on those first before you destroy anything on the board).

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  • 1
    \$\begingroup\$ Not only wrong levels (likely damagingly so) but also inverted. Realistically, for connecting to a D-sub serial port a comparably connectorized consumer USB-RS232 adapter should be used, rather than a logic-level discrete wires one. If the level translator chip on the EVB is going to be bypassed, it will also need to be disabled from driving the MCU RXD line, unless there's already a suitably high value resistor there. Or a different UART on different MCU pins could be used. \$\endgroup\$ – Chris Stratton Mar 22 '18 at 14:27
  • 1
    \$\begingroup\$ @ChrisStratton well I do hope to find RS232 levels on a DB9 connector but encountered TTL on them, so I don't trust them anymore. I think the solder bridges on the board would allow disconnecting the transceiver, but that requires more time to study the schematic than I'm willing to invest right now. \$\endgroup\$ – Arsenal Mar 22 '18 at 14:52

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