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How to redirect the printf function to a UART for debug messages

ST AME Support NF
ST Employee

Redirecting the printf function to a UART for debug messages

1. Introduction

It can be especially useful during code development to display messages in a terminal window about various system parameters to help with debugging. An easy way to do that is to use the printf function and redirect the output to a UART for display in a terminal window.

The ST-LINKs embedded on ST Nucleo boards have a virtual com port feature, and we can easily get debugging information on a terminal using printf redirected to the UART of the STM32 connected to the ST-LINK pins used for the virtual COM port.
In this article I will show you how to redirect the printf output to the STM32 UART peripheral that is connected to the UART pins on the embedded ST-Link that will be transmitted to the host computer and displayed via a windows terminal program, Tera Term.
 

2. Prerequisites

Hardware

  • Micro USB cable: in order to power and program the board
  • Nucleo-G070RB

1891.png
 
Software

  • STM32CubeIDE

3. Theory


On the NUCLEO-G070RB board, the embedded ST-LINK/V2 is connected to PA2 (UART TX) and PA3 (UART RX) of the target STM32G070 device. These port pins have USART2 alternate functions. printf will be redirected to use PA2 and PA3. We will then use a Terminal connected to the Virtual COM port of the ST-LINK of the Nucleo board to display the printf messages.
 

4. Steps

  1. Open STM32CubeIDE
  2. Create a new project using the NUCLEO-G070RB board


1892.png
 

  1. Give a name to the project
  • For this example, the project will be named, “Printf”.

1893.png
 

  1. Initialize all peripherals with their default settings:
  • To do this, click on “Yes”.

1894.png
 

  1. Make sure USART2 has been selected, configured, and mapped to PA2 and PA3 as shown below:
  • By default, this was configured when starting the project from the Nucleo board that was selected.
  • First make sure USART2 is activated as shown here:


1895.png

  • It should be configured in Asynchronous mode with the following settings as shown below in the red rectangles:

1896.png

  • Make sure the USART2 alternate functions have been mapped to PA2 and P3 which are connected to the TX and RX pins (respectively) of the ST-LINK on the Nucleo board.

1897.png

  1. Generate Code
  • Saving the project will generate the code.
  1. Add code for printf:
  • This code will redirect the output of the printf function to the USART2. printf function is calling fputc to transmit the output via the USART.
  • In main.c, add the following code:
/* USER CODE BEGIN PFP */
#define PUTCHAR_PROTOTYPE int __io_putchar(int ch)
/* USER CODE END PFP */
…
  /* USER CODE BEGIN WHILE */
  while (1)
  {
         printf("Hello World\n\r");
         HAL_Delay(1000);
    /* USER CODE END WHILE */

…
/* USER CODE BEGIN 4 */
/**
  * @brief  Retargets the C library printf function to the USART.
  * @param  None
  * @retval None
  */
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(&huart2, (uint8_t *)&ch, 1, 0xFFFF);

  return ch;
}

/* USER CODE END 4 */

 

  1. Build the project, enter debug mode and run the code

1898.png

  • First enter Debug session:

1899.png

  • Then Execute the code:

1900.png

  • Now open a Terminal application like TeraTerm
  • Configure the serial port connection to the ST-LINK Virtual COM port.

1901.png

  • Select the following settings:

1902.png

  • You will now see the printf message, “Hello World”, being displayed every second.

1903.png

5. Related links

STM32G0 - Datasheet
STM32G0x0 advanced Arm®-based 32-bit MCUs - Reference manual
STM32CubeIDE - Integrated Development Environment for STM32 - STMicroelectronics
NUCLEO-G070RB - STM32 Nucleo-64 development board with STM32G070RB MCU, supports Arduino and ST morpho connectivity - STMicroelectronics
Tera Term
 

 
Comments
c1ron
Associate III

I'm not getting the expected result from this article.

I don't see anything printed on the terminal. To test the virtual com port, I use the code:

 /* USER CODE BEGIN WHILE */
char data[] = {"Hey Universe\n\r"};
while (1)
{
	//printf("Hello World\n\r");
	HAL_Delay(1000);
        HAL_UART_Transmit(&huart2, (uint8_t*)&data, 20, 20);
/* USER CODE END WHILE */

This works fine. But if I uncomment the printf-statement, then nothing gets printed to the terminal !

That is, the printf-statement seems to mess up the code.

BTW, I'm using "RealTerm", with 115200 baudrate and 7-bit data-width.

Any ideas ?

YSONG.1
Associate II

Yes, i also have the same problem. I'm also looking for answers.

Use printf will print nothing.

#include "main.h"

#include "stdio.h"

/* Private includes ----------------------------------------------------------*/

/* USER CODE BEGIN Includes */

/* USER CODE END Includes */

/* Private typedef -----------------------------------------------------------*/

/* USER CODE BEGIN PTD */

uint8_t buffer[10] = {'0','1','2','3','4','5','6','7','8','9'};

/* 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;

/* USER CODE BEGIN PV */

/* USER CODE END PV */

/* Private function prototypes -----------------------------------------------*/

void SystemClock_Config(void);

static void MX_GPIO_Init(void);

static void MX_USART2_UART_Init(void);

/* USER CODE BEGIN PFP */

/* USER CODE END PFP */

/* Private user code ---------------------------------------------------------*/

/* USER CODE BEGIN 0 */

/* USER CODE END 0 */

/* USER CODE BEGIN PFP */

#define PUTCHAR_PROTOTYPE int __io_putchar(int ch)

/* USER CODE END PFP */

/**

 * @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_USART2_UART_Init();

 /* USER CODE BEGIN 2 */

 /* USER CODE END 2 */

 /* Infinite loop */

 /* USER CODE BEGIN WHILE */

 while (1)

 {

  /* USER CODE END WHILE */

  /* USER CODE BEGIN 3 */

  /*HAL_UART_Transmit(&huart2,buffer,10,100);*/

     printf("Hello World\n\r");

     HAL_Delay(1000);

  //printf

 }

 /* USER CODE END 3 */

}

/* USER CODE BEGIN 4 */

/**

 * @brief Retargets the C library printf function to the USART.

 * @param None

 * @retval None

 */

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(&huart2, (uint8_t *)&ch, 1, 0xFFFF);

 return ch;

}

/* USER CODE END 4 */

Matth1
Associate II

Thanks Laura, that's very useful for someone like me who's used to this type of debugging with Arduino.

Although, I think it could be helpful to indicate that this work with any Nucleo board, and that USART1 can be used, you just need to provide the proper handler in the PUTCHAR_PROTOTYPE. A newbie like me could be discouraged for not having the exact same board or not USART2 as in the example.

But one thing is weird : in the Pinout & Config section regarding USART1's Parameter Settings, I can only specify Word Length of 8 or 9 bits.

I tried with 8 bits and it worked.

AMuth.3
Associate II

Can I configure the USART in an already created/existing project and generate code as said in this article? If so how to do in STM32F4 Discovery board?

ST AME Support NF
ST Employee

Hi,

The ST-LINK in STM32F4 Discovery supports only SWD and not Virtual COM port.

BR

Nico.

AMuth.3
Associate II


_legacyfs_online_stmicro_images_0693W00000UoafAQAR.pngBut in the product link as given below

https://www.st.com/en/evaluation-tools/stm32f4discovery.html

It is showing as Virtual COM port is supported..

Please look that and confirm.

And what is SWD. Please explain.

ST AME Support NF
ST Employee

OK I see, there are 2 models for the STM32F4 Disco, if you are sure yours support virtual com port then yes you can use it. check what UART and I/Os form the STM32F4 are connected to RX and TX of the ST-LINK and use those when configuring your project.

ST AME Support NF
ST Employee

please refer to the user manual and schematics to see what IOs are connected the ST-LINK on your discovery kit.

AMuth.3
Associate II

I have followed the wiring connection recommended in the user manual of stm32f4 discovery board. I am getting junk characters in the terminal. What are the possible reasons for that. In the .ioc file for GPIO settings I have used "No Pull Up and No Pull down" option. will that be a problem?

AMuth.3
Associate II

I am able to print the desired characters now, but with the following bit-wise manipulation. Before passing the character to the function

HAL_UART_Transmit(&huart2, (uint8_t *)&ch, 1, 0xFFFF);

I had to do this

~(ch<<1);

and use the resulted value to pass to the function above.

Arrived at this solution when I studied the pattern of difference between desired character and the character that was printed in the console.

But don't know the reason for this. Can anybody say what is the reason? I am using STM32F4 Discover board.

ST AME Support NF
ST Employee

Glad to see you are making progress with your printf redirection to UART. Personally I have not seen this issue on my side. May be someone from the community have seen this issue and can help. Glad to see this is working now.

ST AME Support NF
ST Employee

Is there any remaining issue with this article that needs my attention?

AMuth.3
Associate II

Hi @ST AME Support NF​ 

I am using USB to serial DB9 converter to connect my Stm32f4 discovery board with laptop's minicom console. Is it true that I have to use only USB to TTL serial, and not RS232 converter? Please confirm and if so how to select that correct adapter? Thanks.

ST AME Support NF
ST Employee

Correct

AMuth.3
Associate II

can I use usb to ttl converter having builtin PC-PL2303HX Chip. will there be any problems?

ST AME Support NF
ST Employee

Hello, I reviewed the spec and looks like VDD is 5V for this model. This is not going to work or is not adapted to the STM32 board where VDD is 3.3V instead. Try to find a version that works with VDD=3.3V.

TechGuyMike
Associate III

Shouldn't line endings on Windows be "\r\n" instead of "\n\r"? Sure, TeraTerm may be rendering either sequence correctly, but there are no operating systems I know of (not Linux, not Mac) that use "\n\r" for line termination.

ST AME Support NF
ST Employee

Hello @Community member​ both solution should actually work.

Superberti
Associate

On some systems it is not enough to overwrite __io_putchar, if the syscalls.c file is missing or not implemented.

In this case also overwrite _write:

int _write(int file, char *ptr, int len)
{
	int DataIdx;

	for (DataIdx = 0; DataIdx < len; DataIdx++)
	{
		__io_putchar(*ptr++);
	}
	return len;
}

or more efficient (take care of the right UART):

int _write(int fd, char * ptr, int len)
{
  HAL_UART_Transmit(&huart2, (uint8_t *) ptr, len, HAL_MAX_DELAY);
  return len;
}

 

JamesChou
Associate

I'm using NUCLEO-G474RE, and I followed the instructions above (using LPUART1). However, I initially got garbled characters from Tera Term. Later, after changing the word length settings for UART to 8 bits in both the project and Tera Term, I got the correct display.

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Last update:
‎2024-04-29 03:11 AM
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