Debug printf in STM32 (using SWO)

Debug printf via STM32 SWO can be very valuable in tracking application flow and debugging applications that have to run in the long term. You can always log printf based debug logs via SWO or UART and review them later to ensure that your program is functioning properly.

In this note, we go through the process of setting up STM32 CubeIDE to display ITM or SWO debug logs. Note that this applies to CubeIDE and may vary across different IDEs like Keil uVision and IAR. Depending on the IDE, you may have to enable things elsewhere or just do it differently.

Enable SWO Debug Output in STM32 CubeMX

SWO logs can be viewed within CubeIDE (which is built based on Eclipse). To set up the ITM Data Console, you need to do the following:

  • Open Run > Debug Configurations
    In the active debug configuration, under the Debugger tab, enable Serial Wire Viewer (SWV).
  • Make the ITM log terminal visible by going to
    Window > Show View > SWV > SWV ITM Data Console
  • Find SWV settings and enable ITM Stimulus Port 0.
  • Click the red “record” or Start Trace button whenever you want the trace terminal to show you logs.

Add "putchar" Function Code in syscalls.c for SWO printf

Any printf function calls you make will ultimately use a “print character” function that writes to any hardware peripheral where you desire the printf data output to go to.

This printf output destination can be UART, SWO debug output, a flash storage device, or any combination of peripherals like that. Here, we add a snippet necessary to direct characters towards the SWO terminal.

In the user code section of main.c file, you should paste this function. Also, make sure syscalls.c is generated for your project.

int __io_putchar(int ch)
 // Write character to ITM ch.0
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Write a Debug printf Test Program

A simple FreeRTOS task or the main() function can be used to call a printf() function and confirm the working of the SWO debug output and whether it is visible in the STM32 CubeIDE.

For FreeRTOS, for example, a task like this can be set up.

  * @brief  Function implementing the defaultTask thread.
  * @param  argument: Not used
  * @retval None
void StartDefaultTask(void const * argument)
	vTaskDelay (100/portTICK_RATE_MS);

  /* Infinite loop */
	  vTaskDelay (2000/portTICK_RATE_MS);
	  printf ("SWO test!\r\n");
  /* USER CODE END 5 */

Test SWO Debug Output

You can test the SWO printf output by making a printf call. For example, here a printf function is called every 2 seconds from the default FreeRTOS task. The printf is called in an infinite loop and you can see the debug output printed on the SWV ITM console.

debug printf stm32 swo output terminal example
STM32 Debug printf Output Terminal Example (STM32 CubeIDE)
Change Log
  • Initial Release: 19 September 2021

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