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STM32F4 Discovery and printf() redirection to debug viewer in Keil MDK-ARM


The ARM Cortex-M architecture includes a facility for sending/receiving user data through special debug ports. This facility is well suited for typical printf() debug practices, where a typical serial port is utiliced. In theses cases, the idea is to write in my program something like this,

#include <stdio.h>

void main (void) {
   printf("Hello, world!");

and use the «Debug (printf) viewer» option avaible in Keil MDK-ARM  (and other environments) to show a terminal where messages are received. See bellow


These are my notes in order to get this mechanism working on the STM32F4 Discovery kit using the Keil environment.

Some vocabulary.


SWV: Serial Wire Viewer


St-Link/V2 upgrade and drivers

The STM32F4 Discovery includes a built-in St-link/v2 debug interface. According with its manual, the SWV facility is available in this interface and the hardware lines are propagated in the schematic of the Discovery kit.


1 – Download drivers and utilities from the support page for St-Link/v2 for Windows Xp and 7

2 – Install drivers (?Uninstall previous driver from the control panel)

3- Upgrade St-Link/v2 interface of the discovery running the ST-LinkUpgrade.exe utility


 Configuring a Keil project

(But not working completly OK)

Assuming you have a working example for the previous debugger version. (Try to) follow these steps:

1 – Open your project and open the «target options» dialog

2 – Select the second «St-link debugger»


3 – Select «settings» and set options according to the next images.


(Here, be carefull with the speed of the core).


(This a trick for flashing the microcontroller)



Redirecting/retargeting printf() in my program

Create a C module that includes the following code and add it to your project

   @file fputc_debug.c
   @brief Trying to redirect printf() to debug port
   @date 2012/06/25

#include <stdio.h>
#include <stm32f4xx.h>

int fputc(int c, FILE *stream)

And start playing!

To be solved!!!!

The problem now is that the code is not flashed when entering in debug mode. I follow these steps:

1 – Rebuild completly the project (to avoid that the systems assumes that the build has been done)

2 – Flash the microcontroller with the «load» button

3 – Start debugging





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