ST Link error when trying to add LED blink code

NicRoberts · ‎2025-03-03

STM32 NUCLEO 64 -F411RE

From SMT32Cube IDE I build & upload the following code that does nothing ,stripped everything away to show that the ST-Link is working,

#include "stm32f4xx.h"

int main()
{
  while(1)
    {
      /* Do nothing */
    }
}

This builds, uploads & does nothing,

File download complete

Time elapsed during download operation: 00:00:00.326

Verifying .

Download verified successfully

Shutting down...

Exit.

I then add some code to blink LED2 on & off

/* hardware_modules.c 
   light_init: set up GPIOA PA5 as output
   light_on  : set PA5 high
   light_off : set PA5 low
   */

#include "hardware_modules.h"
#include "stm32f4xx.h"

#define GPIOAEN		(1U<<0)

void light_init(void)
{
	/* Enable clk access to GPIOA */
	RCC->AHB1ENR |= GPIOAEN;

	/* Set PA5 as an output pin */
	GPIOA->MODER = (1U<<10);
	GPIOA->MODER &= ~(1U<<11);
}

void light_on(void)
{
	/* Set PA5 output to high*/
	GPIOA->ODR |= (1U<<5);
}

void light_off(void)
{
	/* Set PA5 output to high*/
	GPIOA->ODR &=~(1U<<5);
}

The associated header,

/* hardware_modules.h */

#ifndef __HARDWARE_MODULES_H__
#define __HARDWARE_MODULES_H__

#include <stdint.h>
#include <stdbool.h>

void light_init(void);
void light_on(void);
void light_off(void);

#endif

I now put the init into main()

#include "stm32f4xx.h"
#include "hardware_modules.h"

int main()
{
  light_init();
  while(1)
    {
      /* Do nothing */
    }
}

Builds fine but when I try to upload I get,

Download verified successfully

Error in executing 'cont' command ...

Failed to read all registers from target

Target is not responding, retrying...

Shutting down...

Target is not responding, retrying...

If I remove light_init() it all works fine again.

Sanity checked using older projects & they all build & upload fine.

So what in my code is causing this upload error?

TDK · ‎2025-03-03

PA14/PA15 are used for SWD.

> GPIOA->MODER = ...

This reconfigures ALL GPIOA pins, which makes the SWD pins not work anymore.

Only change the function of the pins you want, in this case pin PA5, so bits 10+11.

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Javier1 · ‎2025-03-03

Working with magic numbers is not the way, code is for humans to read aswel as machines.

Im chatgpting your question, i suggest you do the same, because i dont remember what every register does when you put a random 3 in it.

void light_init(void)
{
    /* Enable clk access to GPIOA */
    RCC->AHB1ENR |= GPIOAEN;

    /* Ensure clock is enabled before modifying GPIOA */
    (void)RCC->AHB1ENR;

    /* Set PA5 as an output pin safely instead of carpet booming the whole MODER register*/
    GPIOA->MODER &= ~(3U << 10);  // Clear bits 10 and 11
    GPIOA->MODER |= (1U << 10);   // Set bit 10 (output mode)
}

Available for consulting/freelancing , hit me up in https://github.com/javiBajoCero