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1. Confirm the Keil product and version in use

2. What Debug Adapter is connected?

3. What is the target selected?

1. Confirm the Keil product and version in use -> mdk538a.exe

2. What Debug Adapter is connected? ->  STM32F401RET6

What is the target selected? -> To add code as below in main.c line 87

To toggle start LED by adding the following application code:

/* USER CODE BEGIN WHILE */
while(1)
{
/*start toggling LED with 100ms delay*/
HAL_GPIO_TogglePin(GPIOA, GPIO_PIN_5);
HAL_Delay(100);
}
/* USER CODE END WHILE */

1. Confirm the Keil product and version in use -> mdk538a.exe

2. What Debug Adapter is connected? ->  STM32F401RET6

What is the target selected? -> To add code as below in main.c

Open main.c and add LED GPIO
toggle application with HAL API in
main.c :
while(1)
{
…
/* USER CODE BEGIN 3 */
HAL_GPIO_TogglePin(GPIOA, GPIO_PIN_5);
HAL_Delay(100); //in ms
}
/* USER CODE END 3 */

and

main.c line 87

To toggle start LED by adding the following application code:

/* USER CODE BEGIN WHILE */
while(1)
{
/*start toggling LED with 100ms delay*/
HAL_GPIO_TogglePin(GPIOA, GPIO_PIN_5);
HAL_Delay(100);
}
/* USER CODE END WHILE */

I recommend comparing your project with the CMSIS-RTOS Blinky example project and its settings, offered by the Pack Installer of Keil MDK