Getting started with EXTI

This article explains the EXTI and its use, with examples.

1 What is an external interrupt/event controller (EXTI)

The EXTI (EXTernal Interrupt/Event) controller consists of up to 40 edge detectors for generating event/interrupt requests on STM32L47x/L48x devices. Each input line can be independently configured to select the type (interrupt or event) and the corresponding trigger event (rising, falling, or both).

2 Configure EXTI to turn on a LED when a user button is pressed

2.1 Objective

Learn how to use the external interrupt and turn ON a LED when user button is pressed.

• Configure the GPIO that is connected to the user Button as External Interrupt (EXTI) with falling edge trigger using STM32CubeIDE
• Learn how to configure the Interrupt Controller : the NVIC
• Verify the correct functionality by pressing a button that turns on a LED

2.2 Create the project in STM32CubeIDE

• File > New > STM32 Project in main panel.
  

This example uses the NUCLEO-L476RG board.

• Select NUCLEO-L476RG using the Board Selector as shown in the figure below:
  

• Save the project

2.3 Configure GPIO

• Configure the LED pin as GPIO_Output (PA5 on NUCLEO-L476RG). For other boards check their user manual.

• Configure a button pin as GPIO_EXTI (PC13 on NUCLEO-L467RG). For other boards check their user manual.

• Check GPIO configuration

Information

Make sure to select 'External interrupt mode with Falling edge trigger detection as GPIO Mode!

• Enable the interrupt for EXTI

2.4 Generate project and edit main.c

• The easiest way to generate the code is to save your current project: Ctrl + S
  

The code is generated so you can see it in the left side of the screen in the project explorer

2.4.1 HAL Library workflow summary

The HAL library provides a high-level access to STM32 peripherals like the EXTI.
The HAL_EXTIX_IRQHandler and EXTIX_IRQHandler are inside stm32l4xx_it.c file.
You must define the Callback function in the main.c : HAL_GPIO_EXTI_Callback.

Warning

Make sure to change X by its value : [15:10]

2.4.2 Configure the Interrupt

Create a function to handle the EXTI interrupts:

• HAL callback function for EXTI: void HAL_GPIO_EXTI_Callback(uint16_t GPIO_Pin)
• To turn an LED on we need to use the function: HAL_GPIO_WritePin

Put the functions into main.c

Information

Insert your code between /* USER CODE BEGIN 4 */ and /* USER CODE END 4 */ tags

/* USER CODE BEGIN 4 */
void HAL_GPIO_EXTI_Callback(uint16_t GPIO_Pin)
{
  if(GPIO_Pin == GPIO_PIN_13) {
    HAL_GPIO_WritePin(GPIOA, GPIO_PIN_5, GPIO_PIN_SET);
  } else {
      __NOP();
  }
}
/* USER CODE END 4 */

2.5 Compile and flash

• Click on Build button
  
• Click on Debug button (to run step by step)
  
• Or on Run button (to execute)
  

=> When you press the blue button on your board the LED must switch on.