/**
  @page GPIO_EXTI GPIO EXTI example
  
  @verbatim
  ******************** (C) COPYRIGHT 2016 STMicroelectronics *******************
  * @file    GPIO/GPIO_EXTI/readme.txt 
  * @author  MCD Application Team
  * @brief   Description of the GPIO EXTI example.
  ******************************************************************************
  * @attention
  *
  * Copyright (c) 2016 STMicroelectronics.
  * All rights reserved.
  *
  * This software is licensed under terms that can be found in the LICENSE file
  * in the root directory of this software component.
  * If no LICENSE file comes with this software, it is provided AS-IS.
  *
  ******************************************************************************
  @endverbatim

@par Example Description

How to configure external interrupt lines.

In this example, one EXTI line (EXTI_Line0) is configured to generate
an interrupt on each falling edge.
In the interrupt routine a led connected to a specific GPIO pin is toggled.

In this example:
    - EXTI_Line0 is connected to PA.00 pin
      - when falling edge is detected on EXTI_Line0 by pressing User push-button, LED3 toggles

On STM32VL-Discovery:
    - EXTI_Line0 is connected to User push-button

In this example, HCLK is configured at 24 MHz.

@note Care must be taken when using HAL_Delay(), this function provides accurate delay (in milliseconds)
      based on variable incremented in SysTick ISR. This implies that if HAL_Delay() is called from
      a peripheral ISR process, then the SysTick interrupt must have higher priority (numerically lower)
      than the peripheral interrupt. Otherwise the caller ISR process will be blocked.
      To change the SysTick interrupt priority you have to use HAL_NVIC_SetPriority() function.
      
@note The application need to ensure that the SysTick time base is always set to 1 millisecond
      to have correct HAL operation.

@par Directory contents 

  - GPIO/GPIO_EXTI/Inc/stm32f1xx_hal_conf.h    HAL configuration file
  - GPIO/GPIO_EXTI/Inc/stm32f1xx_it.h          Interrupt handlers header file
  - GPIO/GPIO_EXTI/Inc/main.h                  Header for main.c module  
  - GPIO/GPIO_EXTI/Src/stm32f1xx_it.c          Interrupt handlers
  - GPIO/GPIO_EXTI/Src/main.c                  Main program
  - GPIO/GPIO_EXTI/Src/system_stm32f1xx.c      STM32F1xx system source file

@par Hardware and Software environment

  - This example runs on STM32F1xx devices.
    
  - This example has been tested with STM32VL-Discovery board and can be
    easily tailored to any other supported device and development board.

@par How to use it ? 

In order to make the program work, you must do the following :
 - Open your preferred toolchain
 - Rebuild all files and load your image into target memory
 - Run the example



 */
