STM32CubeF4

@verbatim
******************** (C) COPYRIGHT 2017 STMicroelectronics *******************
* @file GPIO/GPIO_EXTI/readme.txt
* @author MCD Application Team
* @brief Description of the GPIO EXTI example.
******************************************************************************
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@endverbatim

@par Example Description

How to configure external interrupt lines.

In this example, 1 EXTI lines (EXTI Line0) is configured to generate
an interrupt on each rising and falling edge, respectively.
In the interrupt routine a LED connected to a specific GPIO pin is toggled.

In this example:
- EXTI Line0 is connected to PA0 pin
- when falling edge is detected on EXTI Line0, LED3, LED4, LED5 and LED6 toggles

On STM32F4-Discovery board:
- EXTI Line0 is connected to User and Wake-Up push-button

In this example, HCLK is configured at 168 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 needs 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/stm32f4xx_hal_conf.h HAL configuration file
- GPIO/GPIO_EXTI/Inc/stm32f4xx_it.h Interrupt handlers header file
- GPIO/GPIO_EXTI/Inc/main.h Main program header file
- GPIO/GPIO_EXTI/Src/stm32f4xx_it.c Interrupt handlers
- GPIO/GPIO_EXTI/Src/main.c Main program
- GPIO/GPIO_EXTI/Src/system_stm32f4xx.c STM32F4xx system clock configuration file

@par Hardware and Software environment

- This example runs on STM32F407xx devices.
- This example has been tested with STMicroelectronics STM32F4-Discovery RevB & RevC
boards 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