STM32CubeF4

@verbatim
******************** (C) COPYRIGHT 2017 STMicroelectronics *******************
* @file Examples_LL/RCC/RCC_UseHSEasSystemClock/readme.txt
* @author MCD Application Team
* @brief Description of the RCC HSE start example.
******************************************************************************
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@endverbatim

@par Example Description
This example describes how to use the RCC LL API how to start the HSE
and use it as system clock.

At start-up, HSI clock is used as system clock (default clock after reset) and then Systick is
configured at 1 ms using HSI_VALUE define.

Then, HSE is started and we wait for ready state. System clock is set to HSE thanks to IT
triggered by HSE ready.
- LED2 is turned ON to indicate that HSE is ready.
- LED2 is toggled with a timing of 1 second in case of error to switch SYSCLK to HSE or if HSE CSS
failure is detected (through NMI IT).

@note In HSE bypass mode, input clock will come from the MCO from
ST_LINK MCU. This frequency cannot be changed, and it is fixed
at 8 MHz. To use MCO from ST_LINK you need to check the Board
User Manual to know how to connect the MCO pin to the STM32 device.

@par Directory contents

- RCC/RCC_UseHSEasSystemClock/Inc/stm32f4xx_it.h Interrupt handlers header file
- RCC/RCC_UseHSEasSystemClock/Inc/main.h Header for main.c module
- RCC/RCC_UseHSEasSystemClock/Inc/stm32_assert.h Template file to include assert_failed function
- RCC/RCC_UseHSEasSystemClock/Src/stm32f4xx_it.c Interrupt handlers
- RCC/RCC_UseHSEasSystemClock/Src/main.c Main program
- RCC/RCC_UseHSEasSystemClock/Src/system_stm32f4xx.c STM32F4xx system source file

@par Hardware and Software environment

- This example runs on STM32F411xE devices.

- This example has been tested with NUCLEO-F411RE
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