STM32CubeF4

/**
  @page RCC_ClockConfig RCC Clock Configuration example

  @verbatim
  ******************** (C) COPYRIGHT 2017 STMicroelectronics *******************
  * @file    RCC/RCC_ClockConfig/readme.txt 
  * @author  MCD Application Team
  * @brief   Description of the RCC Clock Configuration example.
  ******************************************************************************
  *
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  *      and/or other materials provided with the distribution.
  *   3. Neither the name of STMicroelectronics nor the names of its contributors
  *      may be used to endorse or promote products derived from this software
  *      without specific prior written permission.
  *
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  ******************************************************************************
  @endverbatim

@par Example Description 

This example describes how to use the RCC HAL API to configure the system clock (SYSCLK)
and modify the clock settings in Run mode.

In this example, after startup SYSCLK is configured to the max frequency using the PLL with
HSE as clock source, the USER Button (connected to EXTI Line15) will be 
used to change the PLL source: from HSE to HSI, and vice versa. 
Each time the Key button is pressed; EXTI_0 interrupt is generated and in the ISR
the PLL oscillator source is checked using __HAL_RCC_GET_PLL_OSCSOURCE() macro:

1- If the HSE oscillator is selected as PLL source, the following steps will be followed to switch 
   the PLL source to HSI oscillator:
     a- Switch the system clock source to HSE to allow modification of the PLL configuration
     b- Enable HSI Oscillator, select it as PLL source and finally activate the PLL
     c- Select the PLL as system clock source and configure the HCLK, PCLK1 and PCLK2 clocks dividers
     d- Disable the HSE oscillator (optional, if the HSE is no more needed by the application)

2- If the HSI oscillator is selected as PLL source, the following steps will be followed to switch 
   the PLL source to HSE oscillator:
     a- Switch the system clock source to HSI to allow modification of the PLL configuration
     b- Enable HSE Oscillator, select it as PLL source and finally activate the PLL
     c- Select the PLL as system clock source and configure the HCLK, PCLK1 and PCLK2 clocks dividers
     d- Disable the HSI oscillator (optional, if the HSI is no more needed by the application)

In this example the SYSCLK divided by 2 is outputted on the MCO2 pin(PC9).

 @note The selected clock to output onto MCO must not exceed 100 MHz (the maximum I/O speed).
 @note The SysTick 1 msec interrupt is required for the HAL process (Timeout management); by default
       the configuration is done using the HAL_Init() API, and when the system clock configuration
       is updated the SysTick configuration will be adjusted by the HAL_RCC_ClockConfig() API.

Three LEDs are toggled with a timing defined by the HAL_Delay() API.


@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 

  - RCC/RCC_ClockConfig/Inc/stm32f4xx_hal_conf.h    HAL configuration file
  - RCC/RCC_ClockConfig/Inc/stm32f4xx_it.h          Interrupt handlers header file
  - RCC/RCC_ClockConfig/Inc/main.h                  Main program header file  
  - RCC/RCC_ClockConfig/Src/stm32f4xx_it.c          Interrupt handlers
  - RCC/RCC_ClockConfig/Src/main.c                  Main program
  - RCC/RCC_ClockConfig/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 STM32F4-Discovery RevB & RevC 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
 
 * <h3><center>&copy; COPYRIGHT STMicroelectronics</center></h3>
 */