/**
  @page RTC_Calendar RTC example
  
  @verbatim
  ******************** (C) COPYRIGHT 2016 STMicroelectronics *******************
  * @file    Examples_LL/RTC/RTC_Calendar/readme.txt 
  * @author  MCD Application Team
  * @brief   Description of the RTC example.
  ******************************************************************************
  *
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  * are permitted provided that the following conditions are met:
  *   1. Redistributions of source code must retain the above copyright notice,
  *      this list of conditions and the following disclaimer.
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  *      this list of conditions and the following disclaimer in the documentation
  *      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
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
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  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  ******************************************************************************
  @endverbatim

@par Example Description

This example guides you through the different configuration steps by mean of HAL API
to configure the RTC calendar. Peripheral initialization done using LL unitary services
functions for optimization purpose (performance and size).

In this example, after start-up, SYSCLK is configured to the max frequency using the PLL with
HSI48 as clock source.

The RTC peripheral configuration is ensured by the Configure_RTC() function 
(configure of the needed RTC resources according to the used hardware CLOCK, 
PWR, RTC clock source and BackUp). You may update this function to change RTC configuration.

@note LSE oscillator clock is used as RTC clock source by default.
      The user can use also LSI as RTC clock source. The user uncomment the adequate 
      line on the main.c file.
      @code
        #define RTC_CLOCK_SOURCE_LSE  
        /* #define RTC_CLOCK_SOURCE_LSI */
      @endcode
      LSI oscillator clock is delivered by a 40 kHz RC.
      LSE (when available on board) is delivered by a 32.768 kHz crystal.

Configure_RTC_Calendar function is then called to initialize the 
time and the date.

A key value is written in backup data register 1 to indicate if the RTC is already configured.  
The RTC is in the backup (BKP) domain. It means that when LSE is enabled, even after a reset
done with debugger,  RTC configuration is not lost.
The program behaves as follows:

1. After startup the program checks the backup data register 1 value:
    - BKP_DR1 value not correct: (value is not correct or has not yet
      been programmed when the program is executed for the first time) the RTC is
      configured.
    
    - BKP_DR1 value correct: this means that the RTC is configured and the time
      and date are displayed on Debugger.
      
2. When a reset (except power on reset) occurs the BKP domain is not reset and the RTC 
   configuration is not lost.
   
3. When power on reset occurs:
    - On Nucleo board, as there are no battery connected to the VBAT pin: 
    the BKP domain is reset and the RTC configuration is lost.

LED2 is turned ON when the RTC configuration is done correctly.


The current time and date are updated and displayed on the debugger in aShowTime 
and aShowDate variables (watch or live watch).

- LED2 is toggling : This indicates that the system generates an error.

@par Directory contents 

  - RTC/RTC_Calendar/Inc/stm32f0xx_it.h          Interrupt handlers header file
  - RTC/RTC_Calendar/Inc/main.h                  Header for main.c module
  - RTC/RTC_Calendar/Inc/stm32_assert.h          Template file to include assert_failed function
  - RTC/RTC_Calendar/Src/stm32f0xx_it.c          Interrupt handlers
  - RTC/RTC_Calendar/Src/main.c                  Main program
  - RTC/RTC_Calendar/Src/system_stm32f0xx.c      STM32F0xx system source file


@par Hardware and Software environment

  - This example runs on STM32F072xB devices.
    
  - This example has been tested with STM32F072RB-Nucleo 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>
 */
