STM32CubeF4

@verbatim
******************** (C) COPYRIGHT 2017 STMicroelectronics *******************
* @file Examples_LL/RTC/RTC_Calendar/readme.txt
* @author MCD Application Team
* @brief Description of the RTC example.
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@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
HSI 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 LSI oscillator clock is used as RTC clock source by default.
The user can use also LSE as RTC clock source. The user uncomment the adequate
line on the main.c file.
@code
#define RTC_CLOCK_SOURCE_LSI
/* #define RTC_CLOCK_SOURCE_LSE */
@endcode
LSI oscillator clock is delivered by a 32 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/stm32f4xx_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/stm32f4xx_it.c Interrupt handlers
- RTC/RTC_Calendar/Src/main.c Main program
- RTC/RTC_Calendar/Src/system_stm32f4xx.c STM32F4xx system source file

@par Hardware and Software environment

- This example runs on STM32F411xx 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