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                        CSS on LSE configuration functions
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  This section provides functions allowing to configure the CSS on LSE capability. 
  
  1. The LSE crystal clock source failures can be monitored when used as RTC clock
     by the mean of the LSI oscillator.
     
  2. The CSS on LSE is a feature implemented externally to RTC peripheral and though 
     has no impact the clock controller registers.

  3. To enable the CSS on LSE you can use CLK_LSEClockSecuritySystemEnable() function
     
  4. To configure the action to perform at RTC clock failure you can use 
     CLK_RTCCLKSwitchOnLSEFailureEnable() function that allows to switch the RTC clock
     from LSE to LSI.