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              Low Speed Internal Clock(LSI) Calibration functions
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  This section provides functions allowing to measure and calibrate the internal 
  low speed clock source to ensure better BEEPER output frequency .
  
  A typical configuration for LSI calibration is done following these steps :
   1. Disable the Beeper to avoid any unwanted interrupt using BEEP_Cmd() function 
   2. Measure the LSI clock frequency by connecting it internally to TIM2 input capture  
      using BEEP_LSClockToTIMConnectCmd() function.
   3. Calibrate the beeper frequency with the measured LSI clock frequency using 
      BEEP_LSICalibrationConfig() function.