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                         Data transfers functions
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  This section provides a set of functions allowing to manage the SPI data transfers
  
  In reception, data are received and then stored into an internal Rx buffer while 
  In transmission, data are first stored into an internal Tx buffer before being 
  transmitted.

  The read access of the SPI_DR register can be done using the SPI_ReceiveData()
  function and returns the Rx buffered value. Whereas a write access to the SPI_DR 
  can be done using SPI_SendData() function and stores the written data into 
  Tx buffer.