Data transfers functions
[USART_Private_Functions]

 ===============================================================================
                            Data transfers functions
 ===============================================================================  

  This subsection provides a set of functions allowing to manage the USART data 
  transfers.
  
  During an USART reception, data shifts in least significant bit first through 
  the RX pin. In this mode, the USART_DR register consists of a buffer (RDR) 
  between the internal bus and the received shift register.

  When a transmission is taking place, a write instruction to the USART_DR register 
  stores the data in the TDR register and which is copied in the shift register 
  at the end of the current transmission.

  The read access of the USART_DR register can be done using the USART_ReceiveData()
  function and returns the RDR buffered value. Whereas a write access to the USART_DR 
  can be done using USART_SendData() function and stores the written data into 
  TDR buffer.