SPI_start_transaction — start a new transaction
void SPI_start_transaction(void)
   SPI_start_transaction starts a new transaction.  It
   can only be called after SPI_commit
   or SPI_rollback, as there is no transaction active at
   that point.  Normally, when an SPI-using procedure is called, there is already a
   transaction active, so attempting to start another one before closing out
   the current one will result in an error.
  
   This function can only be executed if the SPI connection has been set as
   nonatomic in the call to SPI_connect_ext.