+ * The serial 0 on the board_kf board is used to communicate with the
+ * smart card, which has the TX and RX lines connected together. To
+ * allow the smart card to drive the RX line of the CPU the CPU TX has
+ * to be in a high impedance state.
+ * Whenever a transmission is done and there is nothing more to send
+ * the transmitter is turn off. The output pin is held in input with
+ * pull-up enabled, to avoid capturing noise from the nearby RX line.
+ *
+ * The line on the KBus port must keep sending data, even when
+ * there is nothing to transmit, because a burst data transfer
+ * generates noise on the audio channels.
+ * This is accomplished using the multiprocessor mode of the
+ * ATmega64/128 serial.
+ *
+ * The receiver keeps the MPCM bit always on. When useful data
+ * is trasmitted the address bit is set. The receiver hardware
+ * consider the frame as address info and receive it.
+ * When useless fill bytes are sent the address bit is cleared
+ * and the receiver will ignore them, avoiding useless triggering
+ * of RXC interrupt.
+ *
+ * \version $Id$
+ * \author Bernardo Innocenti <bernie@develer.com>