The 68HC11F1 has two types of serial ports namely asynchronous (SCI) and synchronous (SPI). The most common form of communication device used in control is the SCI as this provides the user with a variable baud rate, stop bits, start bits, but no parity. The SCI occupies the same pins as the parallel port D. Similar to the parallel ports the control registers are memory mapped i.e.

      Address   Register
      0x002b  Baud rate
       0x002c    SCI control 1 SCCR1
      0x002d  SCI control 2 SCCR2
       0x002e    SCI status SCSR
      0x002f    SCI Communication data register SCDR
 
Before we use the SCI we must first configure the serial channel with the correct protocol, normally most systems use a baud rate of 9600, 1 stop bit and no parity. The first two registers are used to configure these parameters. The next register is used to handle the status of the SCI i.e. is it ready to transmit or have I received data. Finally data is actually transmitted/ received via the data register SCDR. Let us consider an example.