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.