6.4 Serial Communication Interface
The serial communication interface (SCI) communicates with external devices through
an asynchronous serial bus. The SCI uses a standard nonreturn to zero (NRZ) trans-
mission format. The SCI is fully compatible with other Motorola SCI systems, such as
those in M68HC11 and M68HC05 devices. Figure 6-7 is a block diagram of the SCI
transmitter; Figure 6-8 is a block diagram of the SCI receiver.
6.4.1 SCI Registers
The SCI programming model includes the QSM global and pin control registers, and
four SCI registers. There are two SCI control registers (SCCR0 and SCCR1), one sta-
tus register (SCSR), and one data register (SCDR). Refer to APPENDIX D REGIS-
TER SUMMARY for register bit and field definition.
6.4.1.1 Control Registers
SCCR0 contains the baud rate selection field. Baud rate must be set before the SCI is
enabled. The CPU can read and write this register at any time.
SCCR1 contains a number of SCI configuration parameters, including transmitter and
receiver enable bits, interrupt enable bits, and operating mode enable bits. The CPU
can read and write this register at any time. The SCI can modify the RWU bit under
certain circumstances.
6
Changing the value of SCI control bits during a transfer operation may disrupt opera-
tion. Before changing register values, allow the SCI to complete the current transfer,
then disable the receiver and transmitter.
MOTOROLA
6-22
QUEUED SERIAL MODULE
MC68331
USER’S MANUAL
MOTOROLA [ MOTOROLA ]
