Motorola MPC823e Reference Manual page 945

The port C lines associated with the CDx and CTSx pins have a mode of operation in which
the pin can be internally connected to the corresponding serial communication controller,
but can also generate interrupts. Port C still detects changes on the CTSx and CDx pins and
asserts the corresponding interrupt request, but the serial communication controller
simultaneously uses the CTSx and/or CDx pin to automatically control operation. This
allows you to fully implement protocols V.24, X.21, and X.21 bis with the assistance of other
general-purpose I/O lines. To configure a port C pin as a CTSx or CDx pin that connects to
a serial communication controller and generates interrupts, follow these steps:
1. Write the corresponding PCPAR bit with a zero.
2. Write the corresponding PCDIR bit with a zero.
3. Write the corresponding PCSO bit with a 1.
4. Set the PCINT bit to find out which edges cause the interrupts.
5. Write the corresponding CIMR bit with a 1 so that interrupts can be sent to the core.
6. The pin value can be read at any time using the PCDAT register.
Note: After connecting the CTSx or CDx pins to the corresponding serial
communication controller, you must also choose "normal operation" mode in the
DIAG field of the GSMR_L to enable or disable SCCx transmission and reception
with these pins.
The DREQ1 and DREQ2 pins in port C can assert an external request to the RISC
microcontroller instead of asserting an interrupt to the core. You can program each pin to
assert an interrupt request when a high-to-low change occurs or any change that as
configured in PCINT.
Note: Do not program the DREQ1 and DREQ2 pins to assert external requests to the
RISC microcontroller, unless instructed to do so by Motorola as part of a RAM
microcode package. Otherwise, erratic behavior will occur.
16.14.9 Port C Registers
You can communicate with port C using five registers. The port C interrupt control register
(PCINT) indicates how changes on the pin cause interrupts when they are generated with
that pin. The port C special options register (PCSO) indicates whether certain port C pins
can connect to on-chip peripherals and generate an interrupt at the same time. The
remaining port C registers exist on the other ports as well. However, since port C does not
have an open-drain capability, there is no open-drain register.
MOTOROLA MPC823e REFERENCE MANUAL
Communication Processor Module
16-491