Motorola MPC823e Reference Manual page 656

Communication Processor Module
All the standards provide handshaking signals, but some systems require just three physical
lines—transmit data, receive data, and ground. Many proprietary standards have been built
around the asynchronous character frame and some even implement a multidrop
configuration. In multidrop systems, more than two stations can be present on a network,
each one with a specific address. Frames made up of many characters can be broadcast
with the first character acting as a destination address. To accommodate this standard, the
UART frame is extended one bit to distinguish between an address character and the
normal data characters. Additionally, a synchronous form of the UART protocol exists where
start and stop bits are still present, but because a clock is provided with each bit the
oversampling technique is not required. This mode is called isochronous operation or
synchronous UART.
By appropriately setting the GSMR_L, the SCCx channel can be configured to function in
UART mode, which provides standard serial I/O using asynchronous character-oriented
(start-stop) protocols with RS-232 C-type lines. The SCCx in UART mode, also called the
SCCx UART controller, can be used to communicate with any existing RS-232 type of
device and to provide a port for serial communication to other microprocessors and
terminals (either locally or via modems). It includes facilities for communication using
standard asynchronous bit rates and protocols. The SCCx in UART mode supports a
multidrop mode for master/slave operation with wake-up capability on both the idle signal
and address bit. This mode also supports a synchronous mode of operation in which a clock
must be provided with each bit that is received. It transmits data from memory (either internal
or external) to the TXDx signal and receives data from the RXDx signal into memory. In
synchronous UART mode, the clock must also be supplied and it can be generated internally
or externally. Modem lines are supported via the port B and C pins. The SCCx in UART
mode consists of separate transmit and receive sections whose operations are
asynchronous with the core.
16.9.15.1 FEATURES
The following list summarizes the main features of the SCCx in UART mode:
• Flexible message-oriented data structure
• Implements synchronous and asynchronous UART
• Multidrop operation
• Receiver wake-up on idle line or address mode
• Eight control character comparisons
• Two address comparisons
• Maintenance of four 16-bit error counters
• Received break character length indication
• Programmable data length (5–8 bits)
• Programmable 1 to 2 stop bits in transmission
• Capable of reception without a stop bit
• Programmable fractional stop bit length
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