Motorola MPC823e Reference Manual page 858

Communication Processor Module
CM—Continuous Mode
0 = Normal operation.
1 = The R bit is not cleared by the communication processor module after this buffer
descriptor is closed, thus allowing the associated data buffer to be automatically
retransmitted next time the communication processor module accesses this buffer
descriptor.
P—Preamble
0 = No preamble sequence is sent.
1 = The SMCx UART controller sends one all-ones character before it sends the data
so that the other end detects an idle line before the data is received. If this bit is set
and the data length of this buffer descriptor is zero, only a preamble is sent.
DATA LENGTH
This field represents the number of octets that the communication processor module must
transmit from this buffer descriptor data buffer. However, it is never modified by the
communication processor module. Normally, this value must be greater than zero, but it can
be equal to zero with the P bit set if only a preamble is sent.
If the number of data bits in the SMCx UART character is greater than 8, then the data length
must be even. For example, to transmit three UART characters of 8-bit data, 1 start, and
1 stop, the DATA LENGTH field must be initialized to 3. However, to transmit three SMCx
UART characters of 9-bit data, 1 start, and 1 stop, the DATA LENGTH field must be
initialized to 6, since the three 9-bit data fields occupy three half-words in memory (the 9
least-significant bits of each half-word).
TX DATA BUFFER POINTER
This field always points to the first location of the associated data buffer. It can be even or
odd, unless the number of actual data bits in the SMCx UART character is greater than
8 bits, in which this field is even. For instance, the pointer to 8-bit data, 1 start, and 1 stop
characters can be even or odd, but the pointer to 9-bit data, 1 start, and 1 stop characters
must be even. The buffer can reside in internal or external memory.
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