
Communication Processor Module
MOTOROLA
MPC823 USER’S MANUAL
16-179
SCC2
COMMUNICATION
16
PROCESSOR
MODULE
Bits 0–2—Reserved
These bits are reserved and should be set to 0.
BO—Byte Ordering
You should set these bits to select the required byte ordering of the data buffer. If this
field is modified on-the-fly, it takes effect at the beginning of the next frame or the next
buffer descriptor.
00 = The DEC/Intel convention is used for byte ordering (swapped operation) and
is also called little-endian byte ordering. The transmission order of bytes within
a buffer word is reversed in comparison to the Motorola mode. This mode is
supported only for 32-bit port size memory.
01 = PowerPC little-endian byte ordering. As data is transmitted onto the serial line
from the data buffer, the least-significant byte of the buffer double-word
contains data to be transmitted earlier than the most-significant byte of the
same buffer double-word.
1X = Motorola byte ordering (normal operation) is also called big-endian byte
ordering. As data is transmitted onto the serial line from the data buffer, the
most-significant byte of the buffer word contains data to be transmitted earlier
than the least-significant byte of the same buffer word.
AT—Address Type
These bits contain the function code value used during this SDMA channel memory
access. AT0 is driven with a 1 to identify this SDMA channel access as a DMA type.
MRBLR—The serial communication controller has one maximum receive buffer length
register to define the receive buffer length. The MRBLR defines the maximum number
of bytes that the MPC823 writes to a receive buffer on the serial communication
controller before it moves on to the next buffer. The MPC823 can write fewer bytes to
the buffer than MRBLR if a condition, such as an error or end-of-frame occurs, but it
never writes more bytes than the MRBLR value. It follows then, that buffers you supply
should always be at least as long as the MRBLR. The transmit buffers for the serial
communication controller are not affected in any way by the value programmed into the
MRBLR. Transmit buffers can be individually chosen to have varying lengths. The
number of bytes to be transmitted is chosen by programming the DATA LENGTH field
in the TX buffer descriptor.