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Bits 0–2—Reserved
These bits are reserved and must be set to 0.
BO—Byte Ordering
You must 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—Each serial communication controller has one maximum receive buffer length
register to define the receive buffer length. MRBLR defines the maximum number of
bytes that the MPC823e writes to a receive buffer on a serial communication controller
before it moves on to the next buffer. The MPC823e 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 must always
be at least as long as the MRBLR. The transmit buffers for a 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.
MOTOROLA
MPC823e REFERENCE MANUAL
Communication Processor Module
16-185
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