Motorola MPC823e Reference Manual page 815

• RFCR and TFCR—The USB function code registers control the value that you want to
appear on the AT pins when the associated SDMA channel accesses memory. It also
controls the byte-ordering convention to be used in the transfers.
RFCR AND TFCR
BIT 0 1
FIELD RESERVED
Bits 0–2—Reserved
These bits are reserved and must be set to 0.
BO—Byte Ordering
Set this field to select the required byte ordering for the data buffer. If this field is
modified on-the-fly, it will take effect at the beginning of the next frame.
00 = The DEC/Intel convention is used for byte ordering (swapped operation). It is
also called little-endian byte ordering. The transmission order of bytes within
a buffer word is reversed as compared 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). It 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 1–3
This field contains the function code value used during this SDMA channel's memory
accesses. AT0 will be driven with a one to identify this SDMA channel access as a
DMA-type access.
• MRBLR—The maximum receive buffer length register defines the maximum number of
bytes that the MPC823e will write to the USB receive buffer before moving to the next
buffer. MRBLR must be divisible by four. The MPC823e can write fewer bytes to the
buffer than the MRBLR value if a condition such as an error or end-of-packet occurs,
but it will never write more bytes than the MRBLR value. Your receive buffers must
always be at least as long as MRBLR+2 bytes. The two additional bytes allow for a
6-bit CRC.
The transmit buffers for the USB channel are not affected by the value programmed into
MRBLR. Transmit buffers may be individually chosen to have varying lengths, as
needed. The number of bytes to be transmitted is chosen by programming the DATA
LENGTH field in the TX buffer descriptor.
MOTOROLA
2 3 4
BO
MPC823e REFERENCE MANUAL
Communication Processor Module
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