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Communication Processor Module
If a match is found, the SCCx Ethernet controller fetches the next RX buffer descriptor and,
if it is empty, starts transferring the incoming frame to the RX buffer descriptor associated
data buffer. If a collision is detected during the frame, the RX buffer descriptors associated
with this frame are reused. Thus, there will be no collision frames presented to you except
late collisions, which indicate serious LAN problems. When the data buffer has been filled,
the SCCx Ethernet controller clears the E bit in the RX buffer descriptor and generates an
interrupt if the I bit is set. If the incoming frame exceeds the length of the data buffer, the
Ethernet controller fetches the next RX buffer descriptor in the table and, if it is empty,
continues transferring the rest of the frame to this buffer descriptor associated data buffer.
The RX buffer descriptor length is determined by the MRBLR value in the SCCx parameter
RAM. You must program the MRBLR to be at least 64 bytes.
During reception, the SCCx Ethernet controller checks for a frame that is either too short or
too long. When the frame ends, the receive CRC field is checked and written to the data
buffer. The data length written to the last buffer descriptor in the Ethernet frame is the length
of the entire frame and it enables the software to correctly recognize the frame-too-long
condition.
The SCCx Ethernet controller then sets the L bit in the RX buffer descriptor, writes the other
frame status bits into the RX buffer descriptor, and clears the E bit. Then it generates a
maskable interrupt, which indicates that a frame has been received and is in memory. The
SCCx Ethernet controller then waits for a new frame. It receives serial data least-significant
bit first.
MOTOROLA
MPC823e REFERENCE MANUAL
16-325
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