Ethernet Receive Buffer Descriptor (Rxbd) - Motorola ColdFire MCF5281 User Manual

field for the end of frame buffer will be written with the length of the entire frame, not just
the length of the last buffer.
For simplicity the driver may assign the default receive buffer length to be large enough to
contain an entire frame, keeping in mind that a malfunction on the network or out of spec
implementation could result in giant frames. Frames of 2k (2048) bytes or larger are
truncated by the FEC at 2047 bytes so software is guaranteed never to see a receive frame
larger than 2047 bytes.
Similar to transmit, the FEC will poll the receive descriptor ring after the driver sets up
receive BDs and writes to the RDAR register. As frames are received the FEC will fill
receive buffers and update the associated BDs, then read the next BD in the receive
descriptor ring. If the FEC reads a receive BD and finds the E bit = 0, it will poll this BD
once more. If the BD = 0 a second time the FEC will stop reading receive BDs until the
driver writes to RDAR.

17.6.2 Ethernet Receive Buffer Descriptor (RxBD)

In the RxBD, the user initializes the E and W bits in the first longword and the pointer in
second longword. When the buffer has been DMA'd, the Ethernet controller will modify
the E, L, M, BC, MC, LG, NO, CR, OV, and TR bits and write the length of the used portion
of the buffer in the first longword. The M, BC, MC, LG, NO, CR, OV and TR bits in the
first longword of the buffer descriptor are only modified by the Ethernet controller when
the L bit is set.
15 14
Offset + 0
E RO1
Offset + 2
Offset + 4
Offset + 6
Figure 17-27. Receive Buffer Descriptor (RxBD)
MOTOROLA
13 12 11 10 9
W RO2 L — —
Rx Data Buffer Pointer - A[31:16]
Rx Data Buffer Pointer - A[15:0]
Chapter 17. Fast Ethernet Controller (FEC)
8 7 6 5 4
M BC MC LG NO
Data Length
Buffer Descriptors
3 2 1 0
— CR OV TR
17-47