Motorola MCF5281, MCF5282 user manual Ethernet Receive Buffer Descriptor RxBD, RO1 RO2

Buffer Descriptors

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.

Offset + 0

Offset + 2

Offset + 4

Offset + 6

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

E

RO1

W

RO2

L

—

—

M

BC

MC

LG

NO

—

CR

OV

TR

Data Length

Rx Data Buffer Pointer - A[31:16]

Rx Data Buffer Pointer - A[15:0]

Figure 17-27. Receive Buffer Descriptor (RxBD)

MOTOROLA Chapter 17. Fast Ethernet Controller (FEC) 17-47