MOTOROLA
Chapter 17. Fast Ethernet Controller (FEC)
17-47
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.
Figure 17-27. Receive Buffer Descriptor (RxBD)
0
2
4
6
Rx Data Buffer Pointer - A[15:0]
Data Length
TR
OV
CR
—
NO
LG
MC
BC
M
—
—
L
RO2
W
RO1
E
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Rx Data Buffer Pointer - A[31:16]
Summary of Contents for ColdFire MCF5281
Page 124: ...3 20 MCF5282 User s Manual MOTOROLA EMAC Instruction Set Summary ...
Page 141: ...MOTOROLA Chapter 5 Static RAM SRAM 5 5 SRAM Programming Model ...
Page 142: ...5 6 MCF5282 User s Manual MOTOROLA SRAM Programming Model ...
Page 168: ...6 26 MCF5282 User s Manual MOTOROLA Interrupts ...
Page 186: ...7 18 MCF5282 User s Manual MOTOROLA Functional Description ...
Page 228: ...9 22 MCF5282 User s Manual MOTOROLA Functional Description ...
Page 246: ...10 18 MCF5282 User s Manual MOTOROLA Low Power Wakeup Operation ...
Page 254: ...11 8 MCF5282 User s Manual MOTOROLA Memory Map and Registers ...
Page 264: ...12 10 MCF5282 User s Manual MOTOROLA Chip Select Registers ...
Page 280: ...13 16 MCF5282 User s Manual MOTOROLA Misaligned Operands ...
Page 314: ...14 34 MCF5282 User s Manual MOTOROLA MCF5282 External Signals ...
Page 339: ...MOTOROLA Chapter 15 Synchronous DRAM Controller Module 15 25 SDRAM Example ...
Page 340: ...15 26 MCF5282 User s Manual MOTOROLA SDRAM Example ...
Page 356: ...16 16 MCF5282 User s Manual MOTOROLA DMA Controller Module Functional Description ...
Page 408: ...17 52 MCF5282 User s Manual MOTOROLA Buffer Descriptors ...
Page 446: ...20 24 MCF5282 User s Manual MOTOROLA Interrupts ...
Page 474: ...22 18 MCF5282 User s Manual MOTOROLA Programming Model ...
Page 510: ...23 36 MCF5282 User s Manual MOTOROLA Operation ...
Page 526: ...24 16 MCF5282 User s Manual MOTOROLA I2C Programming Examples ...
Page 672: ...28 12 MCF5282 User s Manual MOTOROLA Functional Description ...
Page 718: ...29 46 MCF5282 User s Manual MOTOROLA Motorola Recommended BDM Pinout ...
Page 750: ...32 8 MCF5282 User s Manual MOTOROLA Ordering Information ...