Ethernet Transmit Buffer Descriptor (Txbd) - Motorola ColdFire MCF5281 User Manual

Table 17-36. Receive Buffer Descriptor Field Definitions (continued)
Word Location
Offset + 0 Bit 1
Offset + 0 Bit 0
Offset + 2 Bits [15:0]
0ffset + 4 Bits [15:0]
Offset + 6 Bits [15:0]
1
The receive buffer pointer, which contains the address of the associated data buffer, must always be evenly
divisible by 16. The buffer must reside in memory external to the FEC. This value is never modified by the
Ethernet controller.
Whenever the software driver sets an E bit in one or more
receive descriptors, the driver should follow that with a write
to RDAR.

17.6.3 Ethernet Transmit Buffer Descriptor (TxBD)

Data is presented to the FEC for transmission by arranging it in buffers referenced by the
channel's TxBDs. The Ethernet controller confirms transmission by clearing the ready bit
(R bit) when DMA of the buffer is complete. In the TxBD the user initializes the R, W, L,
and TC bits and the length (in bytes) in the first longword, and the buffer pointer in the
second longword.
The FEC will set the R bit = 0 in the first longword of the BD when the buffer has been
DMA'd. Status bits for the buffer/frame are not included in the transmit buffer descriptors.
Transmit frame status is indicated via individual interrupt bits (error conditions) and in
statistic counters in the MIB block. See Section 17.5.3, "MIB Block Counters Memory
Map" for more details.
MOTOROLA
Field Name
OV Overrun. Written by the FEC. A receive FIFO overrun occurred
during frame reception. If this bit is set, the other status bits, M,
LG, NO, CR, and CL lose their normal meaning and will be zero.
This bit is valid only if the L-bit is set.
TR Will be set if the receive frame is truncated (frame length > 2047
bytes). If the TR bit is set the frame should be discarded and the
other error bits should be ignored as they may be incorrect.
Data Length Data length. Written by the FEC. Data length is the number of
octets written by the FEC into this BD's data buffer if L = 0 (the
value will be equal to EMRBR), or the length of the frame
including CRC if L = 1. It is written by the FEC once as the BD
is closed.
A[31:16] RX data buffer pointer, bits [31:16]
A[15:0] RX data buffer pointer, bits [15:0]
NOTE
Chapter 17. Fast Ethernet Controller (FEC)
Buffer Descriptors
Description
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