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Fast Ethernet Controllers (FEC0 and FEC1)
Freescale Semiconductor
26-32
NOTE
When the software driver sets an E bit in one or more receive descriptors,
the driver should follow with a write to RDAR.
n
26.5.1.3
Ethernet Transmit Buffer Descriptors (TxBD0 & TxBD1)
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 (TxBD
n
[R]) when DMA of the
0
10–9
Reserved, must be cleared.
0
8
M
Miss. Written by the FEC. This bit is set by the FEC for frames accepted in promiscuous mode, but
flagged as a miss by the internal address recognition. Therefore, while in promiscuous mode, you
can use the M-bit to quickly determine whether the frame was destined to this station. This bit is
valid only if the L-bit is set and the PROM bit is set.
0 The frame was received because of an address recognition hit.
1 The frame was received because of promiscuous mode.
0
7
BC
Set if the DA is broadcast (FFFF_FFFF_FFFF).
0
6
MC
Set if the DA is multicast and not BC.
0
5
LG
Rx frame length violation. Written by the FEC. A frame length greater than RCR
n
[MAX_FL] was
recognized. This bit is valid only if the L-bit is set. The receive data is not altered in any way unless
the length exceeds 2047 bytes.
0
4
NO
Receive non-octet aligned frame. Written by the FEC. A frame that contained a number of bits not
divisible by 8 was received, and the CRC check that occurred at the preceding byte boundary
generated an error. This bit is valid only if the L-bit is set. If this bit is set, the CR bit is not set.
0
3
Reserved, must be cleared.
0
2
CR
Receive CRC error. Written by the FEC. This frame contains a CRC error and is an integral number
of octets in length. This bit is valid only if the L-bit is set.
0
1
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 are zero. This bit
is valid only if the L-bit is set.
0
0
TR
Set if the receive frame is truncated (frame length > 2047 bytes). If the TR bit is set, the frame must
be discarded and the other error bits must be ignored as they may be incorrect.
2
15–0
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 equals 0 (the value is equal to EMRBR), or the length of the frame including
CRC if L is set. It is written by the FEC once as the BD is closed.
4
15–0
A[31:16]
RX data buffer pointer, bits [31:16]
1
6
15–0
A[15:0]
RX data buffer pointer, bits [15:0]
1
The receive buffer pointer, containing the address of the associated data buffer, must always be evenly divisible by 16. The
buffer must reside in memory external to the FEC. The Ethernet controller never modifies this value.
Table 26-29. Receive Buffer Descriptor Field Definitions (continued)
Word
Field
Description
Содержание MCF54455
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