11-36
MCF5272 User’s Manual
Buffer Descriptors
NOTE:
Anytime the software driver sets an E bit in a receive
descriptor, the driver should immediately write to
R_DES_ACTIVE.
11.6.1.2 Ethernet Transmit Buffer Descriptor
Data is presented to the FEC for transmission by arranging it in buffers referenced by the
channel’s TxBDs. The FEC confirms transmission or indicates error conditions using the
BDs to inform the host that the buffers have been serviced. In the TxBD, the user initializes
the R, W, L, and TC bits and the length (in bytes) in the first word and the buffer pointer in
the second words.
If L = 0, then the FEC sets the R bit to 0 in the first word of the BD when the buffer is sent
as a DMA. Status bits are not modified.
If L = 1, then the FEC sets the R bit to 0 and will modify the DEF, HB, LC, RL, RC, UN,
and CSL status bits in the first word of the BD after the buffer is sent as a DMA, and frame
transmission is complete.
The TxBD fields are detailed in Table 11-35.
Offset
+ 2
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 is equal to R_BUFF_SIZE), or the length of the frame
including CRC if L = 1. It is written by the FEC once as the BD is closed.
Offset
+ 4
Rx
Buffer
Pointer
Written by the user. The receive buffer pointer, which always points to the first location of
the associated data buffer, must always be evenly divisible by 16. The buffer must reside in
memory external to the FEC.
15
14
13
12
11
10
9
8
7
6
5
2
1
0
+0
R
TO1
W
TO2
L
TC
DEF
HB
LC
RL
RC
UN
CSL
+2
DATA LENGTH
+4
Tx Data Buffer Pointer A[31–16]
+6
Tx Data Buffer Pointer A[15–0]
Figure 11-27. Transmit Buffer Descriptor (TxBD)
Table 11-34. RxBD Field Descriptions (Continued)
Bits
Name
Description
Содержание DigitalDNA ColdFire MCF5272
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