Fast Ethernet Controller (FEC)
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
45-37
The TxBD is shown in
Figure 45-28
.
Table 45-36
describes TxBD fields.
NOTE
The FEC fetches Tx BDs and the corresponding Tx data continuously until
the Tx FIFO is full. It does not check whether the Tx BD it wants to fetch is
already being processed internally (as a result of a wrap). As it nears the end
of the transmission of the first frame, it starts to DMA the data for the second
frame. At that point, it also fetches the Tx BD for the third frame to keep one
BD ahead of the DMA. For example, in a case in which there are only 2
TxBDs in the ring (1 TxBD per frame), the third BD is also the first BD and
it has not yet been written back, it is read a second time with the R bit still
set. So, the data is fetched and transmitted again. The same thing happens at
the end of the second frame for the second BD. At the end of the third frame,
the first BD is read for a third time, but now it has the R bit cleared, so the
FEC stops.
Since the default Tx FIFO size is 192 bytes and each frame requires at least
8 bytes (1 word of data plus an EOF word), there should be at least 24 Tx
BDs in the ring unless:
•
The SW ensures that there is always at least one TX BD with the R bit
clear.
•
Every frame has more than 1 BD. BDs that are not “last” are written
back immediately after the data is fetched, so there should not be a
problem. It is consecutive “last” BDs that is a condition for the wrap
since a “last” BD is not written back until the frame transmission has
completed in case there are errors to report.
•
The SW has a minimum frame size, N. The minimum number of BDs is
then 192/(N+4) rounded up to the nearest integer, but not less than 3.
0
1
2
3
4
5
6
7
8
9
10
13
14
15
+0
R
TO1
W
TO2
L
TC
DEF
HB
LC
RL
RC
UN
CSL
+2
DATA LENGTH
+4
Tx Data Buffer Pointer A[0–15]
+6
Tx Data Buffer Pointer A[16–31]
Figure 45-28. Transmit Buffer Descriptor (TxBD)
Summary of Contents for PowerQUICC MPC870
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