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Fast Ethernet Controller (FEC)
MCF52235 ColdFire® Integrated Microcontroller Reference Manual, Rev. 6
Freescale Semiconductor
18-41
18.5.4.18 FIFO Transmit FIFO Watermark Register (TFWR)
The TFWR is programmed by the user to control the amount of data required in the transmit FIFO before
transmission of a frame can begin. This allows the user to minimize transmit latency (TFWR = 0x) or allow
for larger bus access latency (TFWR = 11) due to contention for the system bus. Setting the watermark to
a high value minimizes the risk of transmit FIFO underrun due to contention for the system bus. The byte
counts associated with the TFWR field may need to be modified to match a given system requirement
(worst case bus access latency by the transmit data DMA channel).
IPSBAR
Offset: 0x1144 (TFWR)
Access: User read/write
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
R
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
W
Reset
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
R
0
0
0
0
0
0
0
0
0
0
0
0
0
0
X_WMRK
W
Reset
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Figure 18-21. FIFO Transmit FIFO Watermark Register (TFWR)
Table 18-30. TFWR Field Descriptions
Field
Description
31–2
Reserved, should be cleared.
1–0
X_WMRK
Number of bytes written to transmit FIFO before transmission of a frame begins
0x 64 bytes written
10 128 bytes written
11 192 bytes written
Because
of
an
order
from
the
United
States
International
Trade
Commission,
BGA-packaged
product
lines
and
part
numbers
indicated
here
currently
are
not
available
from
Freescale
for
import
or
sale
in
the
United
States
prior
to
September
2010:MCF52234CVM60,
MCF52235CVM60