MOTOROLA
Chapter 6. Programming Model
6-17
PRELIMINARYÑSUBJECT TO CHANGE WITHOUT NOTICE
for the system bus. Setting the watermark to a high value lowers the risk of a transmit FIFO
underrun due to system bus contention.
Table 6-19 bit Þeld descriptions for X_WMRK.
6.2.18 FIFO Transmit Start Register (X_FSTART)
The X_FSTART register, shown in Figure 6-18, can be programmed by the user to indicate
the starting address of the transmit FIFO. X_FSTART is reset to the Þrst available RAM
address. The speciÞc reset value is microcode-dependent. Users do not normally need to
program X_FSTART. If users want to reserve RAM locations for other purposes,
X_FSTART should never be set to value less than reset value.
Bits
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Field
Ñ
Reset
0000_0000_0000_0000
R/W
Read/write
Addr
0xED0
Bits
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
Field
Ñ
X_WMRK
Reset
0000_0000_0000_0000
R/W
Read/write
Addr
0xEDC
Figure 6-17. X_WMRK Register
Table 6-19. X_WMRK Field Descriptions
Bits
Name
Description
0Ð29
Ñ
Reserved. Should be written to zero by the host processor.
30Ð31
X_WMRK Transmit FIFO watermark. Frame transmission begins when the number of bytes selected by
this Þeld have been written into the transmit FIFO or if an end of frame has been written to the
FIFO or if the FIFO is full before the selected number of bytes have been written.
0x 64 bytes written to the transmit FIFO
10 128 bytes written to the transmit FIFO
11 192 bytes written to the transmit FIFO
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