Freescale Semiconductor POWERPC MPC860T - Manual (Page 46)

6-14 MPC860T (Rev. D) Fast Ethernet Controller Supplement MOTOROLA
PRELIMINARYÑSUBJECT TO CHANGE WITHOUT NOTICE
completes. At this time the contents of MII_DATA match the original value written except
for the DATA Þeld, whose contents have been replaced by the value read from the PHY
register.
Writing to MII_DATA during frame generation alters the frame contents. Software should
use the MII_DATAIO_COMPL interrupt to avoid writing to the MII_DATA register during
frame generation.
6.2.14 MII Speed Control Register (MII_SPEED)
The MII_SPEED register, shown in Figure 6-14, provides control of the MII clock (MDC
pin) frequency and allows the MII management frame preamble to be dropped.
MII_SPEED is written by the user.
Table 6-15 describes MII_SPEED Þelds.
The MII_SPEED Þeld must be programmed with a value to provide an MDC frequency of
less than or equal to 2.5 MHz to comply with the IEEE MII speciÞcation. MII_SPEED must
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 0xE84
Bits 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Field Ñ DIS_PREAMBLE MII_SPEED Ñ
Reset 0000_0000_0000_0000
R/W Read/write
Addr 0xE86
Figure 6-14. MII_SPEED Register
Table 6-15. MII_SPEED Field Descriptions
Bits Name Description
0Р23 С Reserved. Should be written to zero by the host processor.
24 DIS_PREAMBLE Discard preamble. The MII standard allows the preamble to be dropped if the attached
PHY devices does not require it.
0 Preamble is not discarded.
1 Causes the preamble (32 1s) not to be prepended to the MII management frame.
25Ð30 MII_SPEED MII_SPEED controls the frequency of the MII management interface clock (MDC)
relative to system clock. Clearing MII_SPEED, turns off the MDC and leaves it in
low-voltage state. Any non-zero value generates an MDC frequency of
1/(MII_SPEED*2) of the system clock frequency.
31 Ñ Reserved. Should be written to zero by the host processor.
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Freescale Semiconductor, Inc.
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