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186 Software Developer’s Manual
PHY Functionality and Features
11.2.1 Polarity Correction (copper only)
The Ethernet controller PHY automatically corrects for polarity errors on the receive pairs in
1000BASE-T and 10BASE-T modes. In 100BASE-TX mode, the polarity does not matter.
In 1000BASE-T mode, receive polarity errors are automatically corrected based on the sequence of
the symbols. Once the descrambler is locked, the polarity is also locked on all pairs. The polarity
becomes unlocked only when the receiver loses lock.
11.2.2 10/100 Downshift (82540EP/EM Only)
Gigabit speed operation requires a 4-pair cable to operate. Some existing cables have only two
pairs. Other cables might have 4 pairs, but one might be broken, leaving three working pairs. Over
two or three pairs, two gigabit link partners might be able to successfully auto-negotiate 1000
Mbps speed, but then be unable to achieve link.
The downshift feature enables the
82540EP/EM PHY to auto-negotiate with another gigabit link
partner using a two or three pair cable and downshift to link at 100 Mbps or 10 Mbps, whichever is
the highest speed below gigabit that the link partner is capable of.
By default, downshift is turned on. Refer to Table 13-39 to disable the downshift feature.