PHY Functionality and Features

11.2.1Polarity 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.

11.2.210/100 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 13-39to disable the downshift feature.

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Software Developer’s Manual

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Intel Intel Gigabit Ethernet Controllers, PCI Polarity Correction copper only, 11.2.2 10/100 Downshift 82540EP/EM Only