LXT971ABC datasheet(42/90 Pages) INTEL

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LXT971A 3.3V Dual-Speed Fast Ethernet Transceiver

Datasheet

Document #: 249414

Revision #: 002

Rev. Date: August 7, 2002

• De-assertion time for CRS is slightly longer than assertion time. This causes IFG intervals to

appear somewhat shorter to the MAC than it actually is on the wire.

• CRS de-assertion is not aligned with TX_EN de-assertion on transmit loopbacks in half-

duplex mode.

3.7.3.2.4 Receive Data Valid

The LXT971A asserts RX_DV to indicate that the received data maps to valid symbols. However,

RXD outputs zeros until the received data is decoded and available for transfer to the controller.

3.7.3.3

Twisted-Pair PMD Sublayer

The twisted-pair Physical Medium Dependent (PMD) layer provides the signal scrambling and de-

scrambling, line coding and decoding (MLT-3 for 100BASE-TX, Manchester for 10BASE-T), as

well as receiving, polarity correction, and baseline wander correction functions.

Scrambler/De-scrambler

The purpose of the scrambler is to spread the signal power spectrum and further reduce EMI using

an 11-bit, data-independent polynomial. The receiver automatically decodes the polynomial

whenever IDLE symbols are received.

Scrambler Seeding

Once the transmit data (or Idle symbols) are properly encoded, they are scrambled to further reduce

EMI and to spread the power spectrum using an 11-bit scrambler seed. Five seed bits are

determined by the PHY address, and the remaining bits are hard coded in the design.

Scrambler Bypass

The scrambler/de-scrambler can be bypassed by setting Register bit 16.12 = 1. The scrambler is

automatically bypassed when the fiber port is enabled. Scrambler bypass is provided for diagnostic

and test support.

3.7.3.3.1 Baseline Wander Correction

The LXT971A provides a baseline wander correction function which makes the device robust

under all network operating conditions. The MLT3 coding scheme used in 100BASE-TX is by

definition “unbalanced”. This means that the average value of the signal voltage can “wander”

significantly over short time intervals (tenths of seconds). This wander can cause receiver errors at

long-line lengths (100 meters) in less robust designs. Exact characteristics of the wander are

completely data dependent.

The LXT971A baseline wander correction characteristics allow the device to recover error-free

data while receiving worst-case “killer” packets over all cable lengths.

3.7.3.3.2 Polarity Correction

The 100BASE-TX de-scrambler automatically detects and corrects for the condition where the

receive signal at TPFIP and TPFIN is inverted.

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