FALXT972MCCA4 datasheet(49/92 Pages) INTEL

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Intel

® LXT972M Single-Port 10/100 Mbps PHY Transceiver

Datasheet

Document Number: 302875-005

Revision Date: 27-Oct-2005

5.7.3.3

Twisted-Pair Physical Medium Dependent Sublayer

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

scrambling functions, line coding and decoding functions (MLT-3 for 100BASE-TX, Manchester

for 10BASE-T), as well as receiving, polarity correction, and baseline wander correction functions.

5.7.3.3.1

Scrambler/Descrambler

The purpose of the scrambler/descrambler 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.

Scrambler bypass is provided for diagnostic and test support.

5.7.3.3.2

Polarity Correction

The 100 Mbps twisted pair signaling is not polarity sensitive. As a result, the polarity status is not a

valid status indicator.

5.7.3.3.3

Baseline Wander Correction

The LXT972M Transceiver provides a baseline wander correction function for when the

LXT972M Transceiver is under network operating conditions. The MLT3 coding scheme used in

100BASE-TX is by definition “unbalanced”. As a result, 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 LXT972M Transceiver baseline wander correction characteristics allow the device to recover

error-free data while receiving worst-case packets over all cable lengths.

5.7.3.3.4

Programmable Slew Rate Control

The LXT972M Transceiver device supports a programmable slew-rate mechanism whereby one of

four pre-selected slew rates can be used. (For details, see Table 56, “Transmit Control Register -

Address 30, Hex 1E” on page 89.) The slew-rate mechanism allows the designer to optimize the

output waveform to match the characteristics of the magnetics.

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