Technical data

1.3.2 100BASE-TX Operation

100BASE-TX specifies operation over two copper media: two pairs of shielded twisted-pair cable

(STP) and two pairs of unshielded twisted-pair cable (Category 5 UTP). 100BASE-TX function

includes the physical coding sub-layer (PCS), the physical medium attachment (PMA) and physical

medium dependent sub-layer (PMD).

When transmitting, 4-bit data nibbles come from the MII interface at a rate of 25 MHz and are

converted to 5-bit encoded data. The data is then serialized and scrambled, subsequently converted

to a NRZI data stream and MLT-3 encoded.

In the receive path the ADC samples the incoming MLT-3 signal at a sampling frequency of 125 MHz.

The resulting MLT-3 signal is reconverted to the NRZI data stream, and then the descrambler

performs the inverse function of the scrambler in the transmit path and parallelizes the data. The

4B/5B decoder completes the processing and supplies the data ready for transmission over the MII

interface.

This section describes the main functions of the 100BASE-TX portion of the PHYs.

<1> Full-duplex or Half-duplex mode

<2> Collision detect indication

<3> Carrier Sense detection

<4> MLT-3 to (from) NRZ Decoding/Encoding

<5> 4B/5B Encoding/Decoding

<6> Scrambler/Descrambler

<7> Adaptive Equalization(DSP)

<8> Baseline Wander Correction

<9> Timing recovery from received data

<10> Support MII, RMII and Symbol interface

(1) Timing recovery

The 125M PLL locks onto the 25 MHz reference clock and generates an internal 125 MHz clock used

to drive the 125 MHz logic and the 100BASE-TX transmitter. The PLL generates multiple phases of

the 125 MHz clock. A multiplexer, controlled by the timing unit of the DSP block, selects the optimum

phase for sampling the data. This is used as the recovered receive clock, which is then used to extract

the serial data from the received signal.

(2) Adaptive equalizer

The adaptive equalizer compensates phase and amplitude distortion caused by the physical transmis-

sion channel consisting of magnetics, connectors, and CAT-5 cable. Thus, the supported cable length

is increased.

(3) Baseline wander correction

If the DC content of the signal is such that the low-frequency components fall below the low frequency

pole of the isolation transformer, then the droop characteristics of the transformer will become signifi-

cant and baseline wander (BLW) on the received signal will result. To prevent corruption of the

received data, the PHY corrects the baseline wander effects using DSP algorithms.

Technical data subject to change Version 1.0.0