Preliminary Data Sheet

12/11/01

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AC104X-DS00-405-R

Section 1: Functional Description

In 100BASE-TX mode, the PHY transmit function converts synchronous 2-bit data to a pair of 125 Mbps differential serial

data streams. The serial data is transmitted over network twisted pair cables via an isolation transformer. Data conversion

includes 4B/5B encoding, scrambling, parallel to serial, NRZ to NRZI, and MLT-3 encoding. The entire operation is

synchronous to 25 MHz and 125 MHz clock. Both clocks are generated by an on-chip PLL clock synthesizer that is locked

on to an external 25 MHz clock source.

The transmit data is transmitted from the MAC to the PHY via the TXD[1:0] signals. The 4B/5B encoder replaces the first

two nibbles of the preamble from the MAC frame with a /J/K/ code-group pair Start-of-Stream Delimiter (SSD), following the

onset of TX_EN signal. The 4B/5B encoder appends a /T/R/ code-group pair End-of-Stream Delimiter (ESD) to the end of

transmission in place of the first two IDLE code-groups that follow the negation of the TX_EN signal. The encapsulated data

stream is converted from 4-bit nibbles to 5-bit code-groups. During the inter-packet gap, when there is no data present, a

continuous stream of IDLE code-groups are transmitted. When TX_ER is asserted while TX_EN is active, the Transmit Error

code-group /H/ is substituted for the translated 5B code word. The 4B/5B encoding is bypassed when Reg. 21.1 is set to “1”,

or the PCSBP pin is strapped high.

In 100BASE-TX mode, the 5-bit transmit data stream is scrambled as defined by the TP-PMD Stream Cipher function in

order to reduce radiated emissions on the twisted pair cable. The scrambler encodes a plain text NRZ bit stream using a key

stream periodic sequence of 2047 bits generated by the recursive linear function:

X[n] = X[n-11] + X[n-9] (modulo 2)

The scrambler reduces peak emissions by randomly spreading the signal energy over the transmitted frequency range, thus

eliminating peaks at any single frequency. For repeater applications, where all ports transmit the same data simultaneously,

signal energy is spread further by using a non-repeating sequence for each PHY, i.e., the scrambled seed is unique for each

different PHY based on the PHY address.

The 5-bit NRZ data is clocked into PHY's shift register with a 25 MHz clock, and clocked out with a 125 MHz clock to convert

it into a serial bit stream. The serial data is converted from NRZ to NRZI format, which produces a transition on Logic 1 and

no transition on Logic 0. To further reduce EMI emissions, the NRZI data is converted to an MLT-3 signal. The conversion

offers a 3dB to 6dB reduction in EMI emissions. This allows system designers to meet the FCC Class B limit. Whenever

there is a transition occurring in NRZI data, there is a corresponding transition occurring in the MLT-3 data. For NRZI data,

it changes the count up/down direction after every single transition. For MLT-3 data, it changes the count up/down direction

after every two transitions. The NRZI to MLT-3 data conversion is implemented without reference to the bit timing or clock

information. The conversion requires detecting the transitions of the incoming NRZI data and setting the count up/down

direction for the MLT-3 data.

The slew rate of the transmitted MLT-3 signal can be controlled to reduce EMI emissions. The MLT-3 signal after the

magnetic has a typical rise/fall time of approximately 4 ns, which is within the target range specified in the ANSI TP- PMD

standard. This is guaranteed with either 1:1 or 1.25:1 transformer.

The 100BASE-TX receive path functions as the inverse of the transmit path. The receive path includes a receiver with

adaptive equalization and DC restoration in the front end. It also includes a MLT-3 to NRZI converter, 125 MHz data and

clock recovery, NRZI/NRZ conversion, Serial-to-Parallel conversion, de-scrambler, and 5B/4B decoder. The receiver circuit

starts with a DC bias for the differential RX+/- inputs, followed with a low-pass filter to filter out high frequency noise from the

transmission channel media. An energy detect circuit is also added to determine whether there is any signal energy on the

media. This is useful in the power-saving mode. The amplification ratio and slicer's threshold is set by the on-chip bandgap

reference.