AC101L-DS03-R datasheet(14/54 Pages) BOARDCOM

14 / 54 page

AC101L

Preliminary Data Sheet

8/9/04

B roa dcom Co rpo rat ion

Page 4

Baseline Wander Correction

Document

AC101L-DS06-R

TX_CLK. This provides a continuously running RX_CLK. At the PCS interface, the 5-bit data RXD[4:0] is synchronized to

the 25-MHz RX_CLK.

BASELINE WANDER CORRECTION

A 100BASE-TX data stream is not always DC-balanced. Because the receive signal must pass through a transformer, the

DC offset of the differential receive input can wander. This effect, known as baseline wander, can greatly reduce the noise

immunity of the receiver. The 100BASE-TX PHY automatically compensates for baseline wander by removing the DC offset

from the input signal, thereby significantly reducing the chance of a receive symbol error.

The baseline wander circuit is not required in 100BASE-FX PHY operation.

MULTIMODE TRANSMITTER

The multimode transmitter transmits MLT3 coded symbols in 100BASE-TX mode, and NRZI coded symbols in 100BASE-FX

mode. It utilizes a current drive output, which is well balanced and produces very low noise transmit signals. PECL voltage

levels are produced with resistive terminations in 100BASE-FX mode.

The serialized data bypasses the scrambler and 4B/5B encoder in FX mode. The output data are NRZI PECL signals. The

PECL level signals are used to drive the fiber-optic transmitter.

STREAM CIPHER SCRAMBLER/DESCRAMBLER

In 100BASE-TX mode, the transmit data stream is scrambled to reduce radiated emissions on the twisted-pair cable. The

data are scrambled by exclusive ORing the NRZ signal with the output of an 11-bit wide Linear Feedback Shift register

(LFSR), which produces a 2047-bit nonrepeating sequence. The scrambler reduces peak emission by randomly spreading

the signal energy over the transmit frequency range and eliminating peaks at certain frequencies.

The receiver descrambles the incoming data stream by exclusive ORing it with the same sequence generated at the

transmitter. The descrambler detects the state of the transmit LFSR by looking for a sequence representing consecutive idle

codes. The descrambler locks to the scrambler state after detecting a sufficient number of consecutive idle code group.

The receiver does not attempt to decode the data stream unless the descrambler is locked. When locked, the descrambler

continuously monitors the data stream to make sure that it has not lost synchronization.

The receive data stream is expected to contain inter-packet idle periods. If the descrambler does not detect enough idle code

within 724

µs, it becomes unlocked and the receive decoder is disabled. The descrambler is always forced into the unlock

state when a link failure condition is detected.

Stream cipher descrambler is not used in the 100BASE-FX mode.

AC101L-DS03-R датащи(PDF) 14 Page - Broadcom Corporation.