5220-DS02-R datasheet(16/90 Pages) BOARDCOM

16 / 90 page

BCM5220

Preliminary Data Sheet

04/20/04

Broa d c om Co rp o r a t i o n

Page 4

Multimode Transmit DAC

Document

5220-DS04-405-R

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 BCM5220 automatically compensates for baseline wander by removing the DC offset from the

input signal, and thereby significantly reduces the chance of a receive symbol error.

The baseline wander correction circuit is not required, and therefore is bypassed, in 10BASE-T and 100BASE-FX operating

modes.

MULTIMODE TRANSMIT DAC

The multimode transmit digital-to-analog converter (DAC) transmits MLT3-coded symbols in 100BASE-TX mode, NRZI-

coded symbols in 100BASE-FX mode, and Manchester-coded symbols in 10BASE-T mode. It allows programmable edge-

rate control in TX mode, which decreases unwanted high frequency signal components, thus reducing EMI. High-frequency

pre-emphasis is performed in 10BASE-T mode; no filtering is performed in 100BASE-FX mode. The transmit DAC 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.

STREAM CIPHER

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

data is 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 non-repeating sequence. The scrambler reduces peak emissions 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-groups.

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 codes within 724

µs, it becomes unlocked,

and the receive decoder is disabled. The descrambler is always forced into the unlocked state when a link failure condition

is detected.

A special node called High Speed Token Ring can be enabled. It increases the scrambler timeout from 724

µs to 5816 µs,

thus allowing frames as large as the Token Ring maximum length to be received without error.

Stream cipher scrambling/descrambling is not used in 100BASE-FX and 10BASE-T modes.

5220-DS02-R 数据表(PDF) 16 Page - Broadcom Corporation.

5220-DS02-R 数据表(HTML) 16 Page - Broadcom Corporation.

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