5228-DS05-R datasheet(17/100 Pages) BOARDCOM

17 / 100 page

Data Sheet

BCM5228

7/12/04

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

Document

5228-DS09-405-R

Multimode Transmit DAC

Page 5

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 performs 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, and 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 in order to reduce radiated emissions on the twisted-pair cable.

The data is scrambled by exclusive ORing the NRZI 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. Signal energy is spread

further by using unique seeds to generate a different non-repeating sequence for each of the eight ports.

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. Once 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. If the receiver is put into Token Ring mode (see bit 10, register 1Bh), the descrambler

monitors the receiver for 5792 µs before unlocking. The descrambler is always forced into the unlocked state when a link

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

FAR-END FAULT

Auto-negotiation provides a Remote Fault capability for detection of asymmetric link failures. Because auto-negotiation is

not available for 100BASE-FX, the BCM5228 implements the IEEE 802.3 standard Far-End Fault mechanism for the

indication and detection of remote error conditions. If the Far-End Fault mechanism is enabled, a transceiver transmits the

Far-End Fault indication whenever a receive channel failure is detected (signal detect is deasserted). Each transceiver also

continuously monitors the receive channel when a valid signal is present (signal detect asserted). When its link partner is

indicating a remote error, the transceiver forces its link monitor into the link fail state and set the Remote Fault bit in the RMII

status register. The Far-End Fault mechanism is on by default in 100BASE-FX mode, off by default in 100BASE-TX and

10BASE-T modes, and can be controlled by software after reset.

REDUCED MEDIA INDEPENDENT INTERFACE (RMII)

The interface in the BCM5228 is based on the low pin count (Reduced) Media Independent Interface (RMII) developed by

the RMII Consortium. A copy of the specification can be found on the consortium Web site at:

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