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RTL8208

REALTEK SINGLE CHIP OCTAL 10/100 MBPS FAST ETHERNET TRANSCEIVER

Realtek Semiconductor 

RTL8208

7.4.4 Baseline Wander Compensation

The RTL8208 is ANSI TP-PMD compliant and supports input and Base Line Wander (BLW) compensation in 100Base-TX

mode. The RTL8208 does not require external attenuation circuitry at its receive inputs, RXIP/RXIN. It accepts TP-PMD

compliant waveforms directly, requiring only a 100Ω termination and a 1:1 transformer.

BLW is the change in the average DC content, over time, of an AC coupled digital transmission over a given transmission

medium. BLW is a result from the interaction between the low frequency components of a transmitted bit stream and the

frequency response of the AC coupling component(s) within the transmission system. If the low frequency content of the digital

bit stream goes below the low frequency pole of the AC coupling transformers, then the droop characteristics of the transformers

will dominate resulting in potentially serious BLW. If BLW is not compensated for, packet loss will occur.

7.5 100Base-FX

The RTL8208 can be configured into 100Base-FX mode through SEL_TXFX[1:0] (RPT_MODE should be 0). According to the

setting of SEL_TXFX[1:0], port 7 or port 6/7 or all eight ports can be configured to 100Base-FX operation.

RPT_MODE=0

SEL_TXFX[1:0] Port 0 Port 1

2’b00

UTP UTP

2’b01

UTP UTP

2’b10

UTP UTP

2’b11

FX FX

UTP: 10Base-T/100Base-TX,

FX: 100Base-FX.

Port2

UTP

UTP

UTP

FX

Medium type

Port3 Port4

UTP UTP

UTP UTP

UTP UTP

FX FX

Port5

UTP

UTP

UTP

FX

Port6

UTP

UTP

FX

FX

Port7

UTP

FX

FX

FX

Compared to common 100Base-FX applications, the RTL8208 lacks a pair of differential SD (signal detect) signals to achieve

its link monitoring function (patent), which significantly reduces the pin count in this octal PHY.

Any of the RTL8208 transceivers may interface with an external 100Base-FX fiber optic device and receiver instead of the

magnetics module used with twisted pair cable. The differential transmit and receive data pairs will operate at PECL voltage

levels instead of those required for twisted-pair transmission. The data will be encoded using two-level NRZI instead of

three-level MLT3. The data stream is not scrambled for fiber-optic transmission.

7.5.1 Transmit Function

In 100Base-FX transmission, TXD is processed as 100Base-TX, except without scrambling, before the NRZI stage. Instead of

converting to MLT-3 signals, as in 100Base-TX, the serial data stream is driven out as NRZI PECL signals, which enter the fiber

transceiver in differential-pairs form. The fiber transceiver should be available working in a 3.3V environment. Refer to the fiber

application section for more information.

PECL DC characteristics

Parameter

PECL Input High Voltage

PECL Input Low Voltage

PECL Output High Voltage

PECL Output Low Voltage

Symbol Min

Max

Unit

Vih

Vdd-1.16 Vdd-0.88

V

Vil

Vdd-1.81 Vdd-1.47

V

Voh

Vdd-1.02

V

Vol

Vdd-1.62

V

7.5.2 Receive Function

Signals are received through PECL receiver inputs from the fiber transceiver, and directly passed to the clock recovery circuit for

data/clock recovery. The scrambler/descrambler is bypassed in 100Base-FX mode.

2003/04/04

23

Rev.1.97

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