ST802RT1B datasheet(45/58 Pages) STMICROELECTRONICS

45 / 58 page

ST802RT1A, ST802RT1B

Device operation

Doc ID 17049 Rev 1

45/58

7.18

FX mode operation

Each port of the ST802RT1x may also be configured for 100BASE-FX transmission over

fiber optics via a pseudo-ECL (PECL) interface.

In 100Base-Fx mode, scrambling and MLT3-to-binary conversion are bypassed when

transmitting, whereas in reception adaptive equalization, binary-to-MLT3 and descrambling

are bypassed.

IEEE standard auto-negotiation functions are also supported unless the device operates

in100Base-Fx mode. When operating in 100Base-Fx the device supports FEF (far-end-fault)

logic to communicate remote fault detection.

The ST802RT1x provides a pseudo-ECL interface suitable for driving a fiber optic interface.

Fiber ports cannot be enabled by auto-negotiation but only either by hardware or through

the MDIO interface. When 100BASE FX is enabled, pins SD+ and SD- indicate the signal

quality status on the fiber optic link. 100 BASE FX mode is automatically selected whenever

a valid differential signal is detected at the SD+ and SD- inputs; when SD+ and SD- are tied

low or left unconnected, the respective PHY is forced in base T mode.

The data flow for 100BASE FX is:

Serialized data -> NRZI encoding -> multimode DAC -> PECL format.

To allow the detection of remote fault conditions in 100BASE FX, the IEEE 802.3 standard

far-end-fault is implemented as in the IEEE 802.3u standard, Clause 24 (24.3.2.1); by

default FEF is on. When FEF is on, a PHY transmits a FEF indication whenever a receive

channel failure is detected and also the PHY continuously monitors the receive channel

when a valid signal is present. When its link partner is indicating a remote error, the PHY

forces its link monitor into the “link fail” state, setting the remote fault bit in the status register

(RN01).

In 100BASE-FX mode there is no scrambling function and the data is only NRZI encoded.

the multimode DAC drives the PECL levels to an external fiber optic transmitter. When there

is no transmission, the device generates “IDLE” symbols.

7.19

FX operation detect circuit

This circuit decodes the information on the status of the optical link. Particularly in the ANSI

specification, it is stated that the signal detect indicates the presence of the optical signal

with sufficient quality to correctly identify a line state.

Both signals tied to ground -> No FX mode required

SD- > SD+ -> FX mode is asserted but no data valid on the line

SD+ > SD- -> FX mode asserted. Link OK

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