LXT970ATC datasheet(19/74 Pages) INTEL

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Dual-Speed Fast Ethernet Transceiver — LXT970A

Datasheet

2.2

Interfaces (Network Media/Protocol Support)

The LXT970A provides the following interfaces:

• A Twisted-Pair Interface which directly supports 100BASE-TX and 10BASE-T applications.

• A pseudo-ECL (PECL) Fiber Interface which supports 100BASE-FX applications through an

external fiber transceiver.

• An MII (Media Independent Interface) for interfacing 10/100 Media Access Controllers

(MACs).

• A Hardware Control Interface to configure various operating characteristics.

2.2.1

Twisted-Pair Interface

The Twisted-Pair Interface directly supports both 100BASE-TX and 10BASE-T applications. The

interface is capable of directly driving an RJ-45 interface through magnetics and termination

resistors. The interface uses two signal pairs - one for transmit and one for receive. A third output,

TREF, connects to the center-tap of the transmit transformer. The same signal pairs, magnetics, and

termination resistors are used for both 10 and 100 Mbps operation.

When the LXT970A is operating as a 100 Mbps device, it transmits and receives a 125 Mbps, 5B-

encoded, scrambled MLT-3 waveform on this interface. The MLT-3 waveform is continuous.

When there is no data to send, “IDLE” symbols are sent and received.

When the LXT970A is operating as a 10 Mbps device, it transmits and receives 10 Mbps

Manchester-encoded data. The waveform is not continuous. When there is no data to send, the

line is left in an idle state. Link pulses are transmitted periodically to keep the link up.

The LXT970A supports both fixed operation and auto-negotiation with parallel detection on this

interface. Fixed operation allows the designer to specify the line speed and duplex mode. With

auto-negotiation enabled, the LXT970A automatically determines line speed and duplex state by

exchanging capability “pages” with its link partner.

A 4 k

Ω passive load is always present across the twisted-pair inputs. When enabled, the twisted-

pair inputs are actively biased to approximately 2.8V.

In applications where the Twisted-Pair Interface is not used, the inputs and outputs may be left

unconnected.

The Twisted-Pair Interface is disabled in power down mode, when the Fiber Interface is selected,

or when the transmit disconnect (bit 19.0) is set. When the Twisted-Pair Interface is disabled its

outputs are tri-stated and inputs are unbiased.

2.2.2

Fiber Interface

The pseudo-ECL Fiber Interface is suitable for driving 100BASE-FX applications through an

external fiber transceiver. This interface consists of a transmit and receive pair. The LXT970A

sends and receives a continuous 125 Mbps, 5B-encoded NRZI stream on this interface.

Scrambling and MLT-3 are not used in fiber connections.

There is no industry standard for auto-negotiation on 100BASE-FX. The LXT970A only supports

forced operation on the Fiber Interface. The LXT970A does not support 10FL (10 Mbps fiber)

applications.

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