LXT971ABC datasheet(26/90 Pages) INTEL

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LXT971A 3.3V Dual-Speed Fast Ethernet Transceiver

Datasheet

Document #: 249414

Revision #: 002

Rev. Date: August 7, 2002

3.3

Operating Requirements

3.3.1

Power Requirements

The LXT971A requires three power supply inputs (VCCD, VCCA, and VCCIO). The digital and

analog circuits require 3.3V supplies (VCCD and VCCA). These inputs may be supplied from a

single source. Each supply input must be de-coupled to ground.

An additional supply may be used for the MII (VCCIO). The supply may be either +2.5V or

+3.3V. Also, the inputs on the MII interface are tolerant to 5V signals from the controller on the

other side of the MII interface. Refer to Table 20 on page 57 for MII I/O characteristics.

As a matter of good practice, these supplies should be as clean as possible.

3.3.2

Clock Requirements

3.3.2.1

External Crystal/Oscillator

The LXT971A requires a reference clock input that is used to generate transmit signals and recover

receive signals. It may be provided by either of two methods: by connecting a crystal across the

oscillator pins (XI and XO), or by connecting an external clock source to pin XI. The connection of

a clock source to the XI pin requires the XO pin to be left open. A crystal-based clock is

recommended over a derived clock (i.e., PLL-based) to minimize transmit jitter. Refer to the

LXT971A/972A Design and Layout Guide for a list of recommended clock sources.

A crystal is typically used in NIC applications. An external 25 MHz clock source, rather than a

crystal, is frequently used in switch applications. Refer to Table 21 on page 57 for clock timing

requirements.

3.3.2.2

MDIO Clock

The MII management channel (MDIO) also requires an external clock. The managed data clock

(MDC) speed is a maximum of 8 MHz. Refer to Table 38 on page 69 for details.

3.4

Initialization

When the LXT971A is first powered on, reset, or encounters a link failure state, it checks the

MDIO register configuration bits to determine the line speed and operating conditions to use for

the network link. The configuration bits may be set by the Hardware Control or MDIO interface as

shown in Figure 7.

3.4.1

MDIO Control Mode

In the MDIO Control mode, the LXT971A reads the Hardware Control Interface pins to set the

initial (default) values of the MDIO registers. Once the initial values are set, bit control reverts to

the MDIO interface.

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