LAN8820 datasheet(13/83 Pages) MICROCHIP

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DS00001871B-page 13

LAN8820/LAN8820I

3.0

FUNCTIONAL DESCRIPTION

This chapter provides functional descriptions of the various device features. These features have been categorized into

the following sections:

• Auto-negotiation

• HP Auto-MDIX

• RGMII Interface

• Serial Management Interface (SMI)

• Interrupt Management

• Resets

• Power-Down modes

• Configuration

• Miscellaneous Functions

• Application Diagrams

3.1

Auto-negotiation

The purpose of the auto-negotiation function is to automatically configure the PHY to the optimum link parameters based

on the capabilities of its link partner. Auto-negotiation is a mechanism for exchanging configuration information between

two link-partners and automatically selecting the highest performance mode of operation supported by both sides. Auto-

negotiation is fully defined in clause 28 and clause 40 of the IEEE 802.3 specification.

Once auto-negotiation has completed, information about the resolved link can be passed back to the controller via the

integrated Serial Management Interface (SMI). The results of the negotiation process are reflected in the Speed Indica-

tion field of the PHY Special Control / Status Register as well as the Auto Negotiation Link Partner Ability Register.

The advertised capabilities of the PHY are stored in Auto Negotiation Advertisement Register. The defaults advertised

by the device are determined as described in Section 3.8.1.2.2, "Configuring the Mode of Operation (CONFIG[3:2])," on

page 26.

The auto-negotiation protocol is a purely physical layer activity and proceeds independently of the MAC controller. When

enabled, auto-negotiation is started by the occurrence of one of the following events:

• Hardware reset

• Software reset

• Power-down reset

• Link status down

• Setting the Restart Auto-Negotiate bit of the Basic Control Register

On detection of one of these events, the device begins auto-negotiation by transmitting bursts of Fast Link Pulses (FLP).

The data transmitted by an FLP burst is known as a “Link Code Word.” This exchange of information allows link partners

to determine the Highest Common Ability (HCD).

Once a capability match has been determined, the link code words are repeated with the acknowledge bit set. Any dif-

ference in the main content of the link code words at this time will cause auto-negotiation to re-start. Auto-negotiation

will also re-start if all of the required FLP bursts are not received.

Writing the 100BASE-TX Full Duplex, 100BASE-TX, 10BASE-T Full Duplex, and 10BASE-T bits of the Auto Negotiation

Advertisement Register allows software control of the advertised capabilities. However, writing the Auto Negotiation

Advertisement Register does not automatically re-start auto-negotiation. The Restart Auto-Negotiate bit of the Basic

Control Register must be set before the new abilities will be advertised. Auto-negotiation can also be disabled via soft-

ware by clearing the Auto-Negotiation Enable bit of the Basic Control Register.

Auto-Negotiation also resolves the Master/Slave clocking relationship between two PHYs for a 1000BASE-T link. Refer

to Section 3.1.4, "Master/Slave," on page 14 for additional information.

3.1.1

RESTARTING AUTO-NEGOTIATION

Auto-negotiation can be restarted at any time by using the Restart Auto-Negotiate bit of the Basic Control Register.

Auto-negotiation will also re-start if the link is broken at any time. A broken link is caused by signal loss. This may occur

because of a cable break, or because of an interruption in the signal transmitted by the Link Partner. Auto-negotiation

resumes in an attempt to determine the new link configuration.

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