LAN8742AI-CZ datasheet(22/124 Pages) MICROCHIP

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LAN8742A/LAN8742Ai

DS00001989A-page 22

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3.2

Auto-Negotiation

The purpose of the auto-negotiation function is to automatically configure the transceiver 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 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

Serial Management Interface (SMI). The results of the negotiation process are reflected in the Speed Indication bits of

the PHY Special Control/Status Register, as well as in the Auto Negotiation Link Partner Ability Register. The auto-nego-

tiation protocol is a purely physical layer activity and proceeds independently of the MAC controller.

The advertised capabilities of the transceiver are stored in the Auto Negotiation Advertisement Register. The default

advertised by the transceiver is determined by user-defined on-chip signal options.

The following blocks are activated during an auto-negotiation session:

• Auto-negotiation (digital)

• 100M ADC (analog)

• 100M PLL (analog)

• 100M equalizer/BLW/clock recovery (DSP)

• 10M SQUELCH (analog)

• 10M PLL (analog)

• 10M Transmitter (analog)

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 transceiver begins auto-negotiation by transmitting bursts of Fast Link Pulses

(FLP), which are bursts of link pulses from the 10M transmitter. They are shaped as Normal Link Pulses and can pass

uncorrupted down CAT-3 or CAT-5 cable. A Fast Link Pulse Burst consists of up to 33 pulses. The 17 odd-numbered

pulses, which are always present, frame the FLP burst. The 16 even-numbered pulses, which may be present or absent,

contain the data word being transmitted. Presence of a data pulse represents a “1”, while absence represents a “0”.

The data transmitted by an FLP burst is known as a “Link Code Word.” These are defined fully in IEEE 802.3 clause 28.

In summary, the transceiver advertises 802.3 compliance in its selector field (the first 5 bits of the Link Code Word). It

advertises its technology ability according to the bits set in the Auto Negotiation Advertisement Register.

There are 4 possible matches of the technology abilities. In the order of priority these are:

• 100M Full Duplex (Highest Priority)

• 100M Half Duplex

• 10M Full Duplex

• 10M Half Duplex (Lowest Priority)

If the full capabilities of the transceiver are advertised (100M, Full Duplex), and if the link partner is capable of 10M and

100M, then auto-negotiation selects 100M as the highest performance mode. If the link partner is capable of half and

full duplex modes, then auto-negotiation selects full duplex as the highest performance operation.

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 not all of the required FLP bursts are received.

The capabilities advertised during auto-negotiation by the transceiver are initially determined by the logic levels latched

on the MODE[2:0] configuration straps after reset completes. These configuration straps can also be used to disable

auto-negotiation on power-up. Refer to Section 3.7.2, "MODE[2:0]: Mode Configuration" for additional information.

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