LAN8720AI datasheet(24/79 Pages) SMSC

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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet

Revision 1.4 (08-23-12)

SMSC LAN8720A/LAN8720Ai

DATASHEET

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-negotiation 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

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 difference 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," on page 31 for additional information.

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