LAN8720AI datasheet(23/79 Pages) SMSC

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

Datasheet

SMSC LAN8720A/LAN8720Ai

Revision 1.4 (08-23-12)

DATASHEET

3.1.4

10BASE-T Receive

The 10BASE-T receiver gets the Manchester- encoded analog signal from the cable via the magnetics.

It recovers the receive clock from the signal and uses this clock to recover the NRZI data stream. This

10M serial data is converted to 4-bit data nibbles which are passed to the controller via MII at a rate

of 2.5MHz.

This 10M receiver uses the following blocks:

Filter and SQUELCH (analog)

10M PLL (analog)

RX 10M (digital)

MII (digital)

3.1.4.1

10M Receive Input and Squelch

The Manchester signal from the cable is fed into the transceiver (on inputs RXP and RXN) via 1:1 ratio

magnetics. It is first filtered to reduce any out-of-band noise. It then passes through a SQUELCH

circuit. The SQUELCH is a set of amplitude and timing comparators that normally reject differential

voltage levels below 300mV and detect and recognize differential voltages above 585mV.

3.1.4.2

Manchester Decoding

The output of the SQUELCH goes to the 10M RX block where it is validated as Manchester encoded

data. The polarity of the signal is also checked. If the polarity is reversed (local RXP is connected to

RXN of the remote partner and vice versa), the condition is identified and corrected. The reversed

condition is indicated by the XPOL bit of the Special Control/Status Indications Register. The 10M PLL

is locked onto the received Manchester signal, from which the 20MHz cock is generated. Using this

clock, the Manchester encoded data is extracted and converted to a 10MHz NRZI data stream. It is

then converted from serial to 4-bit wide parallel data.

The 10M RX block also detects valid 10Base-T IDLE signals - Normal Link Pulses (NLPs) - to maintain

the link.

3.1.4.3

10M Receive Data Across the RMII Interface

The 2-bit data nibbles are sent to the RMII block. These data nibbles are valid on the rising edge of

the RMII REF_CLK.

3.1.4.4

Jabber Detection

Jabber is a condition in which a station transmits for a period of time longer than the maximum

permissible packet length, usually due to a fault condition, which results in holding the TXEN input for

a long period. Special logic is used to detect the jabber state and abort the transmission to the line

within 45ms. Once TXEN is deasserted, the logic resets the jabber condition.

As shown in Section 4.2.2, "Basic Status Register," on page 50, the Jabber Detect bit indicates that a

jabber condition was detected.

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

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