LAN8720AI datasheet(40/79 Pages) SMSC

40 / 79 page

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

Datasheet

Revision 1.4 (08-23-12)

SMSC LAN8720A/LAN8720Ai

DATASHEET

3.8.7

Link Integrity Test

The device performs the link integrity test as outlined in the IEEE 802.3u (Clause 24-15) Link Monitor

state diagram. The link status is multiplexed with the 10Mbps link status to form the Link Status bit in

the Basic Status Register and to drive the LINK LED (LED1).

The DSP indicates a valid MLT-3 waveform present on the RXP and RXN signals as defined by the

ANSI X3.263 TP-PMD standard, to the Link Monitor state-machine, using the internal DATA_VALID

signal. When DATA_VALID is asserted, the control logic moves into a Link-Ready state and waits for

an enable from the auto-negotiation block. When received, the Link-Up state is entered, and the

Transmit and Receive logic blocks become active. Should auto-negotiation be disabled, the link

integrity logic moves immediately to the Link-Up state when the DATA_VALID is asserted.

To allow the line to stabilize, the link integrity logic will wait a minimum of 330

μsec from the time

DATA_VALID is asserted until the Link-Ready state is entered. Should the DATA_VALID input be

negated at any time, this logic will immediately negate the Link signal and enter the Link-Down state.

When the 10/100 digital block is in 10BASE-T mode, the link status is derived from the 10BASE-T

receiver logic.

3.8.8

Loopback Operation

The device may be configured for near-end loopback and far loopback. These loopback modes are

detailed in the following subsections.

3.8.8.1

Near-end Loopback

Near-end loopback mode sends the digital transmit data back out the receive data signals for testing

purposes, as indicated by the blue arrows in Figure 3.12. The near-end loopback mode is enabled by

setting the Loopback bit of the Basic Control Register to “1”. A large percentage of the digital circuitry

is operational in near-end loopback mode because data is routed through the PCS and PMA layers

into the PMD sublayer before it is looped back. The transmitters are powered down regardless of the

state of TXEN.

3.8.8.2

Far Loopback

Far loopback is a special test mode for MDI (analog) loopback as indicated by the blue arrows in

Figure 3.14. The far loopback mode is enabled by setting the FARLOOPBACK bit of the Mode

Control/Status Register to “1”. In this mode, data that is received from the link partner on the MDI is

looped back out to the link partner. The digital interface signals on the local MAC interface are isolated.

Figure 3.12 Near-end Loopback Block Diagram

SMSC

Ethernet Transceiver

10/100

Ethernet

MAC

CAT-5

XFMR

Digital

RXD

TXD

Analog

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