Datasheet

 2013-2015 Microchip Technology Inc. DS00001987A-page 51

LAN8740A/LAN8740Ai

3.8.12 LOOPBACK OPERATION

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

lowing subsections.

3.8.12.1 Near-end Loopback

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

cated by the blue arrows in Figure 3-14. 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 COL signal

will be inactive in this mode, unless Collision Test is enabled in the Basic Control Register. The transmitters are powered

down regardless of the state of TXEN. Refer to Section 5.6.3.1, "100 Mbps Internal Loopback MII Timing" for additional

loopback timing information.

3.8.12.2 Far Loopback

Far loopback is a special test mode for MDI (analog) loopback as indicated by the blue arrows in Figure 3-15. 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 3-14: NEAR-END LOOPBACK BLOCK DIAGRAM

Note: This special test mode is only available when operating in RMII mode.

FIGURE 3-15: FAR LOOPBACK BLOCK DIAGRAM

Microchip

Ethernet Transceiver

10/100

Ethernet

MAC

CAT-5

XFMR

Digital

RXD

TXD

Analog

Far-end system