LAN9252VSLASHML datasheet(140/329 Pages) MICROCHIP

140 / 329 page

LAN9252

DS00001909A-page 140

 2015 Microchip Technology Inc.

11.2.13

LOOPBACK OPERATION

The PHYs may be configured for near-end loopback and connector loopback. These loopback modes are detailed in

the following subsections.

11.2.13.1

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 11-6. The near-end loopback mode is enabled by setting the Loopback (PHY_LOOP-

BACK) bit of the PHY x Basic Control Register (PHY_BASIC_CONTROL_x) 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 Mode

(PHY_COL_TEST) is enabled in the PHY x Basic Control Register (PHY_BASIC_CONTROL_x). The transmitters are

powered down regardless of the state of the internal MII TXEN signal.

11.2.13.2

Connector Loopback

The device maintains reliable transmission over very short cables and can be tested in a connector loopback as shown

in Figure 11-7. An RJ45 loopback cable can be used to route the transmit signals from the output of the transformer

back to the receiver inputs. The loopback works at both 10 and 100 Mbps.

Note:

For a properly terminated cable (Match case), there is no reflected signal. In this case, the TDR Channel

Length field is invalid and should be ignored.

FIGURE 11-6:

NEAR-END LOOPBACK BLOCK DIAGRAM

FIGURE 11-7:

CONNECTION LOOPBACK BLOCK DIAGRAM

10/100

Ethernet

MAC

CAT-5

XFMR

Digital

RXD

TXD

Analog

10/100

Ethernet

MAC

XFMR

Digital

RXD

TXD

Analog

RJ45 Loopback Cable.

Created by connecting pin 1 to pin 3

and connecting pin 2 to pin 6.

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