LAN9250 datasheet(228/421 Pages) MICROCHIP

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LAN9250

DS00001913A-page 228

 2015 Microchip Technology Inc.

12.2.15

LOOPBACK OPERATION

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

following subsections.

12.2.15.1

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 12-6. The near-end loopback mode is enabled by setting the Loopback (PHY_LOOP-

BACK) bit of the PHY Basic Control Register (PHY_BASIC_CONTROL) 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 sub-

layer 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 Basic Control Register (PHY_BASIC_CONTROL). The transmitters are pow-

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

102

113

123

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 12-6:

NEAR-END LOOPBACK BLOCK DIAGRAM

TABLE 12-7:

MATCH CASE ESTIMATED CABLE LENGTH (CBLN) LOOKUP

10/100

Ethernet

MAC

CAT-5

XFMR

Digital

RXD

TXD

Analog

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