LAN9250 datasheet(229/421 Pages) MICROCHIP

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DS00001913A-page 229

LAN9250

12.2.15.2

Connector Loopback

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

in Figure 12-7. 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.

12.2.16

100BASE-FX OPERATION

When set for 100BASE-FX operation, the scrambler and MTL-3 blocks are disable and the analog RX and TX pins are

changed to differential LVPECL pins and connect through external terminations to the external Fiber transceiver. The

differential LVPECL pins support a signal voltage range compatible with SFF (LVPECL) and SFP (reduced LVPECL)

type transceivers.

While in 100BASE-FX operation, the quality of the receive signal is provided by the external transceiver as either an

open-drain, CMOS level, Loss of Signal (SFP) or a LVPECL Signal Detect (SFF).

12.2.16.1

100BASE-FX Far End Fault Indication

Since Auto-Negotiation is not specified for 100BASE-FX, its Remote Fault capability is unavailable. Instead, 100BASE-

FX provides an optional Far-End Fault function.

When no signal is being received, the Far-End Fault feature transmits a special Far-End Fault Indication to its far-end

peer. The Far-End Fault Indication is sent only when a physical error condition is sensed on the receive channel.

The Far-End Fault Indication is comprised of three or more repeating cycles, each of 84 ONEs followed by a single

ZERO. This signal is sent in-band and is readily detectable but is constructed so as to not satisfy the 100BASE-X carrier

sense criterion.

Far-End Fault is implemented through the Far-End Fault Generate, Far-End Fault Detect, and the Link Monitor pro-

cesses. The Far-End Fault Generate process is responsible for sensing a receive channel failure (signal_status=OFF)

and transmitting the Far-End Fault Indication in response. The transmission of the Far-End Fault Indication may start or

stop at any time depending only on signal_status. The Far-End Fault Detect process continuously monitors the RX pro-

cess for the Far-End Fault Indication. Detection of the Far-End Fault Indication disables the station by causing the Link

Monitor process to de-assert link_status, which in turn causes the station to source IDLEs.

Far-End Fault is enabled by default while in 100BASE-FX mode via the Far End Fault Indication Enable (FEFI_EN) of

the PHY Special Control/Status Indication Register (PHY_SPECIAL_CONTROL_STAT_IND).

12.2.16.2

100BASE-FX Enable and LOS/SD Selection

100BASE-FX operation is enabled by the use of the FX mode strap (fx_mode_strap_1) and is reflected in the 100BASE-

FX Mode (FX_MODE) bit in the PHY Special Modes Register (PHY_SPECIAL_MODES).

Loss of Signal mode is selected by the FXLOSEN strap input pin.

FIGURE 12-7:

CONNECTION LOOPBACK BLOCK DIAGRAM

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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