Datasheet

KSZ8721BL/SL

DS00002813A-page 16  2018 Microchip Technology Inc.

To ensure proper operation, the swing of ﬁber module SD should cover the threshold variation. A resistive voltage divider

is recommended to adjust the SD voltage range.

Far End Fault (FEF), repetition of a special pattern which consists of 84-one and 1-zero, is generated under “FX mode

with no signal detected.” The purpose of FEF is to notify the sender of a faulty link. When receiving an FEF, the LINK

will go down to indicate a fault, even with ﬁber signal detected. The transmitter is not affected by receiving an FEF and

still sends out its normal transmit pattern from MAC. FEF can be disabled by strapping pin 27 low. Refer to Ta b le 2- 2 .

3.22 Media Converter Operation

The KSZ8721BL/SL is capable of performing media conversion with two parts in a back-to-back RMII loop-back mode

as indicated in the diagram. Both parts are in RMII mode and with RMII BTB asserted (pins 21 and 22 strapped high).

One part is operating in TX mode and the other is operating in FX mode. Both parts can share a common 50 MHz oscil-

lator.

Under this operation, auto-negotiation on the TX side prohibits 10BASE-T link-up. Additional options can be

implemented under this operation. Disable the transmitter and set it at tri-state by controlling the high TXD2 pin. In order

to do this, RXD2 and TXD2 pins need to be connected via inverter. When TXD2 pin is high in both the copper and ﬁber

operation, it is disabled transmit. Meanwhile, the RXD2 pin on the copper side serves as the energy detect and can

indicate if a line signal is detected. TXD3 should be tied low and RXD3 let ﬂoat.

FIGURE 3-3: FIBER MODULE

TABLE 3-3: 100BT FX MODE

FXSD/FXEN Condition

Less than 0.6V 100TX mode

Less than 1.25V,

but greater than 0.6V

FX mode

No signal detected

FEF generated

Greater than 1.25

FX mode

Signal detected

+/-

Tx +/-

FTx

TxD

(Fiber Mode)

21 22

50MHz

OSC

TxC/

Ref_CLK

TxC/

Ref_CLK

RxD

KS8721BL/SL

To the SD pin of the

Fiber Module

TxD