In 100BASE-FX mode, the DP83822 receive pins connect to an industry standard fiber transceiver through a capacitively coupled circuit. During 100BASE-FX operation, the DP83822 receive path will bypass the MLT-3 decoder and scrambler. This allows for reception of serialized 4B5B encoded NRZI data at 125 MHz.
The DP83822 also has the added feature of a signal detection pin for direct connection to an industry standard fiber transceiver. When enabling 100BASE-FX operation using the FX_EN bootstrap, AMDIX_EN bootstrap turns into SD_EN bootstrap. If 100BASE-FX operation is enabled by setting FX_EN to either bootstrap mode 2 or 3, SD_EN will enable signal detection pin, LED_1, when SD_EN is set to either bootstrap mode 3 or 4. Please see Table 11 for mode information regarding hardware bootstraps.
TI recommends connecting Signal Detect pin from the Optical Transceiver to the LED_1 pin and enable it using SD_EN bootstrap pin in 100BASE-FX mode. The LED_1 pin is not used in design and that, if the electrical link between the fiber module and the DP83822 is broken, disconnected or otherwise disrupted, the link will recover only by initiating a soft reset through MDIO/MDC interface.