The timing diagrams in the HX8872C datasheet are dense but essential. You will find timing requirements for:
[RJ45 + Magnetics] ---TP_TX/RX---> HX8872C ---PECL_TX/RX---> [Fiber Transceiver] | | [25MHz XTAL] [FX_SD pull-up] | | [LEDs with 330R resistors to VDD] hx8872c datasheet