The Serial Gigabit Media Independent Interface (SGMII) is a variant of MII, a standard interface used to connect an Ethernet MAC-block to a PHY. It is used for Gigabit Ethernet but can also carry 10/100 MBit Ethernet.
It uses differential pairs at 625 MHz clock frequency DDR for TX and RX data and TX and RX clocks. It differs from GMII by its low-power and low pin count serial 8B/10B coded interface (commonly referred to as a SerDes). Transmit and receive path each use one differential pair for data and another differential pair for clock. The TX/RX clocks must be generated on device output but are optional on device input (Clock recovery may be used alternatively). 10/100 MBit Ethernet is carried by duplicating data words 100/10 times each, so the clock is always at 625 MHz.
Read more about this topic: Media Independent Interface
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