Pseudo-FIFO and Replication

The MAC rate can be 100 Mbps, 1 Gbps, or 10 Gbps. When the MAC rate is not the same as the SerDes link rate, the XGMII words are replicated. For example, if the MAC rate is 1 Gbps and the SerDes link rate is 10 Gbps, each XGMII 32-bit word is replicated 10 times when going into the 64/66B encoder (which always runs at the SerDes link rate).

To handle the clock domain difference between the MAC and the internal PCS, a pseudo-FIFO bridges between the host mac_tx_clk and the internal pcs_tx_clk. The host clock scales according to the MAC rate/SerDes rate ratio. For example, if the SerDes link rate is 10 Gbps and the MAC rate is 1 Gbps, the mac_tx_clk is 1/10th of the normal operating frequency.

The TX FIFO allows a standard XGMII-style interface to the host with the only change being the clock frequency.