4.4.4. XGMII Encapsulation
By default, the MAC TX inserts 7-byte preamble, 1-byte SFD and 1-byte EFD (0xFD) into frames received from the client.
The MAC TX also supports custom preamble in 10G operations. To use custom preamble, set the tx_preamble_control register to 1. Behavior of the MAC TX in custom preamble mode:
- The MAC TX accepts the first eight bytes in the frame from the client as custom preamble.
- The MAC TX inserts 1-byte EFD (0xFD) into the frame.
- The MAC TX replaces the first byte of the preamble with 1-byte START (0xFB).
- The MAC TX converts the eighth byte of the preamble to a 1-byte SFD (0xD5).
An underflow could occur on the Avalon® streaming TX interface. An underflow occurs when the avalon_st_tx_valid signal is deasserted in the middle of frame transmission. When this happens, the 10GbE MAC TX inserts an error character |E| into the frame and forwards the frame to the XGMII.