Altera Low Latency Ethernet 10G MAC - Manual (Page 29)

An underflow could occur on the Avalon-ST 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.
Inter-Packet Gap Generation and Insertion
The MAC TX maintains an average IPG between TX frames as required by the IEEE 802.3 Ethernet
standard. The average IPG is maintained at 96 bit times (12 byte times) using the deficit idle count (DIC).
The MAC TX inserts or deletes idle bytes depending on the value of the DIC; the DIC must be between 9
to 15 bytes. Averaging the IPG ensures that the MAC utilizes the maximum available bandwidth.
XGMII Transmission
On the XGMII, the MAC TX performs the following:
• Aligns the first byte of the frame to lane 0 of the interface.
• Performs endian conversion. Transmit frames received from the client on the Avalon-ST interface are
big endian. Frames transmitted on the XGMII are little endian; the MAC TX therefore transmits
frames on this interface from the least significant byte.
The following figure shows the timing on the Avalon-ST TX data interface and XGMII. The least signifi‐
cant byte of the value in D5 is transmitted first on the XGMII.
UG-01144
2014.12.15
Inter-Packet Gap Generation and Insertion 3-7
Functional Description of LL Ethernet 10G MAC Altera Corporation
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