Triple-Speed Ethernet Intel® FPGA IP User Guide

ID 683402
Date 5/30/2022
Public
Document Table of Contents

4.1.3. MAC Transmit Datapath

On the transmit path, the MAC function accepts frames from a user application and constructs Ethernet frames before forwarding them to the PHY. Depending on the MAC configuration, the MAC function realigns the payload, modifies the source address, calculates and appends the 32-bit Cyclic Redundancy Check (CRC-32) field, and inserts interpacket gap (IPG) bytes. In half-duplex mode, the MAC function also detects collision and attempts to retransmit frames when a collision occurs. The following conditions trigger transmission:
  • In MAC variations with internal FIFO buffers:
    • Cut-through mode—transmission starts when the level of the FIFO level hits the transmit section-full threshold.
    • Store and forward mode—transmission starts when a full packet is received.
      Note: To use the internal FIFO buffer in the store and forward mode, ensure that the internal FIFO buffer size is larger than the transmitted packet size. If the transmitted packet size is larger than the internal FIFO buffer size, the Triple-Speed Ethernet Intel® FPGA IP locks up with no packet transmission. The lock-up of the IP can only be cleared through a hard reset to the IP.
  • In MAC variations without internal FIFO buffers, transmission starts as soon as data is available on the Avalon® streaming transmit interface.

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