Triple-Speed Ethernet Intel® FPGA IP User Guide

ID 683402
Date 5/30/2022
Document Table of Contents Transmit FIFO Buffer Underflow

If the transmit FIFO buffer hits the almost-empty threshold during transmission and the FIFO buffer does not contain the end-of-packet indication, the MAC function stops reading data from the FIFO buffer and initiates the following actions:
  1. The MAC function asserts the RGMII/GMII/MII error signals (tx_control/gm_tx_err/m_tx_err) to indicate that the fragment transferred is not valid.
  2. The MAC function deasserts the RGMII/GMII/MII transmit enable signals (tx_control/gm_tx_en/m_tx_en) to terminate the frame transmission.
  3. After the underflow, the user application completes the frame transmission.
  4. The transmitter control discards any new data in the FIFO buffer until the end of frame is reached.
  5. The MAC function starts to transfer data on the RGMII/GMII/MII when the user application sends a new frame with an SOP.
Figure 18. Transmit FIFO Buffer UnderflowFigure illustrates the FIFO buffer underflow protection algorithm for gigabit Ethernet system.

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