Triple-Speed Ethernet Intel® FPGA IP User Guide

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

4.1.6.1. Receive Thresholds

Figure 16. Receive FIFO Thresholds


Table 28.  Receive Thresholds
Threshold Register Name Description
Almost empty rx_almost_empty The number of unread entries in the FIFO buffer before the buffer is empty. When the level of the FIFO buffer reaches this threshold, the MAC function asserts the ff_rx_a_empty signal. The MAC function stops reading from the FIFO buffer and subsequently stops transferring data to the user application to avoid buffer underflow.

When the MAC function detects an EOP, it transfers all data to the user application even if the number of unread entries is below this threshold.

Almost full rx_almost_full The number of unwritten entries in the FIFO buffer before the buffer is full. When the level of the FIFO buffer reaches this threshold, the MAC function asserts the ff_rx_a_full signal. If the user application is not ready to receive data (ff_rx_rdy = 0), the MAC function performs the following operations:
  • Stops writing data to the FIFO buffer.
  • Truncates received frames to avoid FIFO buffer overflow.
  • Asserts the rx_err[0] signal when the ff_rx_eop signal is asserted.
  • Marks the truncated frame invalid by setting the rx_err[3] signal to 1.

If the RX_ERR_DISC bit in the command_config register is set to 1 and the section-full (rx_section_full) threshold is set to 0, the MAC function discards frames with error received on the Avalon® streaming interface.

Section empty rx_section_empty An early indication that the FIFO buffer is getting full. When the level of the FIFO buffer hits this threshold, the MAC function generates an XOFF pause frame to indicate FIFO congestion to the remote Ethernet device. When the FIFO level goes below this threshold, the MAC function generates an XON pause frame to indicate its readiness to receive new frames. To avoid data loss, you can use this threshold as an early warning to the remote Ethernet device on the potential FIFO buffer congestion before the buffer level hits the almost-full threshold. The MAC function truncates receive frames when the buffer level hits the almost-full threshold.
Section full rx_section_full The section-full threshold indicates that there are sufficient entries in the FIFO buffer for the user application to start reading from it. The MAC function asserts the ff_rx_dsav signal when the buffer level hits this threshold.

Set this threshold to 0 to enable store and forward on the receive datapath. In the store and forward mode, the ff_rx_dsav signal remains deasserted. The MAC function asserts the ff_rx_dval signal as soon as a complete frame is written to the FIFO buffer.

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