GTS Ethernet Intel® FPGA Hard IP User Guide

ID 817676
Date 8/05/2024
Public
Document Table of Contents

4.4.3.1. Connect the TX MAC Flow Control Interface

Use the i_tx_pause and i_tx_pfc input ports to send XOFF frames to the link partner. Follow the details described in the table and figure below:

Table 31.  TX MAC Flow Control InterfaceAll interface signals are clocked by the i_clk_tx clock. For 10GE/25GE channels, all interface signals are asynchronous.
Signal Name Width Description
i_tx_pause 1 bit
Assert this input to send a PAUSE XOFF frame on the Ethernet link.
Note: For 10GE/25GE channels, you should hold the i_tx_pause signal more than 205ns to get the request captured by the MAC.
i_tx_pfc[7:0] 8 bits

Assert the bits to send a PFC XOFF frame on the Ethernet link for the corresponding priority queue.

Note: For 10GE/25GE channels, you should hold the i_tx_pfc signal more than 205 ns to get the request captured by the MAC.

You must maintain this input asserted until you want the IP core to end the pause period. Refer to the following figure for an example. This signal is functional only if priority flow control is enabled.

The i_tx_pause and i_tx_pfc[7:0] ports are used to request the TX MAC transmit PAUSE and Priority Flow Control packets to the remote link partner.

Figure 35. The i_tx_pause or i_tx_pfc to Request Transmission of a Control Frame
From the above waveform, i_tx_pause/i_tx_pfc[n] are the PAUSE/PFC request ports. Each operates independently.
  • i_tx_pause requests PAUSE frames
  • i_tx_pfc[n] requests PFC frames for queue n, [where n=0 to 7], respectively.

The timing diagram shows that an XOFF frame is transmitted when a packet in-flight is completed and the i_tx_pause input is asserted. When the XOFF frame is transmitted, the ipcore starts a holdoff counter. When the holdoff counter reaches zero, if the i_tx_pause is asserted, a new XOFF frame is transmitted. This sequence continues until the i_tx_pause is removed, at which point XON frame is transmitted.

Note: The IEEE 802.3 standard prohibits using PAUSE and PFC together on the same link. If a PAUSE and PFC occur simultaneously, the PAUSE takes priority.