H-Tile Hard IP Ethernet Intel® FPGA IP User Guide: Stratix® 10 Devices

ID 683430
Date 4/09/2024

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Document Table of Contents Conditions Triggering XOFF Frame Transmission

The H-Tile Hard IP for Ethernet IP core supports retransmission. In retransmission, the IP core retransmits a XOFF frame periodically, extending the pause time, based on signal values.

The TX MAC transmits PAUSE XOFF frames when one of the following conditions occurs:

  • Client requests XOFF transmission—A client can explicitly request that XOFF frames be sent using the i_tx_pause and i_tx_pfc[7:0] signals. When i_tx_pause is asserted, a PAUSE XOFF frame is sent to the Ethernet network when the current frame transmission completes. When i_tx_pfc is asserted, a PFC XOFF packet is transmitted with XOFF requests for each of the Queues that has a bit high in the signal. For example, setting i_tx_pfc to 0x03 sends XOFF requests for Queues 0 and 1.
  • Host (software) requests PAUSE XOFF transmission—Setting the pause request register triggers a request that a PAUSE XOFF frame be sent. Similarly, setting the PFC request register triggers PFC XOFF frame requests for the selected Priority Queues.
  • Retransmission mode—If the retransmit hold-off enable bit has the value of 1, and the i_tx_pause signal remains asserted or the pause request register value remains high, when the time duration specified in the hold-off quanta register has lapsed after the previous PAUSE XOFF transmission, the TX MAC sends another PAUSE XOFF frame to the Ethernet network. The same mechanism applies to PFC. While the IP core is paused in retransmission mode, you cannot use either of the other two methods to trigger a new XOFF frame: the signal or register value is already high.
Note: Intel recommends that you use the flow control ports to backpressure the remote Ethernet node.