Conditions triggering xoff frame transmission, Conditions triggering xon frame transmission, Pause control and generation interface – Altera Low Latency 40-Gbps Ethernet MAC and PHY MegaCore Function User Manual
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Conditions Triggering XOFF Frame Transmission
The LL 40-100GbE IP core supports retransmission. In retransmission mode, the IP core retransmits a
XOFF frame periodically, extending the pause time, based on signal values.
The TX MAC transmits 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
pause control interface signal. When
pause_insert_tx
is asserted, an XOFF frame is sent to the
Ethernet network when the current frame transmission completes.
• Host (software) requests XOFF transmission—Setting the pause request register triggers a request that
an XOFF frame be sent.
• Retransmission mode—If the retransmit hold-off enable bit has the value of 1, and the
pause_insert_tx
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 XOFF transmis‐
sion, the TX MAC sends another XOFF frame to the Ethernet network. 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: Altera recommends that you use the
pause_insert_tx
signal to backpressure the remote Ethernet
node.
Note: Altera recommends that you set and maintain the value of the retransmit hold-off enable bit at 1 to
control the rate of XOFF pause frame transmission.
Conditions Triggering XON Frame Transmission
The TX MAC transmits XON frames when one of the following conditions occurs:
• Client requests XON transmission—A client can explicitly request that XON frames be sent using the
pause control interface signal. When
pause_insert_tx
is de-asserted, an XON frame is sent to the
Ethernet network when the current frame transmission completes.
• Host (software) requests XON transmission—Resetting the pause request register triggers a request
that an XON frame be sent.
Pause Control and Generation Interface
The pause control interface implements flow control as specified by the IEEE 802.3ba 2010 100G Ethernet
Standard. If you turn on priority-based flow control, the interface implements the IEEE Standard
802.1Qbb.The pause logic, upon receiving a pause packet, temporarily stops packet transmission, and can
pass the pause packets through as normal traffic or drop the pause control frames in the RX direction.
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Conditions Triggering XOFF Frame Transmission
UG-01172
2015.05.04
Altera Corporation
Functional Description