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Software Initiated PAUSE Frame Transmission — Intel
®
82575EB Gigabit Ethernet Controller
324632-003 Intel
®
82575EB Gigabit Ethernet Controller
Revision: 2.1 Software Developer’s Manual and EEPROM Guide
January 2011 255
Hardware sends a PAUSE frame if it has previously sent one and the FIFO overflows even if the refresh
timer did not expire. This minimizes the amount of packets dropped if the first PAUSE frame did not
reach its target. Since the manageability receive packets use the same data path, the behavior is
identical when manageability packets are received.
Note: Transmitting Flow Control frames should only be enabled in full duplex mode per the IEEE
802.3 standard. Software should ensure that the transmission of flow control packets is
disabled when the 82575 is operating in half-duplex mode.
9.3.7 Software Initiated PAUSE Frame Transmission
The 82575 has the added capability to transmit an XOFF frame through software. This function is
accomplished by software writing a 1b to the SWXOFF bit of the Transmit Control register (TCTL). Once
this bit is set, hardware initiates the transmission of a PAUSE frame in a manner similar to that
automatically generated by hardware.
The SWXOFF bit is self clearing after the PAUSE frame has been transmitted. Note that the Flow Control
Refresh Threshold mechanism does not work in case of software-initiated flow control. As a result, it is
software’s responsibility to re-generate PAUSE frames before expiration of the pause counter at the
other partner’s end.
The state of the CTRL.TFCE bit or the negotiated flow control configuration does not affect software
generated PAUSE frame transmission.
Note: Software sends an XON frame by programming a 0b in the PAUSE timer field of the FCTTV
register. The software emission of XON packet is not allowed while the hardware flow
control mechanism is active, as both use the FCTIV registers for different purposes.
• XOFF transmission is not supported in 802.3x for half duplex links. Software should not initiate an
XOFF or XON transmission if the 82575 is configured for half duplex operation.
• When flow control is disabled, pause packets (XON/XOFF/other FC) are not detected as Flow
Control packets and can be counted in all kinds of counters (for example, multicast).
9.4 Loopback Support
The 82575 supports four types of loopback in the LAN interfaces:
• MAC Loopback (Point 1)
• Internal PHY Loopback (Point 2)
• Internal SerDes Loopback (Point 3)
• External PHY Loopback (Point 4)
By setting the 82575 to loopback mode, packets that are transmitted towards the line are looped back
to the host. The 82575 is fully functional in these modes, just not transmitting data over the lines.
Figure 28 shows the points of loopback.
Note: For more details about loopback usage and test setup, refer to the Intel® Ethernet
Controllers Loopback Modes application note.