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Intel
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82575EB Gigabit Ethernet Controller — Transmit Control Extended - TCTL_EXT (00404;R/
W)
Intel
®
82575EB Gigabit Ethernet Controller 324632-003
Software Developer’s Manual and EEPROM Guide Revision: 2.1
364 January 2011
14.3.65 Transmit Control Extended - TCTL_EXT
(00404;R/W)
This register controls late collision detection.
COLD is used to determine the latest time in which a collision indication is considered as a valid collision
and not a late collision. When using the internal PHY, the default value of 41h provides a behavior
consistent with the 802.3 spec requested behavior. However, when using an SGMII connected PHY, the
SGMII adds some delay on top of the time budget allowed by the specification (collisions in valid
network topographies even after 512 bit time can be expected). In order to accommodate this
condition, COLD should be updated to take the SGMII inbound and outbound delays. The delay induced
by the 82575 is 16 bit time in 10 Mb/s (add two to the COLD field value) and 40 bit time in 100 Mb/s
(add five to the COLD field value). A delay induced by the specific PHY used should also be added.
CT 11:4 0Fh Collision Threshold
This determines the number of attempts at retransmission prior to giving up on the
packet (not including the first transmission attempt). While this can be varied, it
should be set to a value of 15 in order to comply with the IEEE specification requiring
a total of 16 attempts. The Ethernet back-off algorithm
1
is implemented and clamps to
the maximum number of slot-times after 10 retries. This field only has meaning when
in half-duplex operation.
BST 21:12 3Fh Back-Off Slot Time
This value determines the back-off slot time value in byte time.
SWXOFF 22 0b Software XOFF Transmission
When set to 1b, the 82575 schedules the transmission of an XOFF (PAUSE) frame
using the current value of the PAUSE timer (FCTTV.TTV). This bit self-clears upon
transmission of the XOFF frame.
PBE 23 0b Packet Burst Enable
The 82575 does not support packet bursting for 1Gb/s half-duplex transmit operation.
This bit must be set to 0b.
RTLC 24 0b Re-transmit on Late Collision
When set, enables the 82575 to re-transmit on a late collision event.
Reserved 25 0b Reserved
Reserved 27:26 01h Reserved
Reserved 31:28 0Ah Reserved
1. The 82575’s Back-off Algorithm is not fully compliant with the 802.3 specification.
Field Bit(s)
Initial
Value
Description
Reserved 9:0 40h Reserved
COLD 19:10 42h Collision Distance
Used to determine the latest time in which a collision indication is considered as
a valid collision and not a late collision.
Reserved 31:20 0h Reserved.
Field Bit(s)
Initial
Value
Description