88E1111 Datasheet, PDF (90/252 Pages) List of Unclassifed Manufacturers – Integrated 10/100/1000 Ultra Gigabit Ethernet Transceiver

88E1111
Integrated 10/100/1000 Ultra Gigabit Ethernet Transceiver
2.13 Downshift Feature
Without the downshift feature enabled, connecting between two Gigabit link partners requires a four-pair RJ-45
cable to establish 10, 100, or 1000 Mbps link. However, there are existing cables that have only two-pairs, which
are used to connect 10 Mbps and 100 Mbps Ethernet PHYs. With the availability of only pairs 1, 2 and 3,6, Gigabit
link partners can Auto-Negotiate to 1000 Mbps, but fail to link. The Gigabit PHY will repeatedly go through the
Auto-Negotiation but fail 1000 Mbps link and never try to link at 10 Mbps or 100 Mbps.
With the Marvell® downshift feature enabled, the 88E1111 device is able to Auto-Negotiate with another Gigabit
link partner using cable pairs 1,2 and 3,6 to downshift and link at 10 Mbps or 100 Mbps, whichever is the next
highest advertised speed common between the two Gigabit PHYs.
In the case of a three pair cable (additional pair 4,5 or 7,8 - but not both) the same downshift function for two-pair
cables applies.
By default, the downshift feature is turned off. Refer to Register 20.11:8 which describe how to enable this feature
and how to control the downshift algorithm parameters.
To enable the downshift feature, the following registers must be set:
• Register 20.8 = 1 - enables downshift
• Register 20.11:9 - sets the number of link attempts before downshifting
Doc. No. MV-S100649-00, Rev. E
Page 90
CONFIDENTIAL
Document Classification: Proprietary Information
Copyright © 2004 Marvell
November 19, 2004, Advance