VSC8221 datasheet(50/138 Pages) MICROSEMI

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VSC8221

Data Sheet

VMDS-10106 Revision 4.1

December 2006

50 of 50

16.3 in-Line Powered Device Detection (Cisco Method)

This section describes the flow process an Ethernet switch must perform in order to process in-line power requests made by a

link partner (LP) capable of receiving in-line power.

The in-line powered device detection mode is enabled by setting Mii Register bit 23E.10 = 1 and ensuring that the

Auto-Negotiation Enable Bit is set (Mii Register 0.12 = 1). An interrupt can also be asserted on the MDiNT pin when in-

line power is needed. This is set by Mii Register 25.9 = 1 and ensuring Mii Register 25.15 = 1 in order to enable the

MDiNT pin.

The PHY will then start sending a special Fast Link Pulse (FLP) signal to the LP. Mii Register 23E.9:8 will equal 00

during the search for devices needing in-line power.

The PHY monitors for the special FLP signal looped back by the LP. An LP device capable of receiving in-line power

will loopback the special FLP pulses when it is in a powered-down state. This is reported when Mii Register 23E.9:8 =

01. if enabled, an interrupt on the MDiNT pin will also be asserted. This can be verified as an in-line power detection

interrupt by reading Mii Register 26.9 = 1, which will subsequently be cleared and the interrupt de-asserted after the

read. if an LP device does not loopback the special FLP after a specific time, then Mii Register 23E.9:8 = 10.

if the PHY reports that the LP needs in-line power, then the Ethernet switch needs to enable in-line power to this port

external of the PHY.

The PHY automatically disables in-line powered device detection after Event #3 above and now changes to the normal

Auto-negotiation process. A link is then auto-negotiated and established when the link status register is set (Mii Regis-

ter bit 1.2 = 1).

in a link down event (Mii Register bit 1.2 = 0), the in-line power should be disabled to the in-line powered device exter-

nal of the PHY. The PHY will disable the normal auto-negotiation process and re-enable in-line powered device detec-

tion mode.

16.4 iEEE 802.3af (DTE Power via MDi)

The VSC8221 is fully compatible with switch designs used in systems that supply power to DTE (Data Terminal Equipment) by

means of a MDi (Media Dependent interface, or twisted pair cable), as specified by iEEE 802.3af standard (Clause 33).

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