VSC7440 datasheet(50/444 Pages) MICROSEMI

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Functional Descriptions

VMDS-10492 VSC7440 Datasheet Revision 4.2

3.8.4

Ethernet Inline Powered Devices

The integrated transceivers can detect legacy inline powered devices in Ethernet network applications.

The inline powered detection capability can be part of a system that allows for IP-phone and other

devices, such as wireless access points, to receive power directly from their Ethernet cable, similar to

office digital phones receiving power from a Private Branch Exchange (PBX) office switch over the

telephone cabling. This can eliminate the need of an external power supply for an IP-phone. It also

enables the inline powered device to remain active during a power outage (assuming the Ethernet switch

is connected to an uninterrupted power supply, battery, back-up power generator, or some other

uninterruptable power source).

The following illustration shows an example of this type of application.

Figure 13 •

Inline Powered Ethernet Switch

The following procedure describes the process that an Ethernet switch must perform to process inline

power requests made by a link partner (LP); that is, in turn, capable of receiving inline power.

Enables the inline powered device detection mode on each transceiver using its serial management

interface. Set PHY_CTRL_EXT4.INLINE_POW_DET_ENA to 1.

Ensures that the Auto-Negotiation Enable bit (register 0.12) is also set to 1. In the application, the

device sends a special Fast Link Pulse (FLP) signal to the LP. Reading

PHY_CTRL_EXT4.INLINE_POW_DET_STAT returns 00 during the search for devices that require

Power-over-Ethernet (PoE).

The transceiver monitors its inputs for the FLP signal looped back by the LP. An LP capable of

receiving PoE loops back the FLP pulses when the LP is in a powered down state. This is reported

when PHY_CTRL_EXT4.INLINE_POW_DET_STAT reads back 01. If an LP device does not loop

back the FLP after a specific time, PHY_CTRL_EXT4.INLINE_POW_DET_STAT automatically

resets to 10.

If the transceiver reports that the LP needs PoE, the Ethernet switch must enable inline power on

this port, externally of the PHY.

The PHY automatically disables inline powered device detection if

PHY_CTRL_EXT4.INLINE_POW_DET_STAT automatically resets to 10, and then automatically

changes to its normal auto-negotiation process. A link is then auto-negotiated and established when

the link status bit is set (PHY_STAT.LINK_STAT is set to 1).

Control

PHY_port1

Inline,

Power-Over-Ethernet (PoE)

Power Supply

Cat5

PHY_port0

PHY_portn

Transformer

RJ-45

I/F

Transformer

RJ-45

I/F

Transformer

RJ-45

I/F

Link

Partner

Link

Partner

Link

Partner

Gigabit Switch/PHY

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