VSC8221 datasheet(31/138 Pages) MICROSEMI

31 / 138 page

VSC8221

Data Sheet

VMDS-10106 Revision 4.1

December 2006

31 of 31

10.4 Forcing the PHY into MDi or MDi-X mode in 10/100/1000 Link Speeds

if the Auto-MDi/MDi-X feature is disabled by setting Mii Register 18.5, or by disabling auto-negotiation, the PHY’s default

behavior is to establish a link in MDi mode. To force the PHY into MDi-X mode:

•

Write Mii Register 31 = 0x2A30

•

Write Mii Register 5 = 0x1038

•

Write Mii Register 31 = 0x0000

To force the PHY into MDi mode:

•

Write Mii Register 31 = 0x2A30

•

Write Mii Register 5 = 0x1030

•

Write Mii Register 31 = 0x0000

To disable the forcing of MDi/MDi-X mode:

•

Write Mii Register 31 = 0x2A30

•

Write Mii Register 5 = 0x1020

•

Write Mii Register 31 = 0x0000

10.5 Twisted Pair Link Speed Downshift

in addition to automatic crossover detection, the VSC8221 supports an automatic link speed “downshift” option for operation in

cabling environments incompatible with 1000BASE-T. When this feature is enabled, the VSC8221 will automatically change its

auto-negotiation advertisement to 100BASE-TX after a set number of failed attempts at 1000BASE-T. This is especially useful

in setting up networks using older cable installations which may include only pairs A and B and not pairs C and D. The link

speed downshift feature is configured and monitored through Mii Register 20E (14h) - Extended PHY Control Register #3.

10.6 100Mbps Fiber Support Over Copper Media interface

The VSC8221 supports 100BASE-FX over its copper media interface by using pairs A and B, which provide TX and RX

differential connections, respectively. if the fiber module does not have internal AC coupling capacitors, then they are required

between the PHY and fiber module. The value should be 0.1µF.

The RXLOS/SiGDET signal is not used in this mode.

A separate 1000BASE-X fiber module may be connected to the PHY through the 1000BASE-X SerDes pins.

10.6.1 Register Settings

The PHY can be brought into the 100BASE-FX operation mode using the following configuring sequence:

initialize the PHY into the specific MAC-to-copper operating mode for the MAC interface type required (register 23).

Disable auto-negotiation and force the 100BASE-T FDX mode (register 0).

Run the 100BASE-FX initialization script; for more information, see Section 31.10: "100BASE-FX initialization

Script".

Configure other settings, such as LEDs.

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