VSC8540RT datasheet(22/87 Pages) MICROCHIP

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VSC8540RT

DS60001603E-page 22

 2022 Microchip Technology Inc. and its subsidiaries

configuration. Auto-negotiation also enables a connected MAC to communicate with its link partner MAC through

the VSC8540RT device using optional next pages to set attributes that may not otherwise be defined by the

IEEE standard.

If the Category 5 (Cat5) link partner does not support auto-negotiation, the VSC8540RT device automatically uses par-

allel detection to select the appropriate link speed.

Auto-negotiation is disabled by clearing register 0, bit 12. When auto-negotiation is disabled, the state of register bits

0.6, 0.13, and 0.8 determine the device operating speed and duplex mode.

5.4.3

AUTOMATIC CROSSOVER AND POLARITY DETECTION

For trouble-free configuration and management of Ethernet links, the VSC8540RT device includes a robust automatic

crossover detection feature for allspeeds on the twisted pair interface. Known as HP Auto-MDIX, the function is fully

compliant with Clause 40 of IEEE 802.3-2008.

Additionally, the device detects and corrects polarity errors on all MDI pairs—a useful capability that exceeds the

requirements of the standard.

Both HP Auto-MDIX detection and polarity correction are enabled in the device by default. Default settings can be

changed using device register bits 18.5:4. Status bits for each of these functions are located in register 28.

Note:

The VSC8540RT device can be configured to perform HP Auto-MDIX, even when auto-negotiation is dis-

abled and the link is forced into 10/100 speeds. To enable this feature, set register 18.7 to 0. To use the

feature, also set register 0.12 to 0.

The HP Auto-MDIX algorithm successfully detects, corrects, and operates with any of the MDI wiring pair combinations

listed in the following table, which shows that twisted pair A is connected to the RJ45 connector 1, 2 in normal MDI mode.

5.4.4

MANUAL MDI/MDIX SETTING

As an alternative to HP Auto-MDIX detection, the PHY can be forced to be MDI or MDI-X using register 19E1, bits 3:2.

Setting these bits to 10 forces MDI and setting 11 forces MDI-X. Leaving the bits 00 enables the HP Auto-MDIX setting

to be based on register 18, bits 7 and 5.

5.4.5

ENERGY-EFFICIENT ETHERNET

The VSC8540RT device supports the IEEE 802.3az-2010 Energy-Efficient Ethernet standard to provide a method for

reducing power consumption on an Ethernet link during times of low utilization. It uses low power idles (LPI) to achieve

this objective.

FIGURE 5-7:

LOW POWER IDLE OPERATION

Using LPI, the usage model for the link is to transmit data as fast as possible and then return to a low power idle state.

Energy is saved on the link by cycling between active and low power idle states. During LPI, power is reduced by turning

off unused circuits and using this method, energy use scales with bandwidth utilization. The VSC8540RT device uses

LPI to optimize power dissipation in 100BASE-TXmode.

TABLE 5-11:

SUPPORTED MDI PAIR COMBINATIONS

RJ45 Connections

1, 23, 6

4, 57, 8

Mode

Normal MDI

Normal MDI-X

Active

Quiet

Low Power Idle

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