VSC8540RT datasheet(21/87 Pages) MICROCHIP

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DS60001603E-page 21

VSC8540RT

5.3.5

CLKOUT FREQUENCY SELECTION

The RXD6 and RXD7 pins can be pulled LOW or HIGH to set the bit 0 and bit 1 default values of the CLKOUT frequency

selection register 13G.[14:13] = RXD[7:6]. The following table describes the allowed default CLKOUT frequency set-

tings.

The CLKOUT frequency can be changed from the default setting to any of the other settings on any device variant by

modifying the register bits through the SMI interface.

5.4

Cat5 Twisted Pair Media Interface

The twisted pair interface is compliant with IEEE 802.3-2008 and the IEEE 802.3az-2010 standard for Energy-Efficient

Ethernet. In 10BASE-T and 100BASE-TX modes TX is on the Channel A for MDI and Channel B for MDI-X. Likewise,

RX is on Channel B for MDI and on Channel A for MDI-X.

5.4.1

VOLTAGE MODE LINE DRIVER

The VSC8540RT device uses a patented voltage mode line driver that allows it to fully integrate the series termination

resistors that are required to connect the PHY’s Cat5 interface to an external 1:1 transformer. The interface does not

require the user to place an external voltage on the center tap of the magnetic. The following figure illustrates the con-

nections.

FIGURE 5-6:

CAT5 MEDIA INTERFACE

5.4.2

CAT5 AUTO-NEGOTIATION AND PARALLEL DETECTION

The VSC8540RT device supports twisted pair auto-negotiation, as defined by IEEE 802.3-2008 Clause 28 and

IEEE 802.3az-2010. The auto-negotiation process evaluates the advertised capabilities of the local PHY and its link

partner to determine the best possible operating mode. In particular, auto-negotiation can determine speed and duplex

TABLE 5-10:

CLKOUT FREQUENCY SELECTION

RXD[7:6]

CLKOUT Frequency

25 MHz

50 MHz

125 MHz

Reserved

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