KSZ9897R datasheet(22/190 Pages) MICROCHIP

22 / 190 page

KSZ9897R

DS00002330B-page 22

 2017 Microchip Technology Inc.

Auto-negotiation is enabled by default after power-up or hardware reset. Afterwards, auto-negotiation can be enabled

or disabled via bit 12 of the PHY Basic Control Register. If auto-negotiation is disabled, the speed is set by bits 6 and

13 of the PHY Basic Control Register, and the duplex is set by bit 8.

If the speed is changed on the fly, the link goes down and either auto-negotiation or parallel detection initiate until a

common speed between the KSZ9897R and its link partner is re-established for a link.

If link is already established and there is no change of speed on the fly, the changes (for example, duplex and pause

capabilities) will not take effect unless either auto-negotiation is restarted through bit 9 of the PHY Basic Control Regis-

ter, or a link-down to link-up transition occurs (i.e. disconnecting and reconnecting the cable).

After auto-negotiation is completed, the link status is updated in the PHY Basic Status Register, and the link partner

capabilities are updated in the PHY Auto-Negotiation Link Partner Ability Register, PHY Auto-Negotiation Expansion

Status Register, and PHY 1000BASE-T Status Register.

4.1.8

FAST LINK-UP

Link up time is normally determined by the time it takes to complete auto-negotiation. Additional time may be added by

the auto MDI/MDI-X feature. The total link up time from power-up or cable connect is typically a second or more.

Fast Link-up mode significantly reduces 100BASE-TX link-up time by disabling both auto-negotiation and auto MDI/

MDI-X, and fixing the TX and RX channels. This mode is enabled or disabled by the LED2_1 strapping option. It is not

set by registers, so fast link-up is available immediately upon power-up. Fast Link-up is available at power-up only for

100BASE-TX link speed, which is selected by strapping the LED4_0 pin high. Fast Link-up is also available for 10BASE-

Te, but this link speed must first be selected via a register write.

Fast Link-up is intended for specialized applications where both link partners are known in advance. The link must also

be known so that the fixed transmit channel of one device connects to the fixed receive channel of the other device, and

vice versa. The TX and RX channel assignments are determined by the MDI/MDI-X strapping option on LED2_0.

If a device in Fast Link-up mode is connected to a normal device (auto-negotiate and auto-MDI/MDI-X), there will be no

problems linking, but the speed advantage of Fast Link-up will not be realized.

For more information on configuration straps, refer to Section 3.2.1, "Configuration Straps," on page 16.

4.1.9

LinkMD® CABLE DIAGNOSTICS

The LinkMD® function utilizes Time Domain Reflectometry (TDR) to analyze the cabling for common cabling problems,

such as open circuits, short circuits and impedance mismatches.

LinkMD® works by sending a pulse of known amplitude and duration down the MDI or MDI-X pair, and then analyzing

the shape of the reflected signal to determine the type of fault. The time duration for the reflected signal to return pro-

vides the approximate distance to the cabling fault. The LinkMD® function processes this TDR information and presents

it as a numerical value that can be translated to a cable distance.

4.1.10

REMOTE PHY LOOPBACK

This loopback mode checks the line (differential pairs, transformer, RJ-45 connector, Ethernet cable) transmit and

receive data paths between the KSZ9897R and its Ethernet PHY link partner, and is supported for 10/100/1000 Mbps

at full-duplex.

The loopback data path is shown in Figure 4-2 and functions as follows:

• The Ethernet PHY link partner transmits data to the KSZ9897R PHY port.

• Data received at the external pins of the PHY port is looped back without passing through the MAC and internal

switch fabric.

• The same KSZ9897R PHY port transmits data back to the Ethernet PHY link partner.

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