LAN8820I datasheet(16/83 Pages) MICROCHIP

16 / 83 page

LAN8820/LAN8820I

DS00001871B-page 16

 2009-2015 Microchip Technology Inc.

Note 3-1

The internal CRS signal operates in two modes: Active and Low. When in Active mode, the internal

CRS will transition high and low upon line activity, where a high value indicates a carrier has been

detected. In Low mode, the internal CRS stays low and does not indicate carrier detection.

3.2

HP Auto-MDIX

HP Auto-MDIX facilitates the use of CAT-5 (100BASE-T) media UTP interconnect cable without consideration of inter-

face wiring scheme. If a user plugs in either a direct connect LAN cable, or a cross-over patch cable, as shown in

Figure 3-1, the Auto-MDIX PHY is capable of configuring the twisted pair pins for correct transceiver operation.

The internal logic of the device detects the TX and RX pins of the connecting device. It can automatically re-assign chan-

nel 0 and 1 if required to establish a link. In 1000BASE-T mode, it can re-assign channel 2 and 3. Crossover resolution

precedes the actual auto-negotiation process that involves exchange of FLPs to advertise capabilities. Automatic MDI/

MDIX is described in IEEE 802.3ab Clause 40, section 40.8.2. Since the RX and TX line pairs are interchangeable, spe-

cial PCB design considerations are needed to accommodate the symmetrical magnetics and termination of an Auto-

MDIX design.

Auto-MDIX is enabled by default, and can be disabled by the Auto MDIX Disable bit in the 10/100 Mode Control/Status

MDI-X 2:3 bits in the Extended Mode Control/Status Register.

The device includes an advanced crossover resolution capability called Semi Crossover. This is an extension to HP

Auto-MDIX that corrects for a cable with only two pairs crossed. If Semi Crossover is enabled, after the device has

attempted to establish a link with all four signal pairs normal or crossed, it will attempt to establish a link with pairs 2/3

switched and 0/1 straight, and then with pairs 0/1 switched and pairs 2/3 straight. The Semi Crossover is enabled by

default, and can be disabled by the Semi Crossover Enable bit in the 10/100 Mode Control/Status Register.

After resolution of crossed pairs is complete, using either HP Auto-MDIX or the Semi Crossover function, the MDI/MDI-

X status is reported through the XOVER Resolution 0:1 and XOVER Resolution 2:3 bits of the User Status 2 Register.

Auto-Negotiation

10 Mbps

Full-Duplex

Transmitting

Low

Auto-Negotiation

10 Mbps

Full-Duplex

Receiving

Active

Auto-Negotiation

100 Mbps

Half-Duplex

Transmitting

Active

Auto-Negotiation

100 Mbps

Half-Duplex

Receiving

Active

Auto-Negotiation

100 Mbps

Full-Duplex

Transmitting

Low

Auto-Negotiation

100 Mbps

Full-Duplex

Receiving

Active

TABLE 3-2:

CRS BEHAVIOR (CONTINUED)

Mode

Speed

Duplex

Activity

CRS Behavior

(

Note 3-1)

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