LAN8700 datasheet(23/74 Pages) MICROCHIP

23 / 74 page

 2007-2016 Microchip Technology Inc.

DS00002260A-page 23

LAN8700/LAN8700i

Writing register 4 bits [8:5] allows software control of the capabilities advertised by the PHY. Writing register 4 does not

automatically re-start auto-negotiation. Register 0, bit 9 must be set before the new abilities will be advertised. Auto-

negotiation can also be disabled via software by clearing register 0, bit 12.

The LAN8700/LAN8700i does not support “Next Page” capability.

4.7.1

PARALLEL DETECTION

If the LAN8700/LAN8700i is connected to a device lacking the ability to auto-negotiate (i.e. no FLPs are detected), it is

able to determine the speed of the link based on either 100M MLT-3 symbols or 10M Normal Link Pulses. In this case

the link is presumed to be Half Duplex per the IEEE standard. This ability is known as “Parallel Detection.” This feature

ensures interoperability with legacy link partners. If a link is formed via parallel detection, then bit 0 in register 6 is cleared

to indicate that the Link Partner is not capable of auto-negotiation. The controller has access to this information via the

management interface. If a fault occurs during parallel detection, bit 4 of register 6 is set.

Register 5 is used to store the Link Partner Ability information, which is coded in the received FLPs. If the Link Partner

is not auto-negotiation capable, then register 5 is updated after completion of parallel detection to reflect the speed capa-

bility of the Link Partner.

4.7.2

RE-STARTING AUTO-NEGOTIATION

Auto-negotiation can be re-started at any time by setting register 0, bit 9. Auto-negotiation will also re-start if the link is

broken at any time. A broken link is caused by signal loss. This may occur because of a cable break, or because of an

interruption in the signal transmitted by the Link Partner. Auto-negotiation resumes in an attempt to determine the new

link configuration.

If the management entity re-starts Auto-negotiation by writing to bit 9 of the control register, the LAN8700/LAN8700i will

respond by stopping all transmission/receiving operations. Once the break_link_timer is done, in the Auto-negotiation

state-machine (approximately 1200ms) the auto-negotiation will re-start. The Link Partner will have also dropped the

link due to lack of a received signal, so it too will resume auto-negotiation.

4.7.3

DISABLING AUTO-NEGOTIATION

Auto-negotiation can be disabled by setting register 0, bit 12 to zero. The device will then force its speed of operation

to reflect the information in register 0, bit 13 (speed) and register 0, bit 8 (duplex). The speed and duplex bits in register

0 should be ignored when auto-negotiation is enabled.

4.7.4

HALF VS. FULL DUPLEX

Half Duplex operation relies on the CSMA/CD (Carrier Sense Multiple Access / Collision Detect) protocol to handle net-

work traffic and collisions. In this mode, the carrier sense signal, CRS, responds to both transmit and receive activity.

In this mode, If data is received while the PHY is transmitting, a collision results.

In Full Duplex mode, the PHY is able to transmit and receive data simultaneously. In this mode, CRS responds only to

receive activity. The CSMA/CD protocol does not apply and collision detection is disabled.

4.8

HP Auto-MDIX

HP Auto-MDIX facilitates the use of CAT-3 (10 Base-T) or CAT-5 (100 Base-T) media UTP interconnect cable without

consideration of interface wiring scheme. If a user plugs in either a direct connect LAN cable, or a cross-over patch

cable, as shown in FIGURE 4-4: on page 24, the Microchip LAN8700/LAN8700i Auto-MDIX PHY is capable of config-

uring the TXP/TXN and RXP/RXN pins for correct transceiver operation.

The internal logic of the device detects the TX and RX pins of the connecting device. Since the RX and TX line pairs

are interchangeable, special PCB design considerations are needed to accommodate the symmetrical magnetics and

termination of an Auto-MDIX design.

The Auto-MDIX function can be disabled through an internal register.

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