LAN9250 datasheet(218/421 Pages) MICROCHIP

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LAN9250

DS00001913A-page 218

 2015 Microchip Technology Inc.

If the full capabilities of the transceiver are advertised (100M, full-duplex), and if the link partner is capable of 10M and

100M, then Auto-Negotiation selects 100M as the highest performance mode. If the link partner is capable of half and

full-duplex modes, then Auto-Negotiation selects full-duplex as the highest performance mode.

Once a capability match has been determined, the link code words are repeated with the acknowledge bit set. Any dif-

ference in the main content of the link code words at this time will cause Auto-Negotiation to re-start. Auto-Negotiation

will also re-start if not all of the required FLP bursts are received.

Writing the PHY Auto-Negotiation Advertisement Register (PHY_AN_ADV) bits [8:5] allows software control of the capa-

bilities advertised by the transceiver. Writing the PHY Auto-Negotiation Advertisement Register (PHY_AN_ADV) does

not automatically re-start Auto-Negotiation. The Restart Auto-Negotiation (PHY_RST_AN) bit of the PHY Basic Control

be disabled via software by clearing the Auto-Negotiation Enable (PHY_AN) bit of the PHY Basic Control Register

(PHY_BASIC_CONTROL).

12.2.6.1

Pause Flow Control

The Host MAC is capable of generating and receiving pause flow control frames per the IEEE 802.3 specification. The

PHY’s advertised pause flow control abilities are set via the Asymmetric Pause and Symmetric Pause bits of the PHY

Auto-Negotiation Advertisement Register (PHY_AN_ADV). This allows the PHY to advertise its flow control abilities and

Auto-Negotiate the flow control settings with its link partner. The default values of these bits are determined via config-

uration straps as defined in Section 12.2.18.5, "PHY Auto-Negotiation Advertisement Register (PHY_AN_ADV)," on

page 238.

12.2.6.2

Parallel Detection

If the device is connected to a device lacking the ability to Auto-Negotiate (i.e. no FLPs are detected), it is able to deter-

mine 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 802.3 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 the Link Partner Auto-

Negotiation Able bit of the PHY Auto-Negotiation Expansion Register (PHY_AN_EXP) is cleared to indicate that the link

partner is not capable of Auto-Negotiation. If a fault occurs during parallel detection, the Parallel Detection Fault bit of

the PHY Auto-Negotiation Expansion Register (PHY_AN_EXP) is set.

The PHY Auto-Negotiation Link Partner Base Page Ability Register (PHY_AN_LP_BASE_ABILITY) 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 this register is updated after completion of parallel detection to reflect the speed capability of the link partner.

12.2.6.3

Restarting Auto-Negotiation

Auto-Negotiation can be re-started at any time by setting the Restart Auto-Negotiation (PHY_RST_AN) bit of the PHY

Basic Control Register (PHY_BASIC_CONTROL). 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 setting the Restart Auto-Negotiation (PHY_RST_AN) bit of the

PHY Basic Control Register (PHY_BASIC_CONTROL), the device will respond by stopping all transmission/receiving

operations. Once the internal break_link_time is completed in the Auto-Negotiation state-machine (approximately

1200ms), Auto-Negotiation will re-start. In this case, the link partner will have also dropped the link due to lack of a

received signal, so it too will resume Auto-Negotiation.

Auto-Negotiation is also restarted after the EEPROM Loader updates the straps.

12.2.6.4

Disabling Auto-Negotiation

Auto-Negotiation can be disabled by clearing the Auto-Negotiation Enable (PHY_AN) bit of the PHY Basic Control Reg-

ister (PHY_BASIC_CONTROL). The transceiver will then force its speed of operation to reflect the information in the

PHY Basic Control Register (PHY_BASIC_CONTROL) (Speed Select LSB (PHY_SPEED_SEL_LSB) and Duplex

Mode (PHY_DUPLEX)). These bits are ignored when Auto-Negotiation is enabled.

12.2.6.5

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. If

data is received while the transceiver is transmitting, a collision results.

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