LAN9500AI datasheet(75/213 Pages) MICROCHIP

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DS00001875A-page 75

LAN950x

When enabled, auto-negotiation is started by the occurrence of one of the following events:

• Hardware reset

• Software reset

• Power-down reset

• Link status down

• Setting register 0, bit 9 high (auto-negotiation restart)

On detection of one of these events, the PHY begins auto-negotiation by transmitting bursts of Fast Link Pulses (FLP).

These are bursts of link pulses from the 10M transmitter. They are shaped as Normal Link Pulses and can pass uncor-

rupted down CAT-3 or CAT-5 cable. A Fast Link Pulse Burst consists of up to 33 pulses. The 17 odd-numbered pulses,

which are always present, frame the FLP burst. The 16 even-numbered pulses, which may be present or absent, contain

the data word being transmitted. Presence of a data pulse represents a “1”, while absence represents a “0”.

The data transmitted by an FLP burst is known as a “Link Code Word.” These are defined fully in IEEE 802.3 clause 28.

In summary, the PHY advertises 802.3 compliance in its selector field (the first 5 bits of the Link Code Word). It adver-

tises its technology ability according to the bits set in register 4 of the SMI registers.

There are 4 possible matches of the technology abilities. In the order of priority these are:

• 100M full-duplex (Highest priority)

• 100M half-duplex

• 10M full-duplex

• 10M half-duplex

If the full capabilities of the PHY 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 operation.

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 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 device does not support “Next Page” capability.

5.6.6

PARALLEL DETECTION

If LAN950x 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 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 Ethernet MAC 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.

5.6.6.1

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 device will respond by

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

machine (approximately 1200 mS) 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.

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