LAN8820 Datasheet, PDF (29/83 Pages) Microchip Technology – RGMII 10/100/1000 Ethernet Transceiver

LAN8820/LAN8820I
The device also provides an advanced Auto Link Breaker feature (only for 1000BASE-T links). Using this feature, the
link can be taken down if the bit error rate (BER) exceeds the threshold defined in Link Break Threshold field of the Link
Control Register. The error counting occurs during the idle time for a period commensurate with the specified BER. The
Auto Link Breaker feature is enabled via the Link Break Enable bit of the Link Control Register.
3.9.4.3 100BASE-TX
For 100BASE-TX links, the device and its link partner begin transmitting idle symbols after completion of the auto-nego-
tiation process. Each side continues sending idle symbols whenever it has no data to transmit. The link is maintained
as long as valid idle symbols or data are received.
3.9.4.4 10BASE-T
For 10BASE-T links, the device and its link partner begin exchanging normal link pulses (NLPs) after completion of the
auto-negotiation process. The device transmits an NLP every 16ms and expects to receive an NLP every 8 to 24ms. A
link is maintained as long as NLPs are received.
3.9.4.5 Taking Down a Link
The device takes down an established link when the required conditions are no longer met. When a link is down, data
transmission stops. For 10BASE-T links, the link is taken down after NLPs are no longer received. For 100BASE-TX
and 1000BASE-T links, the link is taken down after valid idle codes are no longer received.
After a link is down, the device does the following:
• If auto-negotiation is enabled, the device re-enters the auto-negotiation phase and begins transmitting FLPs
• If auto-negotiation is not enabled, the device transmits NLPs in 10BASE-T mode, and MLT-3s in 100BASE-TX
mode.
3.9.5 SPEED OPTIMIZER
The Speed Optimize function is designed to resolve the issue of using auto-negotiation to establish a link on impaired
cable plants.
Examples of impaired cable plants for 1000BASE-T (Gigabit) connections include:
• Channel 2 twisted pair cable plant is broken
• Channel 3 twisted pair cable plant is broken
• Channel 2 and 3 twisted pair cable plants are broken
• Cable plant is too long
Examples of impaired cable plants for 100BASE-TX connections include:
• Cable plant is too long
• Using wrong cable plant (such as CAT-3)
The Speed Optimize function requires the MAC to support 1000/100/10 Mbps speeds, 1000/100 Mbps speeds, 1000/
10 Mbps or 100/10 Mbps speeds.
If a link fails to establish after the link partners go through auto-negotiation several times at the HCD (Highest Common
Denominator), the device advertises the next highest-allowable speed (as set in the Auto Negotiation Advertisement
Register) and restarts auto-negotiation with the new speed.
When 1000BASE-T is advertised, the Speed Optimize function can change its advertised speed from 1000BASE-T to
100BASE-TX and from 100BASE-TX to 10BASE-T. When 100BASE-TX is advertised, the Speed Optimize function can
change its advertised speed from 100BASE-TX to 10BASE-T. If a previous link has used the Speed Optimize function
to establish a link, when the link goes down, the device begins advertising with all capable speeds.
The Speed Optimize function resets itself to advertise HCD/all speed capabilities after any of the following occurrences:
• Hardware reset
• Software reset
• While link partners exchange link pulses through the Speed Optimize process, the device does not receive link
pulses for a period of few seconds
• After an established link goes down
The Speed Optimize function is enabled via Speed Optimize Enable bit in the 10/100 Mode Control/Status Register.
When a link (with a speed slower than HCD) is being established through the Speed Optimize process, it is reported via
the Speed Optimize Status bit in the User Status 2 Register.
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DS00001871B-page 29