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Altera_Forum's avatar
Altera_Forum
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13 years ago

Ethernet autonegotiation

I am just wondering what kind of procedures are running during the link establishment in 100Base-T ethernet in case auto negotiation is disabled (i.e. forced mode) at both ends of the ethernet con...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
13 years ago

None :-)

However directly forcing the mode is generally a bad idea - it tends to end up with mismatches, some of which are hard to diagnose.

If you want to force (say) 100M FDX the best thing is to set the the PHYs auto negotiate advertise register (ANAR) with the single bit for the the desired mode.

Then the negotiation will still happen, but the outcome is guaranteed [1].

[1] Unless you connect to an old 10M hub (which doesn't do any detection) or an old dual speed 10/100M hub that uses auto-detect not auto-negotiate. Anything that will do full duplex supports auto-negotiation.