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I'm trying to understand when a 10Base-T / 100Base-T Ethernet PHY device will signal a valid link, specifically by setting the Link Status bit in the Status register, in cases where autonegotiation is used.

I've been looking at the STCube ethernet drivers, and they wait for valid link status before enabling autonegotation and waiting for it to complete. But if autonegotiation is disabled, how can a valid link be established if the device at the other end of the link is set to a different speed or duplex? Or does link status simply indicate the presence of NLPs or Idle signals?

According to 802.3 (22.2.4.2.13) which describes the Link Status bit:

The criteria for determining link validity is PHY specific.

I tried looking at datasheets for a few PHYs, but couldn't find any further information

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The NIC signals a link, after potential autonegotiation, when the actual physical layer is "up". The NIC implements the "Link Integrity Test" to signal the link state to its driver.

See IEEE 802.3 14.2 for details.

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