What is Auto-Negotiation?

Autonegotiation is a function that automatically detects and sets the communication speed and communication method (Full/Half Duplex) of the connected device.
When devices are connected, they will detect the communication speed and method supported by the other device and set the corresponding settings.
For more information on auto-negotiation, see

• 1000 ⁄ 100 ⁄ 10BASE-T ⁄ TX standard: IEEE802.3 Section28 (Clause28)
• 1000 ⁄ 100BASE-T1 ⁄ 10BASE-T1S ⁄ T1L standard: IEEE802.3bp Section28 (Clause98)

It is stipulated in.
The auto-negotiation methods for the T/TX standard and the T1 standard are different, so this article will focus on the T1 standard.
In order for auto-negotiation to occur, both connected devices must be enabled.

When auto-negotiation is performed using the T1 standard, when PHYs with auto-negotiation enabled are connected, communication is carried out using the DME (differential Manchester encoding) method.
Both PHYs send and notify information from their own registers (registers set up for auto-negotiation: AN registers).
In addition, two different communication speed modes (High Speed Mode / Low Speed Mode) are defined for communication, and these are used in combination to set the optimal speed / communication method.

For example, if the PHY with the following configuration is connected,

• Communication speed: 100Mbps
• Communication method: Full Duplex

is set.

Please note that if the auto-negotiation setting on both PHYs is not enabled, linking will not be possible.
When using a PHY with auto-negotiation disabled, the opposing PHY must also have auto-negotiation disabled and must be set to a speed and communication method that will enable communication.

When performing auto-negotiation with a standard other than T1, the priority is as follows:

When performing auto-negotiation, a common speed and communication method is extracted and set from the settings supported by both PHYs.
In this case, the fastest communication method or full duplex will be prioritized as shown in the figure below.