USB Enumeration

Enumeration is the process whereby the Host detects the presence of a Device and takes the necessary steps to ensure that the Device endpoints are added to the list of endpoints serviced by the Host.

Device Enumeration States


Device Detection

The presence of a newly installed Full Speed, High Speed, or Low Speed Device is recognized by changes in the D- or D+ signal. A low-speed device places 5 V on D-, high- and full-speed devices assert 5 V on D+. The connection signals are detected by the Hub and reported to the Host. Once a Device is detected, the Host issues a RESET command to the Device.

Default State

When a RESET control signal sequence is received, the Device will manage its load, per specification, to enumerate. If the attached Device is a High-Speed device, a “chirp” will be returned and the High-Speed detection process will be completed. Once the speed has been settled, the Host reads the Device descriptor and assigns an address.

Addressed State

After setting the address, the Host reads all remaining descriptor tables for the device. If a Host determines it can service the Device’s interface endpoints and provide sufficient power, the Host issues a command informing the Device which of its configurations to activate.

Configured State

After receiving notification from the Host regarding which configuration to activate, the Device is ready to run using the active configuration.

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