Overview

Enumeration is the process by which the host discovers and configures a newly attached USB device. It occurs automatically when a device is plugged in and involves reading descriptors, assigning an address, and loading the appropriate driver.

Enumeration Steps

When a device is connected:

1. Device Attachment and Reset

• The hub detects a change on its downstream port (electrical signaling).
• The hub reports the port status change to the host via an Interrupt IN transfer.
• The host issues a port reset to the hub port.
• After reset, the device operates at its default address (0) and is in the Default state.

2. Read Device Descriptor (Partial)

• The host sends a GET_DESCRIPTOR request to address 0, endpoint 0.
• Initially, only the first 8 bytes of the Device Descriptor are read.
• This reveals the maximum packet size for endpoint 0 (bMaxPacketSize0).

3. Assign Unique Address

• The host issues a SET_ADDRESS request with a unique address (1–127).
• The device transitions to the Address state.
• All subsequent communication uses this new address.

4. Read Full Device Descriptor

• The host reads the complete 18-byte Device Descriptor.
• Key fields retrieved: VID, PID, device class, number of configurations.

5. Read Configuration Descriptors

• The host sends GET_DESCRIPTOR for each configuration.
• The response includes the Configuration Descriptor and all subordinate descriptors:
  • Interface Descriptors
  • Endpoint Descriptors
  • Class-specific descriptors (if any)

6. Select Configuration

• The host sends a SET_CONFIGURATION request.
• The device enters the Configured state and is ready for normal use.

7. Driver Loading

• The host identifies the device class from the descriptors.
• The appropriate class driver is loaded.
• The driver may perform additional class-specific initialization.

Example: USB Keyboard

Descriptor structure:

Device Descriptor
└── Configuration Descriptor
    └── Interface Descriptor (HID)
        ├── HID Descriptor
        └── Endpoint Descriptor (Interrupt IN)

1. Host detects the HID class (bInterfaceClass = 0x03)
2. Loads the built-in HID keyboard driver
3. Driver sets the idle rate and report protocol via class-specific requests

Device States During Enumeration

Host Software Responsibilities

After reading descriptors, the host:

• Allocates memory structures for each endpoint
• Creates communication pipes (logical connections between host buffer and device endpoint)
• Loads the correct driver
• Configures the device for normal operation