USB Enumeration Process

The USB enumeration process identifies and configures a newly connected USB device so that the host can communicate with it. When a device is plugged in, the host controller and USB stack perform a series of control transfers to determine the device’s capabilities and assign it an address.

1. Device Connection

When a device is physically attached, the host’s port logic detects the connection (via a rise in D+ or D– line voltage) and signals the USB stack to begin enumeration.

2. Bus Reset

The host issues a Reset on the bus (holding both D+ and D– low for 10 ms) to force the device into a known state. This ensures:

  • The device’s internal state machines are reset.
  • The device responds at default address 0.

3. Get Device Descriptor (Partial)

  1. Setup Packet
    • bmRequestType: 0x80 (Device‑to‑Host, Standard, Device)
    • bRequest: 0x06 (GET_DESCRIPTOR)
    • wValue: 0x0100 (Descriptor Type: Device, Index 0)
    • wIndex: 0x0000
    • wLength: typically 8 bytes (to read the first part)
  2. Data Stage: Device returns the first 8 bytes of its Device Descriptor, which include:
    • USB version
    • Maximum packet size for endpoint 0
  3. Status Stage: Host sends an ACK.

4. Set Address

  1. Setup Packet
    • bmRequestType: 0x00 (Host‑to‑Device, Standard, Device)
    • bRequest: 0x05 (SET_ADDRESS)
    • wValue: new device address (1–127)
    • wIndex, wLength: 0x0000
  2. Status Stage: Host acknowledges; device switches from address 0 to the assigned address on the next IN.

5. Get Full Device Descriptor

With the new address in effect, the host re-issues a GET_DESCRIPTOR (as in step 3), this time requesting the full length (typically 18 bytes) so it can read:

  • Vendor ID (VID) and Product ID (PID)
  • Device class, subclass, protocol
  • Number of possible configurations

6. Get Configuration Descriptor (Header)

  1. Setup Packet
    • bmRequestType: 0x80
    • bRequest: 0x06
    • wValue: 0x0200 (Descriptor Type: Configuration, Index 0)
    • wIndex, wLength: typically 9 bytes
  2. Data Stage: Host reads the 9-byte configuration header:
    • Total length of all descriptors
    • Number of interfaces
    • Maximum power consumption

7. Get Full Configuration Descriptor

Knowing the total length, the host requests the full configuration block (including all interface and endpoint descriptors) so it can parse each interface.

8. Set Configuration

  1. Setup Packet
    • bmRequestType: 0x00
    • bRequest: 0x09 (SET_CONFIGURATION)
    • wValue: configuration value (from the header)
    • wIndex, wLength: 0x0000
  2. Status Stage: Host and device acknowledge; the device moves from the Default to the Configured state, and its endpoints (other than EP0) become active.

9. Interface and Alternate Setting (if needed)

Some devices support alternate interface settings for different bandwidth/power modes.

  1. Get Interface Descriptor
    • Similar GET_DESCRIPTOR with wValue 0x0400 (Interface, index)
  2. Set Interface
    • bRequest: 0x0B (SET_INTERFACE)
    • wValue: alternate setting number
    • wIndex: interface number

10. Endpoint Descriptors

From the full configuration descriptor (step 7), the host already knows each endpoint’s:

  • Address (direction, endpoint number)
  • Transfer type (control, interrupt, bulk, isochronous)
  • Maximum packet size
  • Polling interval (for interrupt/isochronous)

No further GET_DESCRIPTOR is required unless the host needs to re-read.

11. Driver Binding

Based on the class, subclass, protocol, VID/PID and interface descriptors:

  • The OS loads or binds the correct driver (e.g., USB mass storage, CDC-NCM, HID, etc.)
  • Any class-specific initialization is performed

12. Device Ready

With the configuration set and the driver loaded, the device is in the Configured state. The host can now issue transfers on the data endpoints to send and receive application data.