User's Manual

COMPANY CONFIDENTIAL
33
6. Software Features
6.1 USB Enumeration
When a USB device is attached to or removed from the USB, the host uses a process
known as bus enumeration to identify and manage the device state changes necessary.
When a USB device is attached to a powered port, the following actions are taken:
1. The hub to which the USB device is now attached informs the host of the event via a
reply on its status change pipe. At this point, the USB device is in the Powered state
and the port to which it is attached is disabled.
2. The host determines the exact nature of the change by querying the hub.
3. Now that the host knows the port to which the new device has been attached, the host
then waits for at least 100 ms to allow completion of an insertion process and for power
at the device to become stable. The host then issues a port enable and reset command
to that port.
4. The hub performs the required reset processing for that port. When the reset signal is
released, the port has been enabled. The USB device is now in the Default state and
can draw no more than 100 mA from VBUS. All of its registers and state have been
reset and it answers to the default address.
5. The host assigns a unique address to the USB device, moving the device to the
Address state.
6. Before the USB device receives a unique address, its Default Control Pipe is still
accessible via the default address. The host reads the device descriptor to determine
what actual maximum data payload size this USB device’s default pipe can use.
7. The host reads the configuration information from the device by reading each
configuration zero to n-1, where n is the number of configurations. This process may
take several milliseconds to complete.
8. Based on the configuration information and how the USB device will be used, the host
assigns a configuration value to the device. The device is now in the Configured state
and all of the endpoints in this configuration have taken on their described
characteristics. The USB device may now draw the amount of VBUS power described
in its descriptor for the selected configuration. From the device’s point of view, it is now
ready for use.
When the USB device is removed, the hub again sends a notification to the host. Detaching
a device disables the port to which it had been attached. Upon receiving the detach
notification, the host will update its local topological information.
The following diagram and table describe USB device states. Some of these states are