Memos From the Cube » Blog Archive » USB Device Descriptors - The Diigo Meta page

• 23 Apr 09
  

  York Jong

  There's been a bit of confusion concerning the USB Device Descriptors, so I thought I'd do another informational post to clear things up a bit.

  USB descriptor

  • The Device Descriptor is mandatory, and only one is allowed for each USB device.
  • Configurations are mutually exclusive to one-another; the host may select any one of the configurations at any one time, but cannot use multiple configurations at once.
  • like the Configuration descriptor each interface can have multiple entries which can change the interface’s function depending on which one is selected.
  • even if a device’s features is not selected, bandwidth is still used by the host for checking the interface’s endpoints. Thus, for audio interfaces (which use up a lot of data), an alternate setting with no endpoints is added so that when the microphone is unused, the host can turn off the endpoint and save on USB bandwidth.
  • Address 0 is reserved, and is used for the control endpoint that is present on all USB devices for host-device control requests. Endpoint 0 does not need an explicit Endpoint Descriptor — it is mandatory and thus does not need to be described

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