Response

So how should we respond to the host? As our only goal is to be enumerated we'll respond with the minimum amount of information possible.

✅ First, check the request: Configuration descriptors are requested by index, not by their configuration value. Since we reported a single configuration in our device descriptor the index in the request must be zero. Any other value should be rejected by stalling the endpoint (see section Dealing with unknown requests: Stalling the endpoint for more information).

✅ Next, create and send a response: The response should consist of the configuration descriptor, followed by interface descriptors and then by (optional) endpoint descriptors. We'll include a minimal single interface descriptor in the response. Since endpoints are optional we will include none.

The configuration descriptor and one interface descriptor will be concatenated in a single packet so this response should be completed in a single DATA stage.

The configuration descriptor in the response should contain these fields:

  • bLength = 9, the size of this descriptor (must always be this value)
  • bDescriptorType = 2, configuration descriptor type (must always be this value)
  • wTotalLength = 18 = one configuration descriptor (9 bytes) and one interface descriptor (9 bytes)
  • bNumInterfaces = 1, a single interface (the minimum value)
  • bConfigurationValue = 42, any non-zero value will do
  • iConfiguration = 0, string descriptors are not supported
  • bmAttributes { self_powered: true, remote_wakeup: false }, self-powered due to the debugger connection
  • bMaxPower = 250 (500 mA), this is the maximum allowed value but any (non-zero?) value should do

The interface descriptor in the response should contain these fields:

  • bLength = 9, the size of this descriptor (must always be this value)
  • bDescriptorType = 4, interface descriptor type (must always be this value)
  • bInterfaceNumber = 0, this is the first, and only, interface
  • bAlternateSetting = 0, alternate settings are not supported
  • bNumEndpoints = 0, no endpoint associated to this interface (other than the control endpoint)
  • bInterfaceClass = bInterfaceSubClass = bInterfaceProtocol = 0, does not adhere to any specified USB interface
  • iInterface = 0, string descriptors are not supported

Again, we strongly recommend that you use the usb2::configuration::Descriptor and usb2::interface::Descriptor abstractions here. Each descriptor instance can be transformed into its byte representation using the bytes method -- the method returns an array. To concatenate both arrays you can use an stack-allocated heapless::Vec buffer. If you haven't the heapless crate before you can find example usage in the the src/bin/vec.rs file.

NOTE: the usb2::configuration::Descriptor and usb2::interface::Descriptor structs do not have bLength and bDescriptorType fields. Those fields have fixed values according to the USB spec so you cannot modify or set them. When bytes() is called on the Descriptor value the returned array, the binary representation of the descriptor, will contain those fields set to their correct value.