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nrf52-usb-data-stage.md

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USB-3: DATA Stage

The next step is to respond to the GET_DESCRIPTOR request for our device descriptor, with an actual device descriptor that describes our USB Device.

Handle the request

✅ Open the nrf52-code/usb-app/src/bin/usb-3.rs file

Part of this response is already implemented. We'll go through this.

We'll use the dk::usb::Ep0In abstraction. An instance of it is available in the board value (inside the #[init] function). The first step is to make this Ep0In instance available to the on_event function.

The Ep0In API has two methods: start and end. start is used to start a DATA stage; this method takes a slice of bytes ([u8]) as argument; this argument is the response data. The end method needs to be called after start, when the EP0DATADONE event is raised, to complete the control transfer. Ep0In will automatically issue the STATUS stage that must follow the DATA stage.

✅ Handle the EP0DATADONE event

Do this by calling the end method on the EP0In instance.

✅ Implement the response to the GET_DESCRIPTOR request for device descriptors.

Extend nrf52-code/usb-app/src/bin/usb-3.rs so that it uses Ep0In to respond to the GET_DESCRIPTOR request (but only for device descriptors - no other kind of descriptor).

Values of the device descriptor

The raw values you need to pack into the descriptor are as follows. Note, we won't be doing this by hand, so read on before you start typing!

  • bLength = 18, the size of the descriptor (must always be this value)
  • bDescriptorType = 1, device descriptor type (must always be this value)
  • bDeviceClass = bDeviceSubClass = bDeviceProtocol = 0, these are unimportant for enumeration
  • bMaxPacketSize0 = 64, this is the most performant option (minimizes exchanges between the device and the host) and it's assumed by the Ep0In abstraction
  • idVendor = consts::VID, our example's USB Vendor ID (*)
  • idProduct = consts::PID, our example's USB Product ID (*)
  • bcdDevice = 0x0100, this means version 1.0 but any value should do
  • iManufacturer = iProduct = iSerialNumber = None, string descriptors not supported
  • bNumConfigurations = 1, must be at least 1 so this is the minimum value

(*) the consts crate refers to the crate in the nrf52-code/consts folder. It is already part of the usb-app crate dependencies.

Use the usb2::device::Descriptor abstraction

Although you can create the device descriptor by hand as an array filled with magic values we strongly recommend you use the usb2::device::Descriptor abstraction. The crate is already in the dependency list of the project; browse to the usb2 crate in the cargo doc output you opened earlier.

The length of the device descriptor

The usb2::device::Descriptor struct does 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.

The device descriptor is 18 bytes long but the host may ask for fewer bytes (see wlength field in the SETUP data). In that case you must respond with the amount of bytes the host asked for. The opposite may also happen: wlength may be larger than the size of the device descriptor; in this case your answer must be 18 bytes long (do not pad the response with zeroes).

Expected log output

Once you have successfully responded to the GET_DESCRIPTOR Device request you should get logs like these (if you are logging like our solution does):

USB: UsbReset @ Duration { secs: 0, nanos: 211334227 }
USB: UsbEp0Setup @ Duration { secs: 0, nanos: 252380370 }
SETUP: bmrequesttype: 0, brequest: 5, wlength: 0, windex: 0, wvalue: 52
USB: UsbEp0Setup @ Duration { secs: 0, nanos: 254577635 }
SETUP: bmrequesttype: 128, brequest: 6, wlength: 8, windex: 0, wvalue: 256
GET_DESCRIPTOR Device [length=8]
EP0IN: start 8B transfer
USB: UsbEp0DataDone @ Duration { secs: 0, nanos: 254852293 }
EP0IN: transfer done
USB: UsbEp0Setup @ Duration { secs: 0, nanos: 257568358 }
SETUP: bmrequesttype: 128, brequest: 6, wlength: 18, windex: 0, wvalue: 256
GET_DESCRIPTOR Device [length=18]
EP0IN: start 18B transfer
USB: UsbEp0DataDone @ Duration { secs: 0, nanos: 257843016 }
EP0IN: transfer done
USB: UsbEp0Setup @ Duration { secs: 0, nanos: 259674071 }
SETUP: bmrequesttype: 128, brequest: 6, wlength: 9, windex: 0, wvalue: 512
ERROR unknown request (goal achieved if GET_DESCRIPTOR Device was handled before)
`dk::exit()` called; exiting ...

A solution to this exercise can be found in nrf52-code/usb-app-solutions/src/bin/usb-3.rs.