USB-2: SETUP Stage

At the end of program usb-1 we received a EP0SETUP event. This event signals the end of the SETUP stage of a control transfer. The nRF52840 USBD peripheral will automatically receive the SETUP data and store it in the registers BMREQUESTTYPE, BREQUEST, WVALUE{L,H}, WINDEX{L,H} and WLENGTH{L,H}. In usb-2.rs, you will find a short description of each register above the variable into which it should be read.

For in-depth register documentation, refer to sections 6.35.13.31 to 6.35.13.38 of the nRF52840 Product Specification.

When you need to write some no_std code that does not involve device-specific I/O you should consider writing it as a separate crate. This way, you can test it on your development machine (e.g. x86_64) using the standard cargo test functionality.

So that's what we'll do here. In advanced/common/usb/lib.rs you'll find starter code for writing a no_std SETUP data parser. The starter code contains some unit tests; you can run them with cargo test (from within the usb folder) or you can use Rust Analyzer's "Test" button in VS code.

The definition of Descriptor::Configuration as well as the associated test has been "commented out" using an #[cfg(TODO)] attribute because it is not handled by the firmware yet. Delete the #[cfg(TODO)] so that the unit tests can access it. This pattern is used for enum members and test functions throughout this workshop, so keep it in mind should you see it again.

✅ Parse the data of this SETUP stage.

❗️ Keep the cable connected to the J3 port for the rest of the workshop

Start with the GET_DESCRIPTOR request, which is described in detail in section 9.4.3 of the USB specification. All the constants we'll be using are also described in Tables 9-3, 9-4 and 9-5 of the USB spec.

We can recognize a GET_DESCRIPTOR request by the following properties:

  • bmRequestType is 0b10000000
  • bRequest is 6 (i.e. the GET_DESCRIPTOR Request Code, defined in table 9-4 in the USB spec)

In this task, we only want to parse DEVICE descriptor requests. They have the following properties:

  • the descriptor type is 1 (i.e. DEVICE, defined in table 9-5 of the USB spec)
  • the descriptor index is 0
  • the wIndex is 0 for our purposes
  • ❗️you need to fetch the descriptor type from the high byte of wValue, and the descriptor index from the the low byte of wValue

You will also find this information in the // TODO implement ... comment in the Request::parse() function of lib.rs file.

NOTE: If you'd like to learn more, take a look at Section 9.4.3 Get Descriptor of the USB specification.

To complete the task, proceed like this:

  1. Parse GET_DESCRIPTOR requests for DEVICE descriptors:
    Modify Request::parse() in advanced/common/usb/src/lib.rs to recognize a GET_DESCRIPTOR request of type DEVICE so that the get_descriptor_device test passes. Note that the parser already handles SET_ADDRESS requests.

    • remember the GET_DESCRIPTOR fields described at the start of this section
    • remember that you can define binary literals by prefixing them with 0b
    • you can use bit shifts (>>) and casts (as u8) to get the high/low bytes of wValue

See advanced/common/usb/solution-get-descriptor-device.rs for a solution.

  1. Read incoming request information and pass it to the parser:
    modify usb-2.rs to read USBD registers and parse the SETUP data when an EPSETUP event is received.

    • for a mapping of register names to the USBD API, check the entry for nrf52840_hal::target::usbd in the documentation you've created using cargo doc
    • remember that we've learned how to read registers in events.rs
    • you will need to put together the higher and lower bits of wlength, windex and wvalue to get the whole field
    • Note: If you're using a Mac, you need to catch SetAddress requests returned by the parser as these are sent before the first GetDescriptor request. You can handle them by doing nothing.

  2. when you have successfully received a GET_DESCRIPTOR request for a Device descriptor you are done. You should see an output like this:

INFO:usb_2 -- USB: UsbReset @ 438.842772ms
INFO:usb_2 -- USB: UsbEp0Setup @ 514.984128ms
...
INFO:usb_2 -- SETUP: bmrequesttype: 128, brequest: 6, wlength: 64, windex: 0, wvalue: 256
INFO:usb_2 -- GET_DESCRIPTOR Device [length=64]
INFO:usb_2 -- Goal reached; move to the next section

wlength / length can vary depending on the OS, USB port (USB 2.0 vs USB 3.0) or the presence of a USB hub so you may see a different value.

You can find a solution to step 1. in advanced/common/usb/solution-get-descriptor-device.rs. You can find a solution to step 2. in advanced/firmware/src/bin/usb-2-solution.rs.