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91 lines
5.1 KiB
Markdown
91 lines
5.1 KiB
Markdown
# USB-2: SETUP Stage
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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}.
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In `usb-2.rs`, you will find a short description of each register above the variable into which it should be read.
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> For in-depth register documentation, refer to sections 6.35.13.31 to 6.35.13.38 of the [nRF52840 Product Specification][nrf product spec].
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[nrf product spec]: https://infocenter.nordicsemi.com/pdf/nRF52840_PS_v1.1.pdf
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## Writing a parser for the data of this SETUP stage.
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❗️ Keep the cable connected to the J3 port for the rest of the workshop
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✅ Parse GET_DESCRIPTOR requests for DEVICE descriptors.
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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.
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**Getting Started:**
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**1. Writing code that can be tested**
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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.
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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.
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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.
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**2. Description of GET_DESCRIPTOR request**
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We can recognize a GET_DESCRIPTOR request by the following properties:
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- `bmRequestType` is **0b10000000**
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- `bRequest` is **6** (i.e. the GET_DESCRIPTOR Request Code, defined in table 9-4 in the USB spec)
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**3. Description of DEVICE descriptor requests**
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In this task, we only want to parse DEVICE descriptor requests. They have the following properties:
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- the descriptor type is **1** (i.e. DEVICE, defined in table 9-5 of the USB spec)
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- the descriptor index is **0**
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- the wIndex is **0** for our purposes
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- ❗️you need to fetch the descriptor type from the high byte of `wValue`, and the descriptor index from the the low byte of `wValue`
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Check section 9.4.3 of the [USB specification][usb_spec] for a very detailed description of the requests. All the constants we'll be using are also described in Tables 9-3, 9-4 and 9-5 of the same document.
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**4. Remember that you can define binary literals by prefixing them with `0b`.**
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**5. You can use bit shifts (`>>`) and casts (`as u8`) to get the high/low bytes of `wValue`.**
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**6. Return `Err` if properties aren't met.**
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You will also find this information in the `// TODO implement ...` comment in the `Request::parse()` function of `lib.rs` file.
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> NOTE: If you'd like to learn more, take a look at Section 9.4.3 Get Descriptor of the USB specification.
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See `advanced/common/usb/solution-get-descriptor-device.rs` for a solution.
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✅ Read incoming request information and pass it to the parser:
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modify `usb-2.rs` to read `USBD` registers and parse the SETUP data when an EP0SETUP event is received.
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**Getting Started:**
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- 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`
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- let bmrequesttype = usbd.bmrequesttype.read().bits() as u8;
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- remember that we've learned how to read registers in `events.rs`.
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- you will need to put together the higher and lower bits of `wlength`, `windex` and `wvalue` to get the whole field
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- > 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.
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**Expected Result:**
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When you have successfully received a GET_DESCRIPTOR request for a Device descriptor you are done. You should see an output like this:
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``` console
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USB: UsbReset @ Duration { secs: 0, nanos: 361145018 }
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USB: UsbEp0Setup @ Duration { secs: 0, nanos: 402465820 }
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SETUP: bmrequesttype: 0, brequest: 5, wlength: 0, windex: 0, wvalue: 10
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USB: UsbEp0Setup @ Duration { secs: 0, nanos: 404754637 }
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SETUP: bmrequesttype: 128, brequest: 6, wlength: 8, windex: 0, wvalue: 256
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GET_DESCRIPTOR Device [length=8]
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Goal reached; move to the next section
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`dk::exit()` called; exiting ...
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```
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> Note: `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.
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You can find a solution to this step in `advanced/firmware/src/bin/usb-2-solution.rs`.
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[usb_spec]: https://www.usb.org/document-library/usb-20-specification
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