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61 lines
4.3 KiB
Markdown
61 lines
4.3 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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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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✅ Parse the data of this SETUP stage.
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Start with the GET_DESCRIPTOR request, which is described in detail in section 9.4.3 of the USB specification. All the constants you will need are described in Tables 9-3, 9-4 and 9-5.
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The fields of a GET_DESCRIPTOR request are as follows:
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- `bmRequestType` is 0b10000000
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- `bRequest` is GET_DESCRIPTOR
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- the high byte of `wValue` contains the descriptor type, whereas the low byte contains the descriptor index
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- `wIndex` is set to 0 for our purposes
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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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To complete the task, proceed like this:
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1. **Parse GET_DESCRIPTOR requests:**
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Modify `Request::parse()` in `advanced/common/usb/src/lib.rs` to recognize a GET_DESCRIPTOR request so that the `get_descriptor_device` test passes. Note that the parser already handles SET_ADDRESS requests.
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- check table 9-4 in the USB specification for Request Codes
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- remember that you can define binary literals by prefixing them with `0b`
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- you can use bit shifts (`>>`) and casts (`as u8`) to get the high/low bytes of a `u16`
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See `advanced/common/usb/solution-get-descriptor-device.rs` for a solution.
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2. **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 EPSETUP event is received.
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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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- 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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3. 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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INFO:usb_2 -- USB: UsbReset @ 438.842772ms
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INFO:usb_2 -- USB: UsbEp0Setup @ 514.984128ms
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...
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INFO:usb_2 -- SETUP: bmrequesttype: 128, brequest: 6, wlength: 64, windex: 0, wvalue: 256
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INFO:usb_2 -- GET_DESCRIPTOR Device [length=64]
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INFO:usb_2 -- Goal reached; move to the next section
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```
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`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 step 1. in `advanced/common/usb/solution-get-descriptor-device.rs`.
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You can find a solution to step 2. in `advanced/firmware/src/bin/usb-2-solution.rs`.
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