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advanced/README.md: 9.4.3 usb -> prose
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# 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 following registers: BMREQUESTTYPE, BREQUEST, WVALUE{L,H}, WINDEX{L,H} and WLENGTH{L,H}. These registers are documented in sections 6.35.13.31 to 6.35.13.38 of the [nRF52840 Product Specification][nrf product spec]. 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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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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The format of this setup data is documented in section 9.3 of the USB specification. Your next task is to parse it. We will 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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> NOTE: If you'd like to learn more, take a look at Section 9.4, Standard Descriptor Requests, of the USB specification.
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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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Now, proceed as follows:
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Your task now is to parse the data of this SETUP stage. We will 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 0b00000000
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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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