Add docs

examples/rp/src/bin/usb_raw.rs

@@ -1,6 +1,50 @@
-//! This example shows how to use USB (Universal Serial Bus) in the RP2040 chip.
+//! Example of using USB without a pre-defined class, but instead responding to
+//! raw USB control requests.
 //!
-//! This creates a USB serial port that echos.
+//! The host computer can either:
+//! * send a command, with a 16-bit request ID, a 16-bit value, and an optional data buffer
+//! * request some data, with a 16-bit request ID, a 16-bit value, and a length of data to receive
+//!
+//! For higher throughput data, you can add some bulk endpoints after creating the alternate,
+//! but for low rate command/response, plain control transfers can be very simple and effective.
+//!
+//! Example code to send/receive data using `nusb`:
+//!
+//! ```ignore
+//! use futures_lite::future::block_on;
+//! use nusb::transfer::{ControlIn, ControlOut, ControlType, Recipient};
+//!
+//! fn main() {
+//!     let di = nusb::list_devices()
+//!         .unwrap()
+//!         .find(|d| d.vendor_id() == 0xc0de && d.product_id() == 0xcafe)
+//!         .expect("no device found");
+//!     let device = di.open().expect("error opening device");
+//!     let interface = device.claim_interface(0).expect("error claiming interface");
+//!
+//!     // Send "hello world" to device
+//!     let result = block_on(interface.control_out(ControlOut {
+//!         control_type: ControlType::Vendor,
+//!         recipient: Recipient::Interface,
+//!         request: 100,
+//!         value: 200,
+//!         index: 0,
+//!         data: b"hello world",
+//!     }));
+//!     println!("{result:?}");
+//!
+//!     // Receive "hello" from device
+//!     let result = block_on(interface.control_in(ControlIn {
+//!         control_type: ControlType::Vendor,
+//!         recipient: Recipient::Interface,
+//!         request: 101,
+//!         value: 201,
+//!         index: 0,
+//!         length: 5,
+//!     }));
+//!     println!("{result:?}");
+//! }
+//! ```
 
 #![no_std]
 #![no_main]