Add UART example + core::fmt support to UART (#73)

* Add standalone UART example
* Add UART example to docs in UART module
* Add core::fmt::write_str to UART module to allow easy use of format! macro in user code

rp2040-hal/examples/uart.rs

@@ -0,0 +1,67 @@
+//! Print an incrementing number to UART0 in a loop
+#![no_std]
+#![no_main]
+
+use core::fmt::Write;
+use cortex_m_rt::entry;
+use hal::clocks::init_clocks_and_plls;
+use hal::gpio::{self, Pins};
+use hal::pac;
+use hal::sio::Sio;
+use hal::uart::UartPeripheral;
+use hal::watchdog::Watchdog;
+use panic_halt as _;
+use rp2040_hal as hal;
+
+#[link_section = ".boot2"]
+#[used]
+pub static BOOT2: [u8; 256] = rp2040_boot2::BOOT_LOADER;
+
+#[entry]
+fn main() -> ! {
+    let mut pac = pac::Peripherals::take().unwrap();
+    let mut watchdog = Watchdog::new(pac.WATCHDOG);
+    let sio = Sio::new(pac.SIO);
+
+    // External high-speed crystal on the pico board is 12Mhz
+    let external_xtal_freq_hz = 12_000_000u32;
+    let clocks = init_clocks_and_plls(
+        external_xtal_freq_hz,
+        pac.XOSC,
+        pac.CLOCKS,
+        pac.PLL_SYS,
+        pac.PLL_USB,
+        &mut pac.RESETS,
+        &mut watchdog,
+    )
+    .ok()
+    .unwrap();
+
+    let pins = Pins::new(
+        pac.IO_BANK0,
+        pac.PADS_BANK0,
+        sio.gpio_bank0,
+        &mut pac.RESETS,
+    );
+
+    let mut uart = UartPeripheral::<_, _>::enable(
+        pac.UART0,
+        &mut pac.RESETS,
+        hal::uart::common_configs::_9600_8_N_1,
+        clocks.peripheral_clock.into(),
+    )
+    .unwrap();
+
+    // UART TX (characters sent from pico) on pin 1 (GPIO0) and RX (on pin 2 (GPIO1)
+    let _tx_pin = pins.gpio0.into_mode::<gpio::FunctionUart>();
+    let _rx_pin = pins.gpio1.into_mode::<gpio::FunctionUart>();
+
+    uart.write_full_blocking(b"UART example\r\n");
+
+    let mut value = 0u32;
+    loop {
+        writeln!(uart, "value: {:02}\r", value).unwrap();
+        cortex_m::asm::delay(10_000_000);
+        value += 1
+    }
+}

rp2040-hal/src/uart.rs

@@ -1,7 +1,28 @@
 //! Universal Asynchronous Receiver Transmitter (UART)
-// See [Chapter 4 Section 2](https://datasheets.raspberrypi.org/rp2040/rp2040_datasheet.pdf) for more details
+//!
+//! See [Chapter 4 Section 2](https://datasheets.raspberrypi.org/rp2040/rp2040_datasheet.pdf) of the datasheet for more details
+//!
+//! ## Usage
+//!
+//! See [examples/uart.rs](https://github.com/rp-rs/rp-hal/tree/main/rp2040-hal/examples/uart.rs) for a more complete example
+//! ```rust
+//! // Need to perform clock init before using UART or it will freeze.
+//! // Skipping it here for brevity
+//! let uart = UartPeripheral::<_, _>::enable(
+//!         pac.UART0,
+//!         &mut pac.RESETS,
+//!         hal::uart::common_configs::_9600_8_N_1,
+//!         clocks.peripheral_clock.into(),
+//!     ).unwrap();
+//!
+//! // Set up UART on GP0 and GP1 (Pico pins 1 and 2)
+//! let _tx_pin = pins.gpio0.into_mode::<gpio::FunctionUart>();
+//! let _rx_pin = pins.gpio1.into_mode::<gpio::FunctionUart>();
+//! uart.write_full_blocking(b"Hello World!\r\n");
+//! ```
 
 use core::convert::Infallible;
+use core::fmt;
 use core::ops::Deref;
 use embedded_time::fixed_point::FixedPoint;
 use embedded_time::rate::Baud;
@@ -480,3 +501,11 @@ impl<D: UartDevice> Write<u8> for UartPeripheral<Enabled, D> {
         self.transmit_flushed()
     }
 }
+
+impl<D: UartDevice> fmt::Write for UartPeripheral<Enabled, D> {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        s.bytes()
+            .try_for_each(|c| nb::block!(self.write(c)))
+            .map_err(|_| fmt::Error)
+    }
+}