Add an example for serial communication using DMA

Cargo.toml

@@ -93,3 +93,7 @@ required-features = ["rt", "stm32f303xc", "stm32-usbd"]
 [[example]]
 name = "spi"
 required-features = ["stm32f303"]
+
+[[example]]
+name = "serial_dma"
+required-features = ["stm32f303"]

examples/serial_dma.rs

@@ -0,0 +1,69 @@
+//! Example of transmitting data over serial interface using DMA.
+//! For this to work, the PA9 and PA10 pins must be connected.
+//! Target board: STM32F3DISCOVERY
+
+#![no_std]
+#![no_main]
+
+use panic_semihosting as _;
+
+use core::pin::Pin;
+use cortex_m::{asm, singleton};
+use cortex_m_rt::entry;
+use stm32f3xx_hal::{prelude::*, serial::Serial, stm32};
+
+#[entry]
+fn main() -> ! {
+    let dp = stm32::Peripherals::take().unwrap();
+
+    let mut flash = dp.FLASH.constrain();
+    let mut rcc = dp.RCC.constrain();
+    let clocks = rcc.cfgr.freeze(&mut flash.acr);
+
+    let mut gpioa = dp.GPIOA.split(&mut rcc.ahb);
+
+    let pins = (
+        gpioa.pa9.into_af7(&mut gpioa.moder, &mut gpioa.afrh),
+        gpioa.pa10.into_af7(&mut gpioa.moder, &mut gpioa.afrh),
+    );
+    let serial = Serial::usart1(dp.USART1, pins, 9600.bps(), clocks, &mut rcc.apb2);
+    let (tx, rx) = serial.split();
+
+    let dma1 = dp.DMA1.split(&mut rcc.ahb);
+
+    // the data we are going to send over serial
+    let tx_buf = singleton!(: [u8; 9] = *b"hello DMA").unwrap();
+    // the buffer we are going to receive the transmitted data in
+    let rx_buf = singleton!(: [u8; 9] = [0; 9]).unwrap();
+
+    // DMA channel selection depends on the peripheral:
+    // - USART1: TX = 4, RX = 5
+    // - USART2: TX = 6, RX = 7
+    // - USART3: TX = 3, RX = 2
+    let (tx_channel, rx_channel) = (dma1.ch4, dma1.ch5);
+
+    // start separate DMAs for sending and receiving the data
+    let sending = tx.write_all(Pin::new(tx_buf), tx_channel);
+    let receiving = rx.read_all(Pin::new(rx_buf), rx_channel);
+
+    // block until all data was transmitted and received
+    let (mut tx_buf, tx_channel, tx) = sending.wait();
+    let (rx_buf, rx_channel, rx) = receiving.wait();
+
+    assert_eq!(tx_buf, rx_buf);
+
+    // After a transfer is finished its parts can be re-used for another one.
+    tx_buf.copy_from_slice(b"hi again!");
+
+    let sending = tx.write_all(tx_buf, tx_channel);
+    let receiving = rx.read_all(rx_buf, rx_channel);
+
+    let (tx_buf, ..) = sending.wait();
+    let (rx_buf, ..) = receiving.wait();
+
+    assert_eq!(tx_buf, rx_buf);
+
+    loop {
+        continue;
+    }
+}