implement embedded-hal v1 SetDutyCycle

this also ships with an example making use of it. note that the fading
of the new example is faster than the existing `uno-simple-pwm.rs`
example, since the latter fades with 255 steps while the new example
uses 0 - 100% percent (and thus only sleeps 200 * 10ms instead of
512 * 10ms).

avr-hal-generic/src/simple_pwm.rs

@@ -1,6 +1,8 @@
 //! PWM Implementation
 
 use core::marker::PhantomData;
+use embedded_hal::pwm;
+use embedded_hal::pwm::{ErrorKind, ErrorType, SetDutyCycle};
 
 use crate::port::mode;
 use crate::port::Pin;
@@ -82,6 +84,35 @@ impl<TC, PIN: PwmPinOps<TC>> Pin<mode::PwmOutput<TC>, PIN> {
     }
 }
 
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+pub enum PwmError {
+    /// `embedded-hal` supports duty cycles up to `u16`, however `avr` devices only support up to `u8`.
+    /// Passing a duty cycle larger than [`u8::MAX`] will result in this error.
+    DutyCycleTooLarge,
+}
+
+impl pwm::Error for PwmError {
+    fn kind(&self) -> ErrorKind {
+        ErrorKind::Other
+    }
+}
+
+impl<TC, PIN: PwmPinOps<TC>> ErrorType for Pin<mode::PwmOutput<TC>, PIN> { type Error = PwmError; }
+
+impl<TC, PIN: PwmPinOps<TC, Duty=u8>> SetDutyCycle for Pin<mode::PwmOutput<TC>, PIN> {
+    fn max_duty_cycle(&self) -> u16 {
+        self.get_max_duty() as u16
+    }
+
+    fn set_duty_cycle(&mut self, duty: u16) -> Result<(), Self::Error> {
+        if duty > u8::MAX as u16 {
+            return Err(PwmError::DutyCycleTooLarge);
+        }
+        self.set_duty(duty as u8);
+        Ok(())
+    }
+}
+
 #[macro_export]
 macro_rules! impl_simple_pwm {
     (

examples/arduino-uno/src/bin/uno-simple-pwm-embedded-hal.rs

@@ -0,0 +1,36 @@
+/*!
+ * Example of using simple_pwm to fade a LED in and out on pin d5, but with a twist:
+ * the fade function is not aware of `avr-hal` and only uses `embedded-hal` traits.
+ */
+#![no_std]
+#![no_main]
+
+use embedded_hal::delay::DelayNs;
+use embedded_hal::pwm::SetDutyCycle;
+use arduino_hal::simple_pwm::*;
+use panic_halt as _;
+
+fn fade(led: &mut impl SetDutyCycle, delay: &mut impl DelayNs) -> ! {
+    loop {
+        for pct in (0..=100).chain((0..100).rev()) {
+            led.set_duty_cycle_percent(pct).unwrap();
+            delay.delay_ms(10);
+        }
+    }
+}
+
+#[arduino_hal::entry]
+fn main() -> ! {
+    let dp = arduino_hal::Peripherals::take().unwrap();
+    let pins = arduino_hal::pins!(dp);
+
+    let timer0 = Timer0Pwm::new(dp.TC0, Prescaler::Prescale64);
+
+    // Digital pin 5 is connected to a LED and a resistor in series
+    let mut pwm_led = pins.d5.into_output().into_pwm(&timer0);
+    pwm_led.enable();
+
+    let mut delay = arduino_hal::Delay::new();
+
+    fade(&mut pwm_led, &mut delay);
+}