Pwm: allow get_duty to return the old value for some time

PWM is often implemented in a buffered way to allow glitch-free
operation; as a result, implementing a strict "you get what you last
set" is not feasible for some implementations.

Author: eldruin

src/pwm.rs

@@ -79,6 +79,9 @@ pub trait Pwm {
     fn try_get_period(&self) -> Result<Self::Time, Self::Error>;
 
     /// Returns the current duty cycle
+    ///
+    /// While the pin is transitioning to the new duty cycle after a `try_set_duty` call, this may
+    /// return the old or the new duty cycle depending on the implementation.
     fn try_get_duty(&self, channel: Self::Channel) -> Result<Self::Duty, Self::Error>;
 
     /// Returns the maximum duty cycle value