Add Pwm::clear_output_pin and test it in the demo

Author: jonas-schievink

examples/pwm-demo/src/main.rs

@@ -67,14 +67,20 @@ const APP: () = {
         let led3 = p0.p0_15.into_push_pull_output(Level::High).degrade();
         let led4 = p0.p0_16.into_push_pull_output(Level::High).degrade();
 
-        let pwm = Pwm::new(ctx.device.PWM0);
+        let mut pwm = Pwm::new(ctx.device.PWM0);
         pwm.set_period(500u32.hz())
             .set_output_pin(Channel::C0, led1)
             .set_output_pin(Channel::C1, led2)
             .set_output_pin(Channel::C2, led3)
             .set_output_pin(Channel::C3, led4)
-            .enable_interrupt(PwmEvent::Stopped)
-            .enable();
+            .enable_interrupt(PwmEvent::Stopped);
+
+        // In addition to `set_output_pin`, `swap_output_pin` and `clear_output_pin` can be used to
+        // get the old pin back.
+        let led1 = pwm.clear_output_pin(Channel::C0).unwrap();
+        assert!(pwm.swap_output_pin(Channel::C0, led1).is_none());
+
+        pwm.enable();
 
         let gpiote = Gpiote::new(ctx.device.GPIOTE);
         gpiote.port().input_pin(&btn1).low();

nrf-hal-common/src/pwm.rs

@@ -130,6 +130,8 @@ where
     }
 
     /// Sets the associated output pin for the PWM channel.
+    ///
+    /// Modifying the pin configuration while the PWM instance is enabled is not recommended.
     #[inline(always)]
     pub fn set_output_pin(&self, channel: Channel, pin: Pin<Output<PushPull>>) -> &Self {
         self.pwm.psel.out[usize::from(channel)].write(|w| {
@@ -140,6 +142,8 @@ where
     }
 
     /// Sets the output pin of `channel`, and returns the old pin (if any).
+    ///
+    /// Modifying the pin configuration while the PWM instance is enabled is not recommended.
     pub fn swap_output_pin(
         &mut self,
         channel: Channel,
@@ -157,6 +161,24 @@ where
         old
     }
 
+    /// Disables the output pin of `channel`.
+    ///
+    /// The output pin is returned, if one was previously configured.
+    ///
+    /// Modifying the pin configuration while the PWM instance is enabled is not recommended.
+    pub fn clear_output_pin(&mut self, channel: Channel) -> Option<Pin<Output<PushPull>>> {
+        // (needs `&mut self` because it reads, then writes, to the register)
+        let psel = &self.pwm.psel.out[usize::from(channel)];
+        let old = psel.read();
+        let old = if old.connect().is_connected() {
+            unsafe { Some(Pin::from_psel_bits(old.bits())) }
+        } else {
+            None
+        };
+        psel.reset();
+        old
+    }
+
     /// Enables the PWM generator.
     #[inline(always)]
     pub fn enable(&self) {