Add interrupt exercise.

Author: armandas

src/led_with_interrupt.md

@@ -0,0 +1,55 @@
+# Exercise: toggle LED with interrupt
+
+Now that we have set up the GPIO interrupt, try using it to toggle the LED.
+There are many ways to achieve this, so let's set some specific requirements:
+
+1. Use [`AtomicBool`][1]
+
+   We have already seen how to use a Mutex, so for this exercise, let's try something new.
+
+1. Toggle the LED in the main loop
+
+   In general, we want to avoid doing any work in the interrupt handler,
+   so use the `AtomicBool` to signal to the main loop that the LED should be toggled.
+   Remove any other code from the loop for now.
+
+1. Use `FallingEdge` interrupt type
+1. Take care of debouncing
+
+   You will encounter switch bounce - several interrupt events occurring from a single button press.
+   Try to come up with a solution for this problem so that the LED is only toggled once per button press.
+   There are a multitude of ways to implement this so any solution that works is fine.
+
+<details>
+<summary><strong>Solution</strong></summary>
+
+First, we'll need to import `atomic` and then declare our static object object.
+
+```rust
+use core::sync::atomic;
+
+static BUTTON_EVENT: atomic::AtomicBool = atomic::AtomicBool::new(false);
+```
+
+Next, set the `BUTTON_EVENT` in the `button_handler()`:
+
+```rust
+BUTTON_EVENT.store(true, atomic::Ordering::Relaxed);
+```
+
+Finally, in the main loop, let's check the `BUTTON_EVENT` and if it's `true`, toggle the LED.
+
+```rust
+loop {
+    if BUTTON_EVENT.load(atomic::Ordering::Relaxed) {
+        led.toggle();
+        // Delay before resetting the BUTTON_EVENT acts as debouncing logic.
+        delay.delay_millis(150);
+        BUTTON_EVENT.store(false, atomic::Ordering::Relaxed);
+    }
+}
+```
+
+</details>
+
+[1]: https://doc.rust-lang.org/nightly/std/sync/atomic/struct.AtomicBool.html