Add interrupt example to basics

examples/01.Basics/Interrupt/Interrupt.ino

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+/*
+  Interrupt
+  Use a pushbutton to control an LED; powered by interrupts.
+
+  This sample uses an interrupt service routine (ISR) named `blink` to toggle
+  a global state variable and then uses the built-in LED to display the value
+  of the cached state to the user. The work of updating the cached state happens
+  asynchronously in the interrupt service routine.
+
+  This sample can easily be modified from behaving as a button into behaving as
+  a switch, by changing the interrupt mode from CHANGE to LOW. Furthermore, this
+  example can be modified to update the built-in LED from inside in the ISR,
+  which would allow the loop function to be empty yet still allow actions and
+  reactions to occur.
+
+  This example code is in the public domain (adapted from
+  https://www.arduino.cc/en/Reference/attachInterrupt).
+
+  adapted 1 APR 2017
+  by Zachary J. Fields
+*/
+
+const byte interruptPin = 2;
+volatile byte state = LOW;
+
+void blink() {
+  state = !state;  // toggle the cached state
+}
+
+// the setup function runs once when you press reset or power the board
+void setup() {
+  // initialize digital pin LED_BUILTIN as an output.
+  pinMode(LED_BUILTIN, OUTPUT);
+  // initialize digital pin as input with pull-up resistor (i.e. always HIGH, unless deliberately pulled LOW)
+  pinMode(interruptPin, INPUT_PULLUP);
+  // attach an interrupt service routine to be executed when pin state changes
+  attachInterrupt(digitalPinToInterrupt(interruptPin), blink, CHANGE);
+}
+
+// the loop function runs over and over again forever
+void loop() {
+  // update the LED to reflect the cached state updated with interrupts
+  digitalWrite(LED_BUILTIN, state);
+}
+

examples/01.Basics/Interrupt/Interrupt.txt

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+Use a pushbutton to control an LED; powered by interrupts.