Turns on and off a light emitting diode(LED) connected to digital pin 13, when pressing a pushbutton attached to pin 2. const int buttonPin = 2; // the number of the pushbutton pin const int ledPin = ...

* of the button does not need to be covered. #ifndef F_CPU // this conditional directive ensures that F_CPU has not been defined somewhere else already (without it you might get a warning) ...