Pins configured as INPUT_PULLUP

Pins configured as INPUT_PULLUP with pinMode() are turned in the same state as for INPUT mode, but internal 20 kΩ pull-up resistor is enabled. This prevents floating when nothing is connected to the pin: in such state pin will be in HIGH state. This is same as using external pull-up resistor and INPUT mode. Pins configured as inputs with either INPUT or INPUT_PULLUP can be damaged or destroyed if they are connected to voltages below ground (negative voltages) or above the positive power rail (3.3V).