External Interrupts: 2 (interrupt 0), 3 (interrupt 1), 18 (interrupt 5), 19 (interrupt 4), 20 (interrupt 3), and 21 (interrupt 2).Pins 0 and 1 are also connected to the corresponding pins of the ATmega16U2 USB-to-TTL Serial chip. Used to receive (RX) and transmit (TX) TTL serial data. In addition, some pins have specialized functions: A properly configured shield can read the IOREF pin voltage and select the appropriate power source or enable voltage translators on the outputs for working with the 5V or 3.3V. This pin on the board provides the voltage reference with which the microcontroller operates. A 3.3 volt supply generated by the on-board regulator. Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V).
This pin outputs a regulated 5V from the regulator on the board. You can supply voltage through this pin, or, if supplying voltage via the power jack, access it through this pin.
The input voltage to the board when it's using an external power source (as opposed to 5 volts from the USB connection or other regulated power source).
