arduino ioref

Interesting pins on the Arduino

1. IOREF - This pin is the input/output reference. It provides the voltage reference with which the microcontroller operates.
2. IOREF. This pin on the Arduino/Genuino board provides the voltage reference with which the microcontroller operates. A properly configured shield can read the IOREF pin voltage and select the appropriate power source or enable voltage translators on the outputs to work with the 5V or 3.3V.
3. IOREF: This is a voltage corresponding to the i/o of that board, for example, a Uno would supply 5v to this pin, but a Due would supply 3.3v. Sending a signal to this pin does nothing.
4. 3V3. A 3.3 volt supply generated by the onboard regulator. Maximum current source is 50 mA.
5. Serial: 0 (RX) and 1 (TX); Serial 1: 19 (RX) and 18 (TX); Serial 2: 17 (RX) and 16 (TX); Serial 3: 15 (RX) and 14 (TX). Used to receive (RX) and transmit (TX) TTL serial data. Pins 0 and 1 are also connected to the corresponding pins of the ATmega16U2 USB-to-TTL Serial chip.
6. External Interrupts: 2 (interrupt 0), 3 (interrupt 1), 18 (interrupt 5), 19 (interrupt 4), 20 (interrupt 3), and 21 (interrupt 2). These pins can be configured to trigger an interrupt on a low level, a rising or falling edge, or a change in level. See the attachInterrupt() function for details.
7. Reset. Bring this line LOW to reset the microcontroller. Typically used to add a reset button to shields which physically block the one on the board.

arduino ioref
arduino-uno-pinout
arduino-mega-pinout