Table 1 Synopsis of pinout and other technical characteristics of the Arduino UNO microcontroller

Input and output: Each of the 14 digital pins on the Arduino UNO can be used as an input or output, using the pinMode(), digitalWrite(), and digitalRead() functions. These pins operate at 5 V and each can supply or receive a maximum of 40 mA. Each pin has an internal pull-up resistor, disconnected by default. In addition, some pins have specialized functions, as discussed below

Serial: Pins 0 (RX) and 1 (TX). These are used to receive (RX) and transmit (TX) TTL level serial data. These pins are connected to the corresponding pins of the ATmega8U2 USB-to-TTL Serial chip

External interrupts: Pins 2 and 3. These pins can be configured to trigger an interrupt on a low value, on a rising or falling edge, or on a change in value. See the attachInterrupt() function for details

Pulse width modulation (PWM): Digital pins 3, 5, 6, 9, 10, and 11 can provide an 8-bit PWM output with the analogWrite() function. They are identified with a tilde (~) symbol

SPI: 10 (SS), 11 (MOSI), 12 (MISO), 13 (SCK). These pins support the SPI serial communication protocol using the SPI library

LED: 13. There is a built-in LED connected to digital pin 13. When the pin is in the HIGH state, the LED is on, and when the pin is LOW, the LED is off

Analog inputs: The UNO has 6 analog input lines, labeled A0 through A5, each providing 10-bit resolution (i.e., 1024 different values). By default, they measure from ground to 5 V, though it is possible to change the upper end of their range using the AREF pin and the analogReference() function. Additionally, some pins have specialized functionality

TWI: A4 or SDA pin and A5 or SCL pin. Support TWI communication using the wire library

AREF: Reference voltage for the analog inputs. Used with analogReference()

Reset: Bring this line LOW to reset the microcontroller. Typically used to add a reset button to shields which block the one on the board