Arduino Uno Rev3 Pinout, Projects & Spec

Arduino Uno Rev3 has 14 digital input/output pins labeled from D2 to D13. Also all the analog pins of the board can be used as digital pin. This is explicitly mentioned in the documentation of digitalRead() function.

A digital pin can be used either as input or output pin. This should be specified first using the pinMode function as below:

void setup() {
  pinMode(3, INPUT);        // pin 3 will be used as input
  pinMode(5, OUTPUT);       // pin 5 will be used as ouptut
  pinMode(8, INPUT_PULLUP); // pin 8 will be used as input with pullup resistor internally set by the microcontroller
}

When the pin mode is set, you can read the value of an input pin like this:

pin3Value = digitalRead(3); // read current value of the pin 3 previously set as input

Also to write a digital value to a pin, you can call the digitalWrite function passing one of the constant values HIGH or LOW along with the pin number to write it to:

digitalWrite(5, HIGH); // set pin 5 value to 5V

When a pin mode is set to INPUT_PULLUP, the given pin will be internally connected to the 5V so that if the given pin is floated (i.e. you don't connect it to to either of 5V or GND) it will be read as HIGH.

The first two digital pins D0 and D1 are labeled as RX0 and TX1. It's because they're used for serial communication when the board is connected via USB to computer. So it's better not to use them until you really ran out of digital pin for your project.

The board provides 6 analog input pins labeled A0 to A5 connected to its 10-bit ADC. They can be used for reading analog signals such as temperature sensors, light sensors, and potentiometers.

The VIN pin can be used to power the board with an external voltage source. This pin is regulated on-board, so it can accept a wide range of voltage levels, from 7-12 V.

This pin outputs a regulated 5V voltage. The voltage comes from either VIN pin or the USB. But it can also be used to give the power to the board. When it's used as input, it must be provided with a 5V regulated voltage. When used as voltage output, this pin can be used to power other components or devices in your project, such as sensors or actuators.

When powering the board via 5V pin or VIN pin, remember to connect the GND pin of the board to your power supply ground. There are three GND pins in total in Arduino Uno Rev3 board.

Arduino Uno Rev3 provides a 3.3V pin that is an output of the onboard regulator. You can use it to power the sensor or other modules in the project that require lower voltage level.

Arduino Uno Rev3 has a reset pin that if gets connected to the GND will reset the board. That's exactly what happens when you push the reset button available on the board. The board also contains a small circuit that resets the board when programming a new sketch on it. That's why you don't need to hold the reset button on the board in order to program it.

The AREF pin is used to provide the voltage level that should be used as a reference for ADC module of the processor. The board has a 10 bit ADC which means any analog input voltage will be converted to a number between 0 to 1023. By default the board considers the input voltage (i.e. 5V) as a reference and consider a signal on that level as 1023 and any signal with a value less than 5V will be converted to a number between 0 to 1023 accordingly. When the AREF is provided with a voltage, that voltage will be considered as 1023 and all the voltage level below that will be mapped accordingly.

In order to use it, you should enable it first by calling analogReference(EXTERNAL).

The IOREF (Input/Output Reference) pin on Arduino Uno Rev3 is used as a reference voltage for the input/output voltage levels of the board. It is connected to the voltage regulator and can be used by shields to let them know the voltage level used by the board.

PWM pins are marked as ∿ on Arduino Uno Rev3 pinout as well as the Arduino Uno Rev3 board itself. By default the PWM frequency is 490 Hz, except for pins D3 and D11 that use 980 Hz by default.

The PWM signals are being produced using the Timer/Counter peripherals of the processor. So in order to do any customization on the PWM signal, like changing the frequency or duty cycle, one should initialize registers corresponding to the timer producing those PWM signals.

The registers of each port are named as (replace the 'x' with the port name):

DDRx - The Port x Data Direction Register - read/write

PORTx - The Port x Data Register - read/write

PINx - The Port x Input Pins Register - read only

Each bit in these registers corresponds to a specific I/O pin. For example in order to set pin D8 to HIGH, instead of using digitalWrite(8, HIGH), you can set the corresponding bit on the PORTB register to 1.

PORTB = B00000001; // this is equivalent to "digitalWrite(8, HIGH)"

We set the bit 0 of the PORTB register to 1, because of the PB0 label in front of the D8 pin in the Arduino Uno Rev3 pinout diagram. The PB0 means that the pin 0 of the Port B registers (i.e. DDRB:0, PORTB:0 and PINB:0) correspond to the D8 pin.

The ATmega328 processor of the Arduino Uno Rev3 has a six channel 10-bit ADC referred as ADC0 to ADC5 pins. They are all connected to pins A0 to A5 of the board accordingly. The 10-bit ADC provides the resolution of 1024 value which means any analog input signal between 0V to 5V applied to these pins gets mapped proportionately to a value between 0 to 1023.

The PWM or variable frequency output generated by the Waveform Generator based on the comparison of the current value of the Timer 0 with the value in OCR0A register. The OC0A is accessible in Arduino Uno Rev3 pinout via D6 pin.

The PWM or variable frequency output generated by the Waveform Generator based on the comparison of the current value of the Timer 0 with the value in OCR0B register. The OC0B is accessible in Arduino Uno Rev3 pinout via D5 pin.

The PWM or variable frequency output generated by the Waveform Generator based on the comparison of the current value of the Timer 1 with the value in OCR1A register. The OC1A is accessible via D9 pin on the board.

The PWM or variable frequency output generated by the Waveform Generator based on the comparison of the current value of the Timer 1 with the value in OCR1B register. The OC1B is accessible via D10 pin on the board.

The PWM or variable frequency output generated by the Waveform Generator based on the comparison of the current value of the Timer/Counter 2 with the value in OCR2A/OCR2B registers. The OC2A is accessible on Arduino Uno Rev3 pinout via D11 pin and the OC2B is available on D3.

The ATmega328 can output the system clock on the CLKO pin. To enable the output, the CKOUT Fuse has to be programmed. This mode is suitable when the chip clock is used to drive other circuits on the system. You can not program fuses using Arduino IDE. So in order to use the CLKO output, you need to use another software and program your board using SPI via ICSP headers.

The CLKO is accessible via D8 pin on the board.

The Timer/Counter incorporates an Input Capture unit that can capture external events and give them a timestamp indicating time of occurrence. The external signal indicating an event, or multiple events, can be applied via the ICP1 pin or alternatively, via the analog-comparator unit. In this scenario, the pin acts as an input.

The ICP1 is accessible via D8 pin on the board.

The Timer/Counter 0 can be clocked internally, via the prescaler, or by an external clock source on the T0 pin. When the timer need to be clocked externally, it should be configured via TCCR0B register. The T0 pin is accessible on Arduino Uno Rev3 pinout via D4 pin.

The Timer/Counter 1 can be clocked internally, via the prescaler, or by an external clock source on the T1 pin. When the timer need to be clocked externally, it should be configured via TCCR1B register. The T1 pin is accessible on Arduino Uno Rev3 pinout via D5 pin.

Arduino Uno Rev3 supports 2 external interrupts. The External Interrupts can be triggered by a falling or rising edge or a low level. This is set up as indicated in the specification for the External Interrupt Control Registers - EICRA. The INT0 and INT1 are accessible via D2 and D3 pins on the board.

External interrupts can also be triggered by any of the PCINTxx pins. They only gets triggered when the logic level voltage of the corresponding pin changes.

The Analog Comparator compares the input values on the positive pin AIN0 and negative pin AIN1. When the voltage on the positive pin AIN0 is higher than the voltage on the negative pin AIN1, the Analog Comparator output, ACO, is set. The comparator’s output can be set to trigger the Timer/Counter 1 Input Capture function. In addition, the comparator can trigger a separate interrupt, exclusive to the Analog Comparator. The user can select Interrupt triggering on comparator output rise, fall or toggle.

The AIN0 and AIN1 are accessible via D6 and D7 pins on the board. It is also possible to select any of the A0 to A5 pins to replace the negative input to the Analog Comparator.

MISO (Master In Slave Out) is for sending data from the peripheral (e.g. a SPI LCD module) to the processor. It's available as pin D12 on the board. Recently it's being referred in different documents as CIPO (Controller In Peripheral Out)

The MISO pin is also accessible on ICSP headers on the board.

MOSI (Master Out Slave In) is for sending data from the processor to the peripheral device. It's available as pin D11 on the board. Recently it's being referred in different documents as COPI (Controller Out Peripheral In)

The MOSI pin is also accessible on ICSP headers on the board.

SS (Slave Select) is connected to each peripheral device and controller can enable or disable specific device using that. When controller set this pin for a specific device to low, it means that devices can communicate with the controller. When it's high, it means the device is disabled should ignore any data coming from MOSI. This enables controller to connect to multiple SPI enabled peripherals and sharing the same MISO, MOSI, and SCK. The SS is by default available on pin D10 on Arduino Uno Rev3 pinout. Recently it's being referred in different documents as CS (Chip Select).

SCK (Serial Clock) is a clock signal used to synchronize the processor and peripherals. The line is used as a shared line for all connected peripherals and is available on pin D13 on the board.

The SCK pin is also accessible on ICSP headers on the board.

The SDA is the Serial Data pin and is part of the two-wire serial interface (TWI) that can be used to communicate with other devices using a 2-wire protocol, such as I2C. It is exposed on pin A4 on the board.

The SCL is the Serial Clock pin used as clock signal between two chips communicating via the two-wire serial interface (TWI) also known as I2C. It is exposed on pin A5 on the board.

The Arduino Uno Rev3 exposes the USART serial communication of ATmega328 on pins D0 and D1 (named RXD and TXD respectively on the board). The board also utilizes a dedicate ATmega16U2 processor that is only used for converting the serial communication to USB protocol and vice versa. The USB com drivers on the computer operating system presents the serial communication port of the processor as a virtual com port on the computer. The RX and TX LEDs on the board will flash when data is being transmitted via the ATmega16U2 chip to the computer (but not for serial communication on those pins).

The XCK (Transfer Clock) pin is the input clock for the USART transfer. It is only used by synchronous transfer mode of the USART peripheral. The pin is available on Arduino Uno Rev3 pinout on D4 pin.