Arduino Uno pins and limits

An Arduino Uno has 14 digital pins, six analog inputs and a row of power pins. This chart shows what each pin can do, which pins share a job, and the current limits that keep its ATmega328P chip alive.

Digital pins

Every digital pin can be an input or an output, HIGH (5 V) or LOW (0 V). Some have extra jobs. A ~ on the board marks the PWM pins.

PWM on pin 3 also runs at about 490 Hz. tone() stops PWM on pins 3 and 11, and the Servo library stops it on pins 9 and 10.

Analog pins

A0 to A5 feed the chip's 10-bit analog-to-digital converter. analogRead gives 0 to 1023 for 0 to 5 V, steps of about 4.88 mV, and takes about 100 µs per reading. Each one also works as a digital pin.

To turn a reading into volts: volts ≈ reading × 5 ÷ 1024. A reading of 512 is about 2.5 V.

Talking to other chips

Power pins

Current limits

These come from the ATmega328P's datasheet. Running past the absolute maximum can damage the chip for good.

An input reads HIGH from about 3.0 V and LOW up to about 1.5 V. pinMode(pin, INPUT_PULLUP) adds a built-in resistor of 20–50 kΩ to 5 V, so a button can pull the pin LOW.

Using the chart

1. Pick the pin for the job: a ~ pin for analogWrite, A0–A5 for analogRead, 2 or 3 for attachInterrupt().
2. Work out the current. A red LED through 220 Ω draws about 12.5 mA: fine. A motor, a relay or a long LED strip is not.
3. Add up every load. Under 20 mA a pin, and under 200 mA for the whole chip.