Safe GPIO pins
Arduino specifies a 20 mA maximum per I/O. The ATmega328P ratings of 40 mA per I/O and 200 mA through VCC/GND are absolute maxima, not operating targets; port-group current limits also apply.
Embedded hardware reference for engineers and makers
Arduino Uno R3 (ATmega328P) Pinout & Diagram
44-pin interactive pinoutArduino Uno R3 uses an ATmega328P at 5 V and 16 MHz. The main MCU clock comes from a ceramic resonator; a separate quartz crystal clocks the ATmega16U2 USB bridge.
- Arduino
- Verified source
- Updated Oct 2026
- GPIO
- Power
- Ground
- Analog
- Control
- Bus
- Not connected
Tap a row of pins to see it up close
Arduino Uno R3
ATmega328P
- MCU
- ATmega328P
- CPU core
- 8-bit AVR RISC
- Clock speed
- 16 MHz
- SRAM
- 2 KB
- Flash
- 32 KB
- GPIO count
- 20
- Logic voltage
- 5.0 V
- Form factor
- 44-pin, USB-to-Serial (ATmega16U2)
Before you wire
5 V logic — level-shift 3.3 V peripherals
- Confirm attached sensors and modules accept 5 V logic levels without damage.
- Check per-pin and port-group currents before wiring loads. Use external drivers for motors, relays and other loads that exceed GPIO capability.
- Do not exceed 5 V on the AREF pin; do not connect external voltage if internal analogReference() is configured.
QUICK LINKS
Pin table
Showing 44 of 44 pins| Pin | Name | GPIO | Functions | ADC | Default role | Flags |
|---|---|---|---|---|---|---|
| digital_high:1 digital_high | SCL | GPIO19 | gpioi2cadc | ADC5 | GPIO | |
| digital_high:2 digital_high | SDA | GPIO18 | gpioi2cadc | ADC4 | GPIO | |
| digital_high:3 digital_high | AREF | — | input | — | GPIO | External reference voltage limits |
| digital_high:4 digital_high | GND | — | gnd | — | Ground | |
| digital_high:5 digital_high | D13 / SCK | GPIO13 | gpiospi | — | GPIO | Shared with an onboard component |
| digital_high:6 digital_high | D12 / MISO | GPIO12 | gpiospi | — | GPIO | |
| digital_high:7 digital_high | ~D11 / MOSI | GPIO11 | gpiospipwm | — | GPIO | |
| digital_high:8 digital_high | ~D10 / SS | GPIO10 | gpiospipwm | — | GPIO | |
| digital_high:9 digital_high | ~D9 | GPIO9 | gpiopwm | — | GPIO | |
| digital_high:10 digital_high | D8 | GPIO8 | gpio | — | GPIO | |
| digital_low:1 digital_low | D7 | GPIO7 | gpio | — | GPIO | |
| digital_low:2 digital_low | ~D6 | GPIO6 | gpiopwm | — | GPIO | |
| digital_low:3 digital_low | ~D5 | GPIO5 | gpiopwm | — | GPIO | |
| digital_low:4 digital_low | D4 | GPIO4 | gpio | — | GPIO | |
| digital_low:5 digital_low | ~D3 | GPIO3 | gpiopwm | — | GPIO | |
| digital_low:6 digital_low | D2 | GPIO2 | gpio | — | GPIO | |
| digital_low:7 digital_low | D1 / TX | GPIO1 | gpiouart | — | GPIO | Shared with the USB-UART bridge |
| digital_low:8 digital_low | D0 / RX | GPIO0 | gpiouart | — | GPIO | Shared with the USB-UART bridge |
| power:1 power | NC | — | nc | — | No Connection | |
| power:2 power | IOREF | — | power | — | 5.0 V Power | |
| power:3 power | RESET | — | reset | — | Reset / Enable | |
| power:4 power | 3V3 | — | power | — | 3.3 V Power | |
| power:5 power | 5V | — | power | — | 5.0 V Power | |
| power:6 power | GND | — | gnd | — | Ground | |
| power:7 power | GND | — | gnd | — | Ground | |
| power:8 power | VIN | — | power | — | 7.0–12.0 V Power | |
| analog:1 analog | A0 | GPIO14 | gpioadc | ADC0 | GPIO | |
| analog:2 analog | A1 | GPIO15 | gpioadc | ADC1 | GPIO | |
| analog:3 analog | A2 | GPIO16 | gpioadc | ADC2 | GPIO | |
| analog:4 analog | A3 | GPIO17 | gpioadc | ADC3 | GPIO | |
| analog:5 analog | A4 / SDA | GPIO18 | gpioi2cadc | ADC4 | GPIO | |
| analog:6 analog | A5 / SCL | GPIO19 | gpioi2cadc | ADC5 | GPIO | |
| icsp:1 icsp | MISO | GPIO12 | gpiospi | — | GPIO | |
| icsp:2 icsp | +5V | — | power | — | 5.0 V Power | |
| icsp:3 icsp | SCK | GPIO13 | gpiospi | — | GPIO | |
| icsp:4 icsp | MOSI | GPIO11 | gpiospipwm | — | GPIO | |
| icsp:5 icsp | RESET | — | reset | — | Reset / Enable | |
| icsp:6 icsp | GND | — | gnd | — | Ground | |
| icsp_usb:1 icsp_usb | MISO2 | — | icsp | — | GPIO | |
| icsp_usb:2 icsp_usb | +5V | — | power | — | 5.0 V Power | |
| icsp_usb:3 icsp_usb | SCK2 | — | icsp | — | GPIO | |
| icsp_usb:4 icsp_usb | MOSI2 | — | icsp | — | GPIO | |
| icsp_usb:5 icsp_usb | RESET2 | — | reset | — | Reset / Enable | |
| icsp_usb:6 icsp_usb | GND | — | gnd | — | Ground |
Grounded reference
Board overview
Arduino Uno R3 is the foundational 8-bit development board based on the Microchip ATmega328P. It features standard 0.1" shield headers, 5 V logic, a removable DIP-28 MCU socket, and an onboard ATmega16U2 USB-serial bridge.
Canonical CAD Blueprint
The drawing follows official Arduino A000066 through-hole Rev3e Eagle CAD: socketed ATmega328P, ATmega16U2 USB bridge and two separate six-contact ICSP headers. It shows 44 physical header contacts, including duplicated signals and power contacts.
Power inputs
- USB Type-B connector (5 V DC, 500 mA resettable polyfuse protected)
- VIN or centre-positive 2.1 mm DC jack: 7–12 V recommended; board input limits 6–20 V. Regulator heating limits the available current.
- The 5 V pin is connected to the board supply rail. Available current depends on the power source, regulator temperature and onboard loads; the regulator part rating is not a guaranteed output budget. The 3.3 V pin is rated at 50 mA.
Hardware Variants, Clones & Traps
- The through-hole Uno R3 has a replaceable ATmega328P-PU. The Uno R3 SMD uses a different MCU package and layout; this drawing models the through-hole revision.
- UNO R4 Minima and UNO R4 WiFi are separate Renesas-based boards. Do not apply the Uno R3 MCU pin mapping or current limits to them.
- Compatible boards may use a different USB bridge or connector. Check that board’s schematic and driver; the ATmega16U2 ICSP1 shown here is not present on every clone.
Common use cases
- Introductory robotics, computer science, and electronics education.
- Rapid bench prototyping using standard 2.54mm breadboard jumper wires.
- Stackable Arduino Uno R3 compatible expansion shields.
Notes
- GPIO operate at 5 V. Check the input ratings of each connected device before driving it from a 5 V output; inputs that accept only 3.3 V may need level translation.
- Design below the board’s 20 mA per-I/O ceiling and respect the MCU port-group budgets. The 40 mA per-I/O and 200 mA VCC/GND absolute maxima do not guarantee normal operation.
- Preloaded with Optiboot 115200 baud bootloader; standard firmware uploads use D0 (RX) and D1 (TX) with auto-reset via DTR capacitor.
Before you wire
- Confirm attached sensors and modules accept 5 V logic levels without damage.
- Check per-pin and port-group currents before wiring loads. Use external drivers for motors, relays and other loads that exceed GPIO capability.
- Do not exceed 5 V on the AREF pin; do not connect external voltage if internal analogReference() is configured.
Specifications
- MCU
- ATmega328P
- CPU core
- 8-bit AVR RISC
- Clock
- 16 MHz
- SRAM
- 2 KB
- Flash
- 0.03125 MB
- GPIO count
- 22
- Logic voltage
- 5.0 V
- Operating voltage (VIN)
- 7 - 12 V
Pin usage guide
ADC and analog pins
Pins A0-A5 provide 10-bit analog-to-digital conversion (0-1023) referenced to 5V (or AREF). Pins A4 (SDA) and A5 (SCL) are shared with the I2C bus.
I2C, SPI and UART pins
UART on D0 (RX) and D1 (TX). SPI on D10 (SS), D11 (MOSI), D12 (MISO), D13 (SCK) and the 6-pin ICSP header. I2C on A4 (SDA) and A5 (SCL) plus dedicated top header pins.
Strapping and boot pins
No hardware strapping pins. D13 has an onboard LED buffer. D0/D1 are tied to the USB-to-serial converter (ATmega16U2).
Input-only pins
All digital pins support bidirectional I/O with optional internal pull-up resistors (20-50 kΩ). Analog pins A0-A5 can also function as digital GPIO 14-19.
Frequently asked questions
Short answers to the questions people ask about this board, each checked against the manufacturer or project documents linked under it.
Which Uno R3 pins support hardware PWM?
Use D3, D5, D6, D9, D10 and D11. D8 does not provide hardware PWM. The 16-bit Timer1 drives D9/D10, while the 8-bit Timer0 and Timer2 supply the other PWM outputs. Timer settings may be shared with Arduino timing functions or libraries.
Why does Uno R3 have two ICSP headers?
The ICSP header near the ATmega328P programs the main MCU and exposes MISO=D12, SCK=D13 and MOSI=D11. ICSP1 near the USB connector programs the separate ATmega16U2 USB bridge; its SPI and RESET signals belong to that chip. Both headers also carry 5 V and GND. Choose the header for the processor you intend to program.
How should I power Uno R3 and budget its current?
Use USB 5 V or the recommended 7–12 V at VIN or the centre-positive DC jack. The 5 V rail’s available current depends on the source and regulator heating; do not treat the regulator rating as a guaranteed board output. Arduino specifies 50 mA for the 3.3 V pin and 20 mA maximum per I/O. The MCU absolute maxima and port-group limits are separate constraints.
Help keep this page accurate
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