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Microcontroller
Arduino UNO Rev3
The classic ATmega328P UNO, still the reference target for beginner projects and shields.
- GPIO
- I2C
- SPI
- UART
- ADC
- PWM
- USB
- Processor
- ATmega328P + ATmega16U2 USB
- Logic
- 5 V operating voltage
- Power
- USB-B, barrel jack, VIN, 5 V pin
- Format
- UNO R3 shield layout
Before you wire
4 cautions- Digital IO is 5 V; level-shift 3.3 V-only sensors and modules.
- A4/A5 are the I2C SDA/SCL pins on classic UNO boards.
- SPI is available on D10-D13 and the ICSP header; modern docs call MOSI/MISO COPI/CIPO.
- The 3.3 V regulator output is current-limited; do not power large peripherals from it.
Pin map
Connector
Arduino UNO R3/R4 shield headers
Component side up, shield headers along the long edges
Simplified technical illustration. Pin data is source-backed; verify against the linked documentation before wiring.
Pin
Hover, tap, or arrow to a pad to inspect it.
- UNO Rev3 and UNO R4 share the classic shield header footprint and 5 V operating voltage, but MCU-specific alternate functions differ.
- SPI is exposed on the ICSP header; D10 through D13 are commonly used with SPI shields.
- A4 and A5 are the default I2C SDA/SCL pins on the UNO-style header.
All 25 pins (table)
| # | Signal | Role | Net | Also / notes |
|---|---|---|---|---|
| Digital | ||||
| 1 | UART | UART | ||
| 2 | UART | UART | ||
| 3 | GPIO | |||
| 4 | PWM | |||
| 5 | GPIO | |||
| 6 | PWM | |||
| 7 | PWM | |||
| 8 | GPIO | |||
| 9 | GPIO | |||
| 10 | PWM | |||
| 11 | SPI | SPI | ||
| 12 | SPI | SPI | ||
| 13 | SPI | SPI | ||
| 14 | SPI | SPI | ||
| Analog / Power | ||||
| 14 | ADC | |||
| 15 | ADC | |||
| 16 | ADC | |||
| 17 | ADC | |||
| 18 | I2C | I2C | ||
| 19 | I2C | I2C | ||
| 20 | Power | 3V3 | ||
| 21 | Power | 5V | ||
| 22 | Ground | GND | ||
| 23 | Power | VIN | ||
| 24 | System | |||
Source references
Why it matters
- Official full pinout PDF documents ATmega328P ports, shield headers, ICSP, and current limits.
- Most widely documented Arduino shield footprint for examples and classroom hardware.
- ATmega16U2 provides USB serial while the ATmega328P runs sketches.
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