Leonardo R3 (ATmega32U4): specs and pinout
Arduino Leonardo-style board built on the ATmega32u4, which has USB built in, so the board can appear to a computer as a keyboard, mouse or virtual serial port.
Specs
| Spec | Value | Source |
|---|---|---|
| Microcontroller | ATmega32u4, 16 MHz | Arduino official documentation Tech Specs: Microcontroller, Clock speed |
| Memory | 32 KB flash, 2.5 KB SRAM, 1 KB EEPROM | Arduino official documentation Tech Specs: Memory |
| I/O voltage | 5 V | Arduino official documentation Tech Specs: Power |
| Input voltage | 7-12 V nominal; the pinout sheet states VIN 6-20 V input | Arduino official documentation Tech Specs: Power; Arduino datasheet A000057 page 1 |
| DC current per I/O pin | 10 mA in Tech Specs; the pinout sheet marks 20 mA as the maximum current per I/O pin | Arduino official documentation Tech Specs: Power; Arduino datasheet A000057 page 1 |
| Current from 3.3 V pin | 50 mA maximum | Arduino datasheet A000057 Pinout PDF page 1 |
| Pins | 20 digital I/O (7 PWM), 12 analog inputs | Arduino official documentation Tech Specs: Pins |
| Communication | UART, I2C, SPI; native USB (USB 2.0 full-speed device) | Arduino official documentation Tech Specs: Communication; https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-7766-8-bit-AVR-ATmega16U4-32U4_Datasheet.pdf Features, p.1 |
| USB connector | Micro USB | Arduino official documentation Tech Specs: USB connector |
Power
| Value | Source | |
|---|---|---|
| Supply | 7-12 V nominal on the barrel plug or VIN (pinout sheet says 6-20 V on VIN); 5 V from USB | Arduino official documentation Tech Specs: Power; no typical current given |
Pinout
Pin rows are from the official Arduino Leonardo pinout sheet (A000057). If the listed board is a compatible clone, the Arduino document is used as the reference and the clone's own layout is not covered.
| Pin | Function | Voltage | Notes | Source |
|---|---|---|---|---|
| Power 1 | NC | Arduino datasheet A000057 Pinout PDF A000057, page 1 | ||
| Power 2 | IOREF | Arduino datasheet A000057 Pinout PDF A000057, page 1 | ||
| Power 3 | RESET | Arduino datasheet A000057 Pinout PDF A000057, page 1 | ||
| Power 4 | +3V3 | 3.3 V | 50 mA maximum from 3.3 V per the pinout sheet | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Power 5 | +5V | 5 V | Arduino datasheet A000057 Pinout PDF A000057, page 1 | |
| Power 6 | GND | Arduino datasheet A000057 Pinout PDF A000057, page 1 | ||
| Power 7 | GND | Arduino datasheet A000057 Pinout PDF A000057, page 1 | ||
| Power 8 | VIN | 6-20 V input per the pinout sheet | Arduino datasheet A000057 Pinout PDF A000057, page 1 | |
| Analog 1 | A0 / D18 | 5 V logic | PF7, ADC[7] | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Analog 2 | A1 / D19 | 5 V logic | PF6, ADC[6] | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Analog 3 | A2 / D20 | 5 V logic | PF5, ADC[5] | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Analog 4 | A3 / D21 | 5 V logic | PF4, ADC[4] | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Analog 5 | A4 / D22 | 5 V logic | PF1, ADC[1] | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Analog 6 | A5 / D23 | 5 V logic | PF0, ADC[0] | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Digital header 1 | D0 / RX | 5 V logic | PD2 | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Digital header 2 | D1 / TX | 5 V logic | PD3 | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Digital header 3 | D2 / SDA | 5 V logic | PD1 | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Digital header 4 | D3 / SCL (PWM) | 5 V logic | PD0 | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Digital header 5 | D4 / A6 | 5 V logic | PD4 | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Digital header 6 | D5 (PWM) | 5 V logic | PC6 | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Digital header 7 | D6 / A7 (PWM) | 5 V logic | PD7 | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Digital header 8 | D7 | 5 V logic | PE6 | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Digital header 9 | D8 / A8 | 5 V logic | PB4 | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Digital header 10 | D9 / A9 (PWM) | 5 V logic | PB5 | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Digital header 11 | D10 / A10 (PWM) | 5 V logic | PB6 | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Digital header 12 | D11 (PWM) | 5 V logic | PB7 | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Digital header 13 | D12 / A11 | 5 V logic | PD6 | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Digital header 14 | D13 (PWM, LED_BUILTIN) | 5 V logic | PC7 | Arduino datasheet A000057 Pinout PDF A000057, page 1 |
| Digital header 15 | AREF | Arduino datasheet A000057 Pinout PDF A000057, page 1 | ||
| Digital header 16 | GND | Arduino datasheet A000057 Pinout PDF A000057, page 1 | ||
| ICSP 1 | CIPO (PB3) | Arduino datasheet A000057 Pinout PDF A000057, page 3 | ||
| ICSP 2 | +5V | 5 V | Arduino datasheet A000057 Pinout PDF A000057, page 3 | |
| ICSP 3 | SCK (PB1) | Arduino datasheet A000057 Pinout PDF A000057, page 3 | ||
| ICSP 4 | COPI (PB2) | Arduino datasheet A000057 Pinout PDF A000057, page 3 | ||
| ICSP 5 | RESET | Arduino datasheet A000057 Pinout PDF A000057, page 3 | ||
| ICSP 6 | GND | Arduino datasheet A000057 Pinout PDF A000057, page 3 |
Build with it
Project ideas for the Leonardo R3 (ATmega32U4). Each link opens Soldr with the prompt filled in; Soldr assigns the pins, checks power and shows how to wire every part.
- USB macro keypad: The 8 keypad lines fit on the 20 digital pins at 5 V logic, and the Leonardo's USB keyboard support sends the key presses.
- USB volume knob: Encoder and button use digital pins; the board sends media-key presses over USB.
- Presentation page-turner pedal: Each button is one input; the board appears as a keyboard and sends page-up and page-down.
Common mistakes
- Sending constant keystrokes or mouse moves from a sketch: the Arduino page warns that if the Mouse or Keyboard library runs constantly, it becomes difficult to program the board; add a start/stop switch (source: Arduino official documentation)
- Drawing more than the per-pin limit: Tech Specs list 10 mA per I/O pin while the pinout sheet marks 20 mA as the maximum; keep loads small and use a driver (source: Arduino datasheet A000057)
- Pulling more than 50 mA from the 3.3 V pin (source: Arduino datasheet A000057)
Not covered by the official source
- Arduino publishes no PDF datasheet for this board; the Tech Specs page and the pinout sheet are the sources.
- The Tech Specs (10 mA per I/O pin) and the pinout sheet (20 mA maximum per I/O pin) disagree; both are shown.
- The schematic is not used as a source on this page.
Sources
- Arduino official documentation (Arduino Leonardo hardware page, Tech Specs)
- Arduino datasheet A000057 (Arduino Leonardo full pinout, last update 25/11/2025)
- link (Microchip ATmega16U4/ATmega32U4 datasheet, Atmel-7766J 04/2016)