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Arduino Leonardo

Category: Arduino
Manufacturer: Arduino
SKU: A000057
Status: Active
Release Year: 2012
Form Factor: Arduino Uno
Read Time: ~9 min read

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Arduino Leonardo top view
Arduino Leonardo bottom view
Arduino Leonardo pinout
Arduino Leonardo top view
Arduino Leonardo bottom view
Arduino Leonardo pinout

Overview

The Arduino Leonardo is an 8-bit AVR board built around the ATmega32U4 microcontroller — the key distinction being that the ATmega32U4 includes a built-in USB controller. Unlike the Arduino Uno, which uses a separate ATmega16U2 chip to bridge USB-to-serial, the Leonardo's MCU handles USB communication directly. This native USB capability enables the board to appear to a host computer as a USB keyboard, mouse, joystick, MIDI device, or any other USB HID class device — entirely in software, without additional hardware.

Released in 2012, the Leonardo shares the same physical form factor as the Arduino Uno and is compatible with most Uno shields. It features 20 digital I/O pins (with 7 PWM-capable), 12 analog inputs, and one hardware UART (Serial1). The USB virtual serial port (Serial) is independent from the hardware UART, meaning both can be used simultaneously.

The native USB design also introduces one notable behavioral difference from the Uno: there is no separate serial chip to maintain the USB connection during reset, so a brief disconnect occurs when the board resets during upload.

Quick Overview

Property Value
Microcontroller ATmega32U4 (8-bit AVR)
Clock Speed 16 MHz
Operating Voltage 5V
Input Voltage (recommended) 7–12V
Digital I/O Pins 20 (7 with PWM)
Analog Input Pins 12 (A0–A11)
Flash Memory 32 KB (4 KB used by bootloader)
SRAM 2.5 KB
EEPROM 1 KB
Hardware UART 1 (Serial1 on D0/D1)
USB micro-USB (native HID — ATmega32U4 is the USB controller)
I2C 1 (D2 SDA, D3 SCL)
SPI 1 (ICSP header)
Dimensions 68.6 × 53.4 mm

Key Features

  • Native USB HID via the ATmega32U4's built-in USB controller — no separate bridge chip
  • Emulate USB keyboard, mouse, joystick, or MIDI device with standard Arduino libraries
  • 20 digital I/O pins with 7 PWM outputs
  • 12 analog input channels (A0–A5 on dedicated pins; A6–A11 shared with digital pins D4, D6, D8, D9, D10, D12)
  • Simultaneous USB serial (CDC) and hardware UART (Serial1) — useful for debugging HID sketches
  • Compatible with many Arduino Uno shields (same 5V logic and header layout)
  • Single hardware UART (Serial1) on D0/D1 for communication with external serial devices
  • On-board ICSP header for SPI peripherals and bootloader programming

Technical Specifications

Specification Value
Microcontroller ATmega32U4
Architecture 8-bit AVR
Clock Speed 16 MHz
Operating Voltage 5V
Recommended Input Voltage 7–12V
Maximum Input Voltage 20V
Digital I/O Pins 20
PWM Pins 7 (D3, D5, D6, D9, D10, D11, D13)
Analog Input Pins 12 (A0–A5 dedicated; A6–A11 on digital pins)
DC Current per I/O Pin 40 mA
DC Current for 3.3V Pin 50 mA
Flash Memory 32 KB (4 KB bootloader)
SRAM 2.5 KB
EEPROM 1 KB
Hardware UART 1 (Serial1, D0=RX1, D1=TX1)
USB Serial (CDC) Serial (via USB — separate from Serial1)
I2C 1 (D2 SDA, D3 SCL)
SPI 1 (ICSP header MISO, MOSI, SCK, SS)
USB micro-USB (native, ATmega32U4 integrated controller)
On-Board LED Pin 13
Dimensions 68.6 × 53.4 mm

Pinout

Arduino Leonardo top view

Digital I/O Pins

Pin Function Notes
D0 (RX1) Hardware Serial1 RX Also general digital I/O
D1 (TX1) Hardware Serial1 TX Also general digital I/O
D2 Digital I/O / SDA I2C data line
D3 Digital I/O / SCL / PWM I2C clock line, PWM output
D4 Digital I/O / A6 Dual-function analog input
D5 Digital I/O / PWM PWM output
D6 Digital I/O / A7 / PWM Dual-function analog, PWM
D7 Digital I/O
D8 Digital I/O / A8 Dual-function analog input
D9 Digital I/O / A9 / PWM Dual-function analog, PWM
D10 Digital I/O / A10 / PWM Dual-function analog, PWM
D11 Digital I/O / PWM PWM output
D12 Digital I/O / A11 Dual-function analog input
D13 Digital I/O / PWM / LED Built-in LED, PWM output

Analog Pins

Pin Notes
A0–A5 Dedicated analog input pins (also usable as digital I/O)
A6–A11 Shared with D4, D6, D8, D9, D10, D12 — analog input only on those physical pins

Power Pins

Pin Description
VIN 7–12V input (barrel jack equivalent)
5V Regulated 5V output
3.3V 3.3V output (50 mA max)
GND Ground
RESET Pull LOW to reset
IOREF 5V reference for shields
AREF Analog reference for ADC

Power

The Arduino Leonardo can be powered from:

  1. micro-USB: 5V from the connected host computer.
  2. DC Barrel Jack: 7–12V DC, regulated on-board to 5V.
  3. VIN Pin: 7–12V from an external supply on the headers.

Each I/O pin can source or sink up to 40 mA. The 3.3V pin supplies up to 50 mA. Total I/O current draw should be managed to avoid overloading the on-board regulator.

HID Support

The Leonardo's most distinctive feature is its native USB HID capability. Because the ATmega32U4 contains the USB controller itself, it can enumerate as any standard USB HID class device without additional chips or libraries beyond what is built into the Arduino framework.

Available HID Classes

HID Class Library Example Use
USB Keyboard Keyboard.h Automate keystrokes, macros, text input
USB Mouse Mouse.h Move cursor, click buttons, scroll
USB Joystick Joystick.h (third-party) Game controller emulation
USB MIDI MIDIUSB.h Musical instrument interface

Keyboard Emulation Example

#include <Keyboard.h> void setup() { // Wait for the user to press a button on pin 2 pinMode(2, INPUT_PULLUP); Keyboard.begin(); } void loop() { if (digitalRead(2) == LOW) { // Type a string when button is pressed Keyboard.println("Hello from Arduino Leonardo!"); delay(500); // Debounce } }

Mouse Emulation Example

#include <Mouse.h> void setup() { Mouse.begin(); } void loop() { // Move the mouse cursor in a small circle Mouse.move(5, 0, 0); delay(50); Mouse.move(0, 5, 0); delay(50); Mouse.move(-5, 0, 0); delay(50); Mouse.move(0, -5, 0); delay(50); }

Important HID Note

When uploading HID sketches, the board may temporarily disconnect from the IDE as the USB device re-enumerates. If the board enumerates as a HID device and loses its serial port, press the RESET button once to interrupt the sketch and regain the serial port for a new upload.

Getting Started

What You Need

  • Arduino Leonardo (with or without headers)
  • Micro-USB cable
  • Computer with Arduino IDE installed

Steps

  1. Connect the Leonardo to your computer with a micro-USB cable.
  2. Open the Arduino IDE.
  3. Select Tools > Board > Arduino AVR Boards > Arduino Leonardo.
  4. Select the correct COM port under Tools > Port.
  5. Upload a sketch.

Blink Example

// Blink the built-in LED on pin 13 void setup() { pinMode(13, OUTPUT); } void loop() { digitalWrite(13, HIGH); delay(1000); digitalWrite(13, LOW); delay(1000); }

Serial Note

On the Leonardo, Serial refers to the USB CDC virtual serial port. Serial1 refers to the hardware UART on D0/D1. When using the Serial Monitor, wait for the USB serial connection to be established:

void setup() { Serial.begin(9600); while (!Serial) { ; } // Wait for USB serial port to open Serial.println("Leonardo ready"); } void loop() {}

Programming

  • Bootloader: Caterina bootloader (USB CDC, 1200-baud touch reset)
  • Upload method: IDE sends a 1200-baud "touch" on the CDC port to trigger reset and enter bootloader mode
  • No auto-reset via DTR: Unlike the Uno, the Leonardo lacks a hardware DTR reset circuit on a bridge chip. The 1200-baud touch protocol handles this via software.
  • If upload fails: Press the RESET button on the board at the moment the IDE indicates "Uploading…" to manually trigger the bootloader

There is no separate FTDI or ATmega16U2 bridge — the ATmega32U4 IS the USB device at all times.

Shield Compatibility

The Leonardo uses the same physical headers as the Arduino Uno R3 and operates at 5V logic, making it compatible with most Uno shields.

Shield Type Compatibility
Arduino Uno R3 shields Generally compatible (same 5V logic, same headers)
Shields using D0/D1 for Serial Verify — Serial1 is on D0/D1; USB CDC is the primary serial
Shields assuming auto-reset on DTR May require manual reset for re-upload

Note that a few shields that directly configure the UART or assume specific bootloader behavior may need minor adjustments.

Package Contents

Item Quantity
Arduino Leonardo with headers board 1

The board is also sold in a "without headers" variant for custom installations. A micro-USB cable is not included.

Applications

  • USB keyboard and mouse automation (macro pads, accessibility tools)
  • Custom USB game controllers and joysticks
  • USB MIDI controller instruments
  • USB HID badge readers and input automators
  • Educational projects introducing native USB concepts
  • Multi-device serial communication (USB CDC + Serial1 simultaneously)
  • Wearable input devices and gesture controllers

Where to Buy

Equivalent Boards

Board Key Difference
Arduino Micro Same ATmega32U4, same native HID, much smaller breadboard-compatible form factor
Arduino Uno R3 Same Uno form factor, ATmega328P (no native HID), separate USB bridge chip
Arduino UNO R4 Minima 32-bit ARM Cortex-M4 (RA4M1), has USB HID, same Uno form factor

Documentation

Notes

  • The Serial object on the Leonardo is the USB virtual COM port. Unlike on the Uno, it does not map to pins D0/D1. Hardware serial on D0/D1 is Serial1.
  • Because the ATmega32U4 manages its own USB connection, the board briefly disconnects and reconnects during each reset/upload cycle.
  • If a sketch enumerates the board as a pure HID device with no CDC serial port, the COM port disappears from the OS. Press RESET once to interrupt and restore CDC.
  • The Leonardo is sold both "with headers" (pre-soldered) and "without headers" (bare board). The SKU A000057 refers to the with-headers version.
  • Pin A6–A11 on the Leonardo are not available on separate analog header pins; they are multiplexed with digital pins D4, D6, D8, D9, D10, and D12.

Community

Revision History

Version Date Notes
v1.0 2026-06 Initial entry