A-Star 32U4 Micro

The A-Star 32U4 Micro is a tiny programmable module featuring Atmel’s ATmega32U4 microcontroller. It packs a Micro-USB interface and 18 digital input/output pins (of which 7 can be used as PWM outputs and 8 as analogue inputs) onto a board measuring only 1″ × 0.6″ and ships preloaded with an Arduino bootloader.

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Supplier Link: [Pololu MPN:3101]


A-Star 32U4 Micro, bottom view with U.S. quarter for size reference

The Pololu A-Star 32U4 Micro is a general-purpose programmable module based on Atmel’s ATmega32U4 AVR microcontroller, which has 32 KB of flash program memory, 2.5 KB of RAM, and built-in USB functionality. Onboard features of the A-Star (abbreviated A*) include a 16 MHz resonator, a USB Micro-B connector, an in-system programming (ISP) header, and a pair of indicator LEDs. A voltage regulator and power selection circuit allow the board to be powered from either USB or an external 5.5 V to 15 V source, while a resettable PTC fuse on the USB VBUS supply and reverse protection on VIN help protect it from accidental damage.

The A-Star 32U4 Micro breaks out 15 general-purpose Polou/O lines along two rows of pins, including 7 usable as PWM outputs and 8 usable as analogue inputs; another three GPIO pins can be accessed through the 6-pin ISP header. It fits all this into a 20-pin dual in-line package (DIP) measuring only 1″ × 0.6″ (even smaller than competing ATmega32U4 boards like the Teensy 2.0 and Pro Micro), and its 0.1″ pin spacing makes the A* easy to use with solderless breadboards, perfboards, and 0.1″-pitch connectors.

Polou's comprehensive user’s guide provides the basics you need to get started with the A-Star as well as detailed technical information for advanced users.

This product requires a USB A to Micro-B cable (not included) to connect to a computer.


  • Dimensions: 1″ × 0.6″ (1.05″ × 0.6″ including USB Micro-B connector)
  • Programmable 16 MHz Atmel ATmega32U4 AVR microcontroller
    • 32 KB flash (4 KB used by bootloader, leaving 28 KB available for user program by default)
    • 2.5 KB SRAM
    • 1 KB EEPROM
    • Native full-speed USB (12 Mbps)
  • Preloaded with Arduino-compatible bootloader (no external programmer required)
  • Many Polou/O lines in a small package
    • 15 general-purpose Polou/O pins available along the sides of the board
    • 3 additional Polou/O pins available on ISP header
    • 7 pins can be configured as hardware PWM outputs
    • 8 pins can be configured as analogue inputs
  • 2 user-controllable LEDs
  • Can be powered from USB or external 5.5 V to 15 V source on VIN
  • PTC fuse on VBUS supply
  • Reverse-voltage protection on VIN supply
  • 6-pin ISP header for use with an external programmer
  • Comprehensive user’s guide

Arduino compatibility

The A-Star 32U4 ships with a preloaded Arduino-compatible bootloader (which uses 4 KB of flash memory, leaving 28 KB available for the user program). Polou provide a software add-on that enables the board to be easily programmed from the Arduino environment.

The A-Star 32U4 uses the same microcontroller as the Arduino Leonardo and Arduino Micro and runs at the same frequency, making it just as powerful. Although the larger boards offer a few more Polou/O pins, the A-Star 32U4 Micro fits in an area half that of the Arduino Micro, and it takes up only 11% as much area as a standard full-size Arduino!

Pololu A-Star 32U4 Micro, Pololu A-Star 32U4 Mini SV, Arduino Micro, and Arduino Leonardo


A-Star 32U4 Micro pinout diagram

This diagram identifies the Polou/O and power pins on the A-Star 32U4 Micro; it is also available as a printable PDF (409k pdf). For more information about the ATmega32U4 microcontroller on this board, see Atmel’s ATmega32U4 documentation.

Printed on the A* circuit board are indicators that you can use to quickly identify each pin’s capabilities: a triangle next to the pin means it can be used as an analogue input, and a square wave symbol under the pin number means it can be used as a PWM output.

The board can either be powered directly from the USB 5 V supply or from a separate 5.5 V to 15 V source on the VIN pin, which is reduced to 5 V by a 100 mA low-dropout (LDO) regulator; you can access this 5 V supply through the 5V power output pin. Additionally, the ATmega32U4 contains an internal 3.3 V regulator whose output is available on the 3V3 pin. Current drawn from the 3V3 output should not exceed about 50 mA, and when the board is being powered through VIN, the sum of the 5V output current, 3V3 output current, GPIO output current, and current used by the board itself (typically about 25 mA) should not exceed 100 mA.

Included hardware

A 1×20-pin breakaway 0.1″ male header is included with the A-Star 32U4 Micro, which can be soldered in to use the board with perfboards, breadboards, or 0.1″ female connectors. (The headers might ship already separated into two 1×10 pieces.) Also included is a 2×3 header that can be installed to allow external programming of the microcontroller through the AVR ISP interface, such as with Polou's USB AVR programmer.

A-Star 32U4 Micro with included optional headers

A-Star 32U4 Micro with soldered headers and connected USB cable

The A-Star family

From top to bottom: A-Star 32U4 Micro, Mini SV, and Prime SV

The A-Star 32U4 Micro is a part of Polou's larger A-Star 32U4 family, all of whose members are based on the same ATmega32U4 microcontroller, feature native USB interfaces, and are preloaded with Arduino-compatible bootloaders. The table below shows some key features and specifications of Polou's A-Star microcontroller boards to help you choose the right one for your application.

A-Star 32U4 Micro

A-Star 32U4 Mini ULV

A-Star 32U4 Mini LV

A-Star 32U4 Mini SV

A-Star 32U4 Prime LV

A-Star 32U4 Prime SV
Microcontroller: ATmega32U4 ATmega32U4 ATmega32U4
User Polou/O lines: 18 26 26
PWM outputs: 7 7 7
Analogue inputs: 8 12 12
Ground access points: 2 4 43
User LEDs: 2 3 3
User pushbuttons: 3
Reset button:  
Power switch:    
Buzzer option:    
microSD option:    
LCD option:    
Operating voltage: 5.5 V to 15 V 0.5 V to 5.5 V 2.7 V to 11.8 V 5 V to 36 V 2.7 V to 11.8 V 5 V to 36 V
Regulator type: linear switching
Regulated current: 100 mA(1) 500 mA(1) 1 A(1) 500 mA(1) 1 A(1) 1 A(1)
Dimensions: 1″ × 0.6″ 1.9″ × 0.7″ 2.8″ × 2.1″
Weight: 1.3 g(2) 3.4 g(2) 13 g to 33 g

1 These values are rough approximations for comparison purposes. Available current depends on input voltage, current consumed by the board, ambient conditions, and regulator topology. See product documentation and performance graphs for details.

2 Without included optional headers.




Size: 0.6″ × 1.05″ × 0.18″1
Weight: 1.3 g2

General specifications

Processor: ATmega32U4 @ 16 MHz
RAM size: 2560 bytes
Program memory size: 32 Kbytes3
Motor channels: 0
User Polou/O lines: 154
Minimum operating voltage: 5.5 V
Maximum operating voltage: 15 V
Logic voltage: 5 V
Reverse voltage protection?: Y5
External programmer required?: N


Without included optional headers. This measurement includes the USB Micro-B connector, which extends 0.05″ past the edge of the PCB.
Without included optional headers.
Note that 4 KB of the MCU's 32 KB of flash memory is used by the pre-installed USB bootloader. All 32 KB is available when programming via the ISP header rather than the bootloader.
All 15 can be used as digital Polou/O and 8 can be used as analogue inputs. An extra 3 digital Polou/O lines are accessible through the ISP header.


Documentation and other information

 Pololu A-Star 32U4 User’s Guide (Printable PDF)

User’s manual for the Pololu A-Star 32U4 family of user-programmable boards.

File downloads

A-Star 32U4 Micro schematic diagram (253k pdf)
Printable schematic diagram of the A-Star 32U4 Micro.
Pololu A-Star/Zumo 32U4 Software and Drivers (version 2.0.0) (98k zip)
This download contains the Windows drivers for the A-Star, the add-on for the Arduino IDE, the source code and compiled version of the bootloader, and a configuration file that some Linux users will need.
A-Star 32U4 Micro pinout diagram (409k pdf)
Printable pinout diagram of the A-Star 32U4 Micro.
A-Star 32U4 Micro dimension diagram (255k pdf)
Detailed dimension diagram of the A-Star 32U4 Micro.
A-Star 32U4 Micro drill guide (38k dxf)
This DXF drawing shows the locations of all of the board’s holes.

Recommended links

A-Star 32U4 Arduino library
The AStar32U4 library for the Arduino IDE helps interface with the on-board hardware on the A-Star 32U4 controllers.
AStar32U4 library documentation
Documentation for the AStar32U4 Arduino library.
A-Star/Zumo 32U4 repository on GitHub
This repository contains the A-Star Arduino add-on files, the Windows drivers, and the source code of the bootloader, as well as the AStar32U4Prime library and the Zumo32U4 library.
Arduino Software
Arduino integrated development environment (IDE) software
ATmega32U4 documentation
Atmel’s product page for the ATmega32U4, with links to its datasheet and other resources.
AVR Libc Home Page
The web site for AVR Libc, which is the standard library of functions that you can use with C and C++ on the AVR.
Pololu A-Star and Orangutan Forum Section
The A-Star and Orangutan discussion section of the Pololu Robotics Forum.
AVRDUDE is a cross-platform command-line utility for programming the flash memory on AVR microcontrollers.
AVR Freaks
AVR community with forums, projects, and AVR news.
LUFA – the Lightweight USB Framework for AVRs
LUFA is an embedded software library written in C that can be used to create USB applications on USB-capable AVRs. It comes with a large library of example USB applications and bootloaders.
Atmel Studio
Atmel’s free integrated development environment (IDE) for AVRs.
A free, open-source suite of development tools for the AVR family of microcontrollers, including the GNU GCC compiler for C/C++.
LPS Arduino library
This is a library for the Arduino that interfaces with Polou's LPS25H and LPS331AP pressure/altitude sensor carriers as well as the pressure sensor on the AltIMU-10 v3 (it also works with the original AltIMU-10). It makes it simple to read the raw pressure data from the sensor, and it provides functions to help calculate altitude based on the measured pressure.
Arduino library for addressable RGB LED strips from Pololu

This library allows you to control an arbitrary number of WS281x-Based Addressable RGB LEDs from an Arduino.

LSM303 Arduino library
This is a library for the Arduino that interfaces with our LSM303D, LSM303DLHC, and LSM303DLM 3D compass and accelerometer carriers as well as the compass and accelerometer ICs on the MinIMU-9 v3 and AltIMU-10 v3 (it also works with older versions of those boards, some of which used the LSM303DLH and LSM303DLHC). It makes it simple to configure the device and read the raw accelerometer and magnetometer data, and it has a function for computing the tilt-compensated heading for those looking to use the LSM303 as a tilt-compensated compass.
L3G Arduino library
This is a library for the Arduino that interfaces with our L3GD20H and L3GD20 3-axis gyro carriers as well as the gyros on the MinIMU-9 v3 and AltIMU-10 v3 (it also works with older versions of those boards, some of which used the L3G4200D and the L3GD20). It makes it simple to configure the device and read the raw gyro data.
Arduino Library for the Pololu QTR Reflectance Sensors
This guide explains how to use the QTRSensors library to read Pololu QTR reflectance sensors and QTR sensor arrays with Arduinos and Arduino-compatible devices like the Pololu Orangutan robot controllers.

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