Lisp for microcontrollers

Lisp for Arduino, Adafruit M0/M4, Micro:bit, ESP8266/32, RISC-V, and Teensy 4.x boards.


New Version 4.0 for all platforms

Version 4.0 of uLisp is a major update of uLisp for all platforms that totally overhauls the way that symbols are implemented. It now uses the same representation for long symbols as for strings, making the symbol table unnecessary, with the following benefits:

  • The space previously allocated to the symbol table is no longer needed, freeing up this memory to be available for the Lisp workspace.
  • Previously programs that made heavy use of long symbols could fail with a “no room for long symbols” error message, because they filled the symbol table. This can no longer happen.
  • Long symbols that are no longer used are automatically garbage-collected. For example, if you redefine a function with different local variable names, the space used by the previous names is freed up.
  • In general, programs run in less memory. For example, many of the uLisp examples that previously required a ATSAMD51 ARM M4 processor will now run on an ATSAMD21 ARM M0 processor.
  • Platforms with limited RAM, such as the Arduino Uno and Nano Every, could previously only use short symbols of up to three characters. They can now use long symbols like any other platform.
  • Short symbols of up to three characters on 8/16-bit platforms, or up to six alphanumeric characters on 32-bit platforms, are packed into the object cell, making them as memory efficient as before.

uLisp® is a version of the Lisp programming language specifically designed to run on microcontrollers with a limited amount of RAM, from the Arduino Uno based on the ATmega328 up to the Teensy 4.0/4.1. You can use exactly the same uLisp program, irrespective of the platform.

Because uLisp is an interpreter you can type commands in, and see the effect immediately, without having to compile and upload your program. This makes it an ideal environment for learning to program, or for setting up simple electronic devices.

Lisp is also an ideal language for learning about fundamental programming concepts. It incorporates string handling, list processing, and garbage collection, and so is also an excellent language for expressing complex ideas, such as teaching a robot to solve mazes or finding the shortest route on a map. As well as supporting a core set of Lisp functions uLisp includes Arduino extensions, making it ideal as a control language for the Arduino.

You can download the current version of uLisp free from the Download uLisp page.

uLisp projects

Snake2.jpgSimple arcade game

GPSPlotter2.jpgGPS mapping application

NeoPixelsSmall.jpgDriving NeoPixels using assembler

ClueRayTrace2.jpgRay tracing with uLisp

LispBadgeHand2.jpgLisp Badge

DotMatrixClock2.jpgDot-matrix clock


RAM: At least 2 Kbytes.

Program memory: At least 32 Kbytes.

EEPROM, flash, or FRAM memory: If available, used for saving and loading the uLisp workspace.

The versions of uLisp for 8-bit and 16-bit platforms support integers between -32768 and 32767.

AVR version

The AVR version of uLisp supports the following boards:

Arduino Uno or other ATmega328-based cards. These will give you enough memory for a simple uLisp application using short symbol names. All the Simple examples will run on the Arduino Uno.

Arduino Mega 2560 or other ATmega2560-based boards. These will give you enough memory for a fairly complex application; for examples see AnimalsTweetmazeRoute finder, and Infinite precision arithmetic.

ATmega1284. Although there isn't an official Arduino board based on it, the ATmega1284 is easy to wire up on a prototyping board, and provides a generous 16 Kbytes RAM.

ATmega4809 boards. The Arduino Nano Every and Microchip Curiosity Nano evaluation board are low-cost platforms based on the ATmega4809.

32/64-bit platforms

The versions of uLisp for 32-bit platforms support integers between 2147483647 and -2147483648, and 32-bit floating-point numbers.

ARM version

The ARM version of uLisp supports the following boards:

Arduino Zero. This board is based on the SAMD21 ARM Cortex-M0+ core and provides 256 Kbytes of flash and 32 Kbytes of RAM.

Arduino MKRZero. This is similar to the Arduino Zero, based on the SAMD21 ARM Cortex-M0+ core and with 256 Kbytes of flash and 32 Kbytes of RAM. It incorporates an SD-card socket, allowing you to use an SD card for saving and loading uLisp images.

Adafruit M0 boards. The Adafruit Gemma M0, Adafruit ItsyBitsy M0, and Adafruit Feather M0 are each based on the ATSAMD21 48 MHz ARM Cortex M0+ microcontroller. They have similar features and performance; the main difference is the form-factor of each board.

Adafruit M4 boards. The Adafruit Metro M4 Grand Central, Adafruit Metro M4, Adafruit ItsyBitsy M4, and Adafruit Feather M4 are each based on the ATSAMD51 120 MHz ARM Cortex M4 microcontroller.

Adafruit PyGamer and PyBadge. The Adafruit PyGamer and PyBadge are handheld game platforms based on the ATSAMD51 120 MHz ARM Cortex M4 microcontroller and incorporating a 160x128 colour TFT display.

Adafruit nRF52840 boards. The Adafruit CLUE and Adafruit ItsyBitsy nRF52840 are each based on the Nordic Semiconductor nRF52840 64 MHz ARM Cortex-M4 microcontroller, with 1 Mbyte of flash program memory and 256 Kbytes of RAM.

BBC Micro:bitThis is based on a Nordic Semiconductor nRF51822 ARM Cortex-M0 microcontroller. It runs at 16 MHz and provides 256 Kbytes of flash program memory and 16 Kbytes of RAM.

Maxim MAX32620FTHRThis is based on a Maxim MAX32620 ARM Cortex-M4F microcontroller running at 96 MHz, with 2048 Kbytes of flash memory and 256 Kbytes of RAM.

Teensy 4.0 and 4.1. These are based on the NXP iMXRT1062 ARM M7 processor running at 600 MHz, and with 1 Mbytes of RAM.

ESP8266/ESP32 version

The ESP8266/ESP32 version of uLisp supports the following boards:

ESP8266 boards. These boards are based on the 32-bit Tensilica Xtensa L106 microprocessor running at 80 MHz, with 4 Mbytes of flash and 80 Kbytes of RAM. They include integrated Wi-Fi.

ESP32 boards. These boards are based on the 32-bit Tensilica Xtensa LX6 microprocessor running at 160 or 240 MHz, with 4 Mbytes of flash and 520 Kbytes of RAM. They include integrated Wi-Fi and dual-mode Bluetooth.

RISC-V version

The RISC-V version of uLisp supports the following boards:

Sipeed MAiX RISC-V boards. These boards are based on the Kendryte K210 RISC-V Dual Core 64 bit 400 MHz processor and provide 8 Mbytes RAM and 16 Mbytes flash. They are similar in performance.

Other platforms

For additional platforms supported by earlier versions of uLisp see: Older versions.


See Performance.


The language is generally a subset of Common Lisp, and uLisp programs should also run under Common Lisp. But note that there is one namespace for functions and variables; in other words, you cannot use the same name for a function and a variable. 

The 8/16 bit platforms provide lists, symbols, integers, characters, strings, (32-bit platforms), and streams.

In addition the 32-bit platforms provide floating-point numbers and arrays, including bit-arrays.

An integer is a sequence of digits, optionally prefixed with "+" or "-". On the 8/16-bit platforms integers can be between -32768 and 32767. On the 32-bit platforms integers can be between 2147483647 and -2147483648. You can enter integers in hexadecimal, octal, or binary with the notations #x2A, #o52, or #b101010, all of which represent 42.

All platforms support long user-defined symbol names. Almost any sequence of characters that doesn't represent a number can be used as a symbol; so, for example, 12a and *2 are valid symbols.

uLisp provides tail-call optimization, so applications written using recursive functions can be as efficient as using iteration.

Strings can consist of an arbitrary sequence of ASCII characters. Strings can be unlimited length, and are automatically garbage-collected.

uLisp includes a mark and sweep garbage collector. Garbage collection takes under 1 msec on an Arduino Uno or under 3 msec on an Arduino Mega 2560 (see Performance).

uLisp also includes a simple program editor (see Using the program editor), a trace facility, and a pretty printer (see Debugging in uLisp).

Saving the workspace

On most platforms you can save the workspace, containing the functions and variables you have defined, using the save-image function, and then reload it with load-image. These functions take advantage of whatever non-volatile storage is available on each platform:

  • On platforms with a DataFlash chip, such as Adafruit's ATSAMD21, ATSAMD51, and nRF52840 Express boards, you can save the entire workspace to DataFlash.
  • On the AVR128DA48 and AVR128DB48, the Raspberry Pi Pico, and platforms based on the ATSAMD21 such as the Arduino Zero and Seeeduino XIAO, you can save the workspace to the flash program memory.
  • On older AVR-based platforms, such as the Arduino Uno and Arduino Mega 2560, you can save the workspace to EEPROM.
  • On the ESP8266 and ESP32 boards you can save the workspace to flash memory using the EEPROM emulation.
  • On any platform you can add an SD-card interface, and then save the workspace to an SD card.


The following example illustrates how you might use uLisp.

After uploading uLisp to your microcontroller board you can communicate it via by typing or pasting commands into the Serial Monitor, or using a serial terminal. For more information see Using uLisp.

Suppose you have a red LED connected to the analogue output pin 9 on an Arduino Uno. Then you can type in the Lisp command:

(analogwrite 9 128)

to set the LED to 128, which corresponds to half brightness.

To save having to write this command every time you want to set the red LED you can define a function called red:

(defun red (x) (analogwrite 9 x))

Now you can achieve the same effect simply by writing:

(red 128)

In each case the LED changes immediately, as soon as you type in the command.

Suppose you've got a potentiometer connected to vary the voltage on the analogue input A0. You could define a function dim to make the potentiometer adjust the brightness of the LED with:

(defun dim () (loop (red (/ (analogread 0) 4)))

and run it by typing:


Finally, you could save the uLisp image to EEPROM, and specify that dim should be run on load, by entering:

(save-image 'dim)

When you reset the Arduino dim will now load and run automatically.

This is a simple example showing how uLisp allows you to build up complex programs from simpler components, testing each of the components as you go along.

Next: Performance