Learn Lisp Programming Using MCUs - Part 2: Extending µLisp with Custom Functions

In the second installment of this series, Raul delves into the world of µLisp and shows how to extend capabilities by adding custom functions such as "pulsein" and "servo-create", enabling developers to interface with ultrasonic sensors and control servomotors. In Part 1, Raul presented µLisp concepts and syntax, and extensions specific to MCUs.

- Raul Alvarez-Torrico

μLisp is a dialect of the Lisp programming language. It is tailored to microcontrollers having limited amounts of RAM, and can be run on various platforms—from the Arduino Uno based on the ATmega328, up to the Teensy 4.0/4.1 [1]. To follow the topics discussed here, you need to be acquainted with the Arduino platform and any programming language from the Lisp family. Please see Part 1 of this article series (“Crash Course Offers Insight into Pioneering Language,” Circuit Cellar 411, October, 2024) for a brief introduction to Lisp programming and the µLisp dialect [2].

With µLisp, it is possible to add your own extension functions, such that they appear as built into the language after re-compiling and flashing the µLisp interpreter to your Arduino board. There’s an excellent tutorial on how to do this on the µLisp website [3]. I will build on that tutorial to discuss how to add custom functions. For the examples presented here, I will continue to use Seeed Studio’s Wio Terminal MCU development board; but the information discussed also can be applied to other µLisp-supported Arduino boards.

EXTENDING µLISP

In general, any function not supported by default in µLisp can possibly be added as an extension to the language—for instance, hardware access functions for servomotor control. Let’s explore a simple example to help us understand how the language extension system works.

Listing 1 shows the Arduino C code for adding the pulsein extension to µLisp, which is not available by default. This extension will make it possible to measure digital pulse widths. Five sections of the listing show the µLisp extensions structure. Like the core µLisp interpreter, extensions are also written in Arduino C language. So, any extensions to the interpreter imply basically writing C code. That makes adding them straightforward, using the Arduino C language built-in functions and external libraries.