David Johnson-Davies' New Universal TinyI2C Routines Can Slash Your AVR Project's RAM Requirements

Embedded engineer David Johnson-Davies has released an update to his popular TinyI2C routines, offering "universal" support for "just about any Microchip/Atmel AVR processor" — and taking up just a fifth the RAM required by default libraries.

"The main difference between these routines and the standard Arduino Wire library is that these don't use buffers," Johnson-Davies explains of the next-generation version of the routines originally released back in 2018, "so have much smaller memory requirements and don't impose a limit on transmissions."

While the original release was entirely functional, the new version aims to address a key issue: Fragmentation of I2C peripherals within the AVR ecosystem. "Over the years different generations of AVR chips have featured three different, incompatible peripherals to handle I2C," Johnson-Davies explains. "[The] Universal Serial Interface (USI) peripheral; 2-Wire Serial Interface (TWI) peripheral; [and the] Two-Wire Interface (TWI) peripheral," the latter being a new TWI variant available in the company's latest parts.

The new universal release of the TinyI2C routines works around this incompatibility in a simple way: Providing three separate blocks of code, selected during compilation via #if defined statements. Just set up your development environment correctly, and the routines will handle the rest.

For those who had previously been using Johnson-Davies' TinyI2C routines, the new release effectively replaces the original TinyI2C as well as the newer TinyMegaI2C while bringing major benefits over the Arduino Wire I2C routines: No need to assign buffers, meaning RAM usage is considerably lower; no limits on transmission length; a flexible read routine; and support for polling rather than interrupts.

To test the new release out, Johnson-Davies put together two sample programs. One uses a pair of LED matrices as a display for an I2C bus scanner; the other uses them to display thermometer data. The latter also serves as a benchmark: "Depending on the platform," Johnson-Davies finds, "the TinyI2C code is typically about half the size of the Arduino Wire code, with 5 per cent of the RAM usage."

A full write-up of the routines is available on Johnson-Davies' website, with source code available on the project's GitHub repository under the permissive Creative Commons-Attribution 4.0 International license.