David Johnson-Davies published another great and detailed project based on ATtiny85. It’s an harmonic function generator with an OLED display.
This article describes a simple function generator based on an ATtiny85 which allows you to generate a virtually unlimited number of waveforms using additive harmonic synthesis, by specifying the amplitude of each of the waveform’s harmonics.
It includes a volume control, audio amplifier, and loudspeaker so you can hear the waveforms. It’s not only a useful waveform generator, but also a good introduction to the composition of musical notes.
Harmonic Function Generator using ATtiny85 – [Link]