Just USB? :-)
My first home computer was a ZX-80 with 1KB RAM - total. That included shared video & OS memory. Learned a lot trying to cram tokenized BASIC programs into what little was available.
Ya, there is open source USB code for the ATTiny2313 written in C that can be used to get your bitty little processor talking to the rest of the world. It’s fun to read; there is rather a lot going on with USB.