We also have an extensive concept for cooperative, stackless multitasking, based on Dunkels Protothreads [19], which we expanded on with Resumable Functions [20] which are used in all device drivers to make communication non-blocking and allow multiple device drivers on the same bus to safely access it.

This form of multithreading is very resource friendly since it does not require context switches and only the main call stack (literally 2 bytes of RAM per Protothread).
However, this is quite an advanced topic and there are disadvantages to this approach compared to a full real-time operating system (power efficiency, hard real-time), such as mbed os or riot-os, however, they do not necessarily work well on AVRs.
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