ROS on ARM Cortex-M microcontrollers?

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There are various in-progress projects to implement the ROS communication stack in a way that has no dependencies and could work with, for example, LWIP. The most complete at this point is probably rosc.

That said, my preferred solution is to use rosserial—you run a lightweight server node on your Linux/ROS machine, which proxies topics and services from the "real" ROS network over a serial or socket connection to the microcontroller. There are several examples of rosserial client libs; unfortunately none for a Cortex at this point, but it wouldn't take much to port the Arduino example to it.

Disclosure: I'm the author of the performance- and reliability-oriented C++ rosserial_server implementation.