How do ROS nodes in microcontrollers work?

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rosserial is a protocol for wrapping standard ROS serialized messages and multiplexing multiple topics and services over a character device such as a serial port or network socket. It has Client libraries that allow users to easily get ROS nodes up and running on various systems. And it also has ROS-side Interfaces allows you setup a node on the host machine to bridge the connection from the serial protocol to the more general ROS network. It's not ROS master discovery mechanism works between the host machine and the microcontroller. When you write your own project on microcontroller, you will depend on rosserial 's client libraries. If you just work on a host machine with a non-ROS microcontroller, you have to write a package which is able to connect with the device.