Many microcontrollers live their lives in isolation: Embedded in your toaster, washing machine or other utility device, they communicate only with the sensors that give them information and the actuators that let them control things. Another class of microcontroller communicates with peer devices via buses such as the CAN bus (particularly in cars) or MOD bus in an industrial environment. In this way, a group of devices can cooperate to provide a distributed functionality: In the case of modules on the CAN bus in a car, one might look after the buttons on the steering wheel, whilst another will accept commands to activate a turn signal or open the sunroof. Communication between the modules is the key here.

Larger, more powerful devices might have an Ethernet or WiFi port for communication with a remote server or for configuration. The devices I will discuss today have a USB interface that allows configuration, uploading or downloading data (such as music in a simple, portable music player), or software updates. USB was the natural successor to the once ubiquitous RS-232 serial interface. RS-232 was not well-defined, with many variants in the protocol and the hardware implementation. Its early use was in communicating between computers and modems (sometimes via acoustic couplers), and computers and "dumb" terminals.

USB was designed to fix some of those issues, as well as to increase bandwidth and general flexibility. Unlike RS-232, USB is more than a point-to-point link. Multiple devices can be supported by a single host via hubs. USB is also more or less plug-and-play for the majority of USB setups, such as serial port emulation, block devices (e.g., mass storage such as thumb drives and SD card adapters), and audio devices such as microphones and speakers. Devices whose requirements don't fall into those categories can implement their own transfer protocols using the "bulk transfer" class.

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