Single chip microcomputer is a kind of microcomputer. Intuitively speaking, a single chip microcomputer is generally a chip or an integrated circuit (IC), which integrates resources such as cpu, ram, rom, io and flash, and can be used to control external circuits, and then realize your functions through external circuits.

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Arduino is a platform composed of single chip microcomputer and external circuit, which can realize some functions. Its data is open source and is a classic of open source hardware.

If it is not easy to understand, the relationship between MCU and Arduino is the relationship between cpu and a certain type of motherboard (plug in CPU)