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What is it ?
What is a Microcontroller ?
You would get the answer directly from the word itself. As you see, the word 'Microcontroller' is made up of two parts : Micro and Controller.
What does 'Micro' mean ? You might have learned it as a millionth (1/1000000) in your school physics. However, you don't need to use Physics to explain this word. It just mean 'very small'.
Then, what does 'Controller' mean ? It would mean many things.. but in this context, it would mean 'a Processor' which is a kind of IC (Integrated Chip) that can control various things outside of the chip by executing a predefined program.
Now just combine these two terms, a Microcontroller can be defined as 'Very small processor that can control various things outside of the chip by executing a predefined program'.
If you google 'Microcontroller' definition, you would find a lot of variations of the definition and I put down just a couple of definition that I think is good to me.
The Wikipedia defines a Microcontroller as follows :
A microcontroller (or MCU for microcontroller unit) is a small computer on a single integrated circuit.
The IoTAgenda defines a Microcontroller as follows :
A microcontroller is a compact microcomputer designed to govern the operation of embedded systems in motor vehicles, robots, office machines, complex medical devices, mobile radio transceivers, vending machines, home appliances, and various other devices. A typical microcontroller includes a processor, memory, and peripherals.
Even though you find many 'written definition', the practical definition of Microcontroller is not that clear because there are so many different types of Microcontroller with various level of performance. As you see in the definition, one keyword is 'Small' or 'Very Small' or 'Compact'. But these terms are not an absolute concept.. they are relative concept. It means there is no clear answer to 'how small it should be for a controller to be a 'Micro' controller'. In this kind of situation, the best way to understand the concept is just go through as many real life example as possible and try to build your own intuitive understanding. In this way, you would get more and more familiar with the concept. Once you get familiar with the concept, you wouldn't need any written definition. You would just know it.
Following are some of the examples of Microcontrollers. See if you can draw any intuitive image in your brain about Microcontroller. (Try search more examples with web search on your own).