The driving factor for the microcontroller functions, speed, power consumption and cost(!) are the specific volume applications it is used for. Like Microprocessors for typical data processing application, such as a desktop computer, need high data throughput - resulting in high clock rates. A microcontroller for embedded applications, like controlling a washing machine motor, need specific peripheral I/O functions and memory all on one chip. Cost at the performance needed is here the driving factor.
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