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Over the years, dozens of serial protocols have been crafted to meet particular needs of embedded systems.
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So, we often opt for serial communication, sacrificing potential speed for pin real estate. If you've ever had to move a project from a basic Arduino Uno to a Mega, you know that the I/O lines on a microprocessor can be precious and few. But it requires many more input/output (I/O) lines. It's fast, straightforward, and relatively easy to implement. Parallel communication certainly has its benefits. Over a set amount of time, the mega-highway potentially gets more people to their destinations, but that rural two-laner serves its purpose and costs a fraction of the funds to build. Think of the two interfaces as a stream of cars: a parallel interface would be the 8+ lane mega-highway, while a serial interface is more like a two-lane rural country road. Example of a serial interface, transmitting one bit every clock pulse.