Introduction to some hardware terms
The Linux kernel GPIO subsystem is not just about GPIO toggling. It is tightly coupled to the pin controller subsystem; they share some terms and concepts that we need to introduce:
- Pin and pad: A pin is a physical input or output wire/line that transports an electrical signal from or to a component. In schematics, the term "pin" is widely used. Contact pads, on the other hand, are the contact surface areas of a printed circuit board or an integrated circuit. As a result, a pin comes from a pad, and a pin is a pad by default.
- GPIO: Most MCUs and CPUs can share one pad among several functional blocks. This is accomplished by multiplexing the input and output signals of the pad. The different modes the pin/pad can operate in are known as ALT modes (or alternate modes), and it is common for CPUs to support up to eight settings (or modes) per pad. GPIO is one of these modes. It allows changing the pin direction and reading its value...
