Alternate Functions

Most GPIO pins feature alternate functions
Each pin has one or several predefined uses
PWM, UART, SPI, I2C, ...
Most pins are part of the GPIO subsystem
Usually, high-speed signals (e.g. USB) live on dedicated pins

When using GPIO, programs should be portable
We need an API that always works.
The same program should build and work everywhere
The code should never refer to CPU specifics
When using alternate functions, it's a device-specific issue
The driver (UART, SPI, whatever) is machine specific,
It must know the device registers
It can know the device pins (and AF) too
Still, we want a consistent API for GPIO use

When offering and alternate function API, GPIO must be 0
Portability is paramount