Alternate Functions

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

When using GPIO, programs should be portable
We need an API that always works, with any CPU family
The same program should build and work everywhere
The applicatin code (and data) 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