Alternate Functions


Most GPIO pins have 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 matter of the driver
The driver (UART, SPI, whatever) is machine specific, so it can know the AF
Still, we want a consistent API offered by the GPIO API

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