23.2.2 Physical and Logical GPIO Pins

SAM devices use two naming conventions for the I/O pins in the device; one physical and one logical. Each physical pin on a device package is assigned both a physical port and pin identifier (e.g. "PORTA.0") as well as a monotonically incrementing logical GPIO number (e.g. "GPIO0"). While the former is used to map physical pins to their physical internal device module counterparts, for simplicity the design of this driver uses the logical GPIO numbers instead.