13.1.1.4.4.1 No Synchronization (Asynchronous Input)
This mode is used for peripherals
where input synchronization is not required or the peripheral performs the
synchronization. Examples include communication ports SPI, and I2C.
Note: Using input synchronization when
the peripheral performs the synchronization can cause unexpected results. The user
must make sure that the GPIO pin is configured for asynchronous in this case.