6.4.1.4 Configuring Analog and Digital Port Pins (ANSELx)

The ANSELx register controls the operation of the analog port pins. The port pins that are to function as analog inputs must have their corresponding ANSEL and TRISx bits set. To use port pins for I/O functionality with digital modules, such as Timers, SERCOMs and so on, the corresponding ANSELx bit must be cleared. The ANSELx register has a default value of 0xFFFF; therefore, all pins that share analog functions are, by default, analog and not digital.

If the TRISx bit is cleared (output) while the ANSELx bit is set, the digital output level (V_OH or V_OL) is converted by an analog peripheral, such as the ADC module or the comparator module. When the PORTx register is read, all pins configured as analog input channels are read as cleared (a low-level). Pins configured as digital inputs do not convert an analog input. Analog levels on any pin defined as a digital input (including the ANx pins) can cause the input buffer to consume current that exceeds the device specifications.