Instead of manually starting conversions by setting one or more of the Start Conversion bits (CHnSTART) in Control Register A (CTRLA) or assigning events to virtual channels, the ADC can be put in free-running mode. This means that a number of channels are repeatedly converted in sequence as long as the mode is active.
The Channel Sweep Selection bitfield (SWEEP) in the Event Control register (EVCTRL) selects which channels to include in free-running mode. You can choose between channel 0 only, channel 0 and 1, channel 0 to 2, or all four channels.
Care should be taken not to change any involved MUX settings when in free-running mode, as this would corrupt conversion results.