The minimum capture is enabled by writing the CAPTMIN mode in the Channel n Capture Mode bits in the Control A register (CTRLA.CAPTMODEn = CAPTMIN).
CCx Content:
In CAPTMIN operations, CCx keeps the Minimum captured values. Before enabling this mode of capture, the user must initialize the corresponding CCx register value to a value different from zero. If the CCx register initial value is zero, no captures will be performed using the corresponding channel.
MCx Behaviour:
In CAPTMIN operation, capture is performed only when on capture event time, the counter value is lower than the last captured value. The MCx interrupt flag is set only when on capture event time, the counter value is upper or equal to the value captured on the previous event. So interrupt flag is set when a new absolute local Minimum value has been detected.