25.2.1 Block Diagram
The timer/counter can be clocked from multiple clock sources, including the Peripheral Clock (CLK_PER), internal oscillators, and Event System (EVSYS).
The Clock Select (CLKSEL) bit field in the Control C (TCFn.CTRLC) register selects one of the clock sources used as an input for the prescaler.
By using the EVSYS, any event source, such as an external clock signal on any I/O pin, may be used as the counter clock input. In addition, the timer/counter can be controlled via the event system.