14.4.7.3 External Reference Frequency Setup

When using an external reference frequency, the signal must be connected to pin 26 (XTAL1) as indicated in the figure below and the XTAL_MODE bits in the XOSC_CTRL register (XOSC_CTRL.XTAL_MODE) need to be set to the external oscillator mode for power saving reasons. The oscillation peak-to-peak amplitude shall be between 100mV and 500mV, the optimum range is between 400mV and 500mV. Pin 25 (XTAL2) should not be wired. It is possible, among other waveforms, to use sine and square wave signals.