Figure 5-16 shows the recommended oscillator connections when OSC1 is not used. An internal 10-kΩ pullup on the RTC_XTALIN terminal is turned on when OSC1 is disabled to prevent this input from floating to an invalid logic level which may increase leakage current through the oscillator input buffer. OSC1 is disabled by default after power is applied. Therefore, both RTC_XTALIN and RTC_XTALOUT terminals should be a no connect (NC) when OSC1 is not used.
For more information on disabling OSC1, see the device-specific technical reference manual.