The RTC crystal connected on the device supplies the free-running slow clock. The accuracy of the slow clock frequency must be 32.768 kHz ±150 ppm. In this mode of operation, the crystal is tied between RTC_XTAL_P (pin 51) and RTC_XTAL_N (pin 52) with a suitable load capacitance to meet the ppm requirement.
Figure 8-9 shows the crystal connections for the slow clock.
Section 18.104.22.168.1 lists the RTC crystal requirements.