The peripheral clock (CLK_PER) is required to be at least four times faster than the RTC clock (CLK_RTC) for reading the counter value, regardless of the prescaler setting.
A 32.768 kHz crystal can be connected to the TOSC1 or TOSC2 pins, along with any required load capacitors. Alternatively, an external digital clock can be connected to the TOSC1 pin.