SAM L21/C20/C21 Clock Setup

The RTC clock can be selected from OSC32K, XOSC32K or OSCULP32K , and a 32KHz or 1KHz oscillator clock frequency is required. This clock must be configured and enabled in the 32KHz oscillator controller before using the RTC.

Table 1 lists the available RTC clock.

Table 1. RTC Clocks Source
RTC clock frequency	Clock source	Description
1.024kHz	ULP1K	1.024kHz from 32KHz internal ULP oscillator
32.768kHz	ULP32K	32.768kHz from 32KHz internal ULP oscillator
1.024kHz	OSC1K	1.024kHz from 32KHz internal oscillator
32.768kHz	OSC32K	32.768kHz from 3KkHz internal oscillator
1.024kHz	XOSC1K	1.024kHz from 32KHz internal oscillator
32.768kHz	XOSC32K	32.768kHz from 32KHz external crystal oscillator

Note: For the calendar to operate correctly, an asynchronous clock of 1Hz should be used.