SAM D20/D21/R21/D10/D11/DA1 Clock Setup

The RTC is typically clocked by a specialized GCLK generator that has a smaller prescaler than the others. By default the RTC clock is on, selected to use the internal 32KHz RC-oscillator with a prescaler of 32, giving a resulting clock frequency of 1024Hz to the RTC. When the internal RTC prescaler is set to 1024, this yields an end-frequency of 1Hz for correct time keeping operations.

The implementer also has the option to set other end-frequencies. Table 1 lists the available RTC frequencies for each possible GCLK and RTC input prescaler options.

Table 1. RTC Output Frequencies from Allowable Input Clocks
End-frequency	GCLK prescaler	RTC prescaler
32KHz	1	1
1KHz	32	1
1Hz	32	1024

The overall RTC module clocking scheme is shown in Figure 1.

Figure 1. SAM D20/D21/R21/D10/D11/DA1 Clock Setup

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