14.4.1.1 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 1KHz to the RTC. When the internal RTC prescaler is set to 1024, this yields an end-frequency of 1Hz.

The implementer also has the option to set other end-frequencies. Table 14-2 lists the available RTC frequencies for each possible GCLK and RTC input prescaler options.

Table 14-2. 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 14-1.

Figure 14-1. SAM D20/D21/R21/D10/D11/DA1 Clock Setup