The RTC bus clock (CLK_RTC_APB) can be enabled and disabled
in the Main Clock module MCLK, and the default state of CLK_RTC_APB can be found in the
Peripheral Clock Masking section.
A 32KHz or 1KHz oscillator clock (CLK_RTC_OSC) is required
to clock the RTC. This clock must be configured and enabled in the 32KHz oscillator
controller (OSC32KCTRL) before using the RTC.
This oscillator clock is asynchronous to the bus clock
(CLK_RTC_APB). Due to this asynchronicity, writing to certain
registers will require synchronization between the clock domains. Refer to Section
Synchronization for further details.