25.6.2.1 Initialization

The following bits are enable-protected, meaning that they can only be written when the RTC is disabled (CTRLA.ENABLE=0):

  • Operating Mode bits in the Control A register (CTRLA.MODE)
  • Prescaler bits in the Control A register (CTRLA.PRESCALER)
  • Clear on Match bit in the Control A register (CTRLA.MATCHCLR)
  • Clock Representation bit in the Control A register (CTRLA.CLKREP)
  • BKUP registers Reset On Tamper bit in Control A register (CTRLA.BKTRST)
  • GP registers Reset On Tamper Enable in Control A register (CTRLA.GPTRST)

The following registers are enable-protected:

  • Control B register (CTRLB)
  • Event Control register (EVCTRL)
  • Tamper Control register (TAMPCTRL)

Enable-protected bits and registers can be changed only when the RTC is disabled (CTRLA.ENABLE=0). If the RTC is enabled (CTRLA.ENABLE=1), these operations are necessary: first, write CTRLA.ENABLE=0, then, check whether the write synchronization has finished, then, change the desired bit field value. Enable-protected bits in the CTRLA register can be written at the same time as CTRLA.ENABLE is written to ‘1’, but not at the same time as CTRLA.ENABLE is written to ‘0’.

Enable protection is denoted by the Enable-Protected property in the register description.

The RTC prescaler divides the source clock for the RTC counter.

Note: In the Clock/Calendar mode, the prescaler must be configured to provide a 1 Hz clock to the counter for correct operation.

The frequency of the RTC clock (CLK_RTC_CNT) is derived by the following formula:

fCLK_RTC_CNT=fCLK_RTC_OSC2PRESCALER

The frequency of the oscillator clock, CLK_RTC_OSC, is given by fCLK_RTC_OSC, and fCLK_RTC_CNT is the frequency of the internal prescaled RTC clock, CLK_RTC_CNT.