21.6.2.1 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:

  • Event Control register (EVCTRL)
  • Control B register (CTRLB)
  • 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 and check whether the write synchronization has finished, then change the desired bit field value. Enable-protected bits in 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 Clock/Calendar mode, the prescaler must be configured to provide a 1.024 kHz clock to the counter for correct operation.

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

f CLK_RTC_CNT = f CLK_RTC_OSC 2 PRESCALER

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.