Atmel Xplained Pro Evaluation Kit for the ATSAMD21J18A Microcontroller ATSAMD21-XPRO ATSAMD21-XPRO Data Sheet

221

Atmel | SMART SAM D21 [DATASHEET]

Atmel-42181C–SAM-D21_Datasheet–07/2014

The RTC keeps track of time in the system and enables periodic events, as well as interrupts and events at a specified 
time. The RTC consists of a 10-bit prescaler that feeds a 32-bit counter. The actual format of the 32-bit counter depends 
on the RTC operating mode.

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

z

Operating Mode bits in the Control register (CTRL.MODE) 

z

Prescaler bits in the Control register (CTRL.PRESCALER) 

z

Clear on Match bit in the Control register (CTRL.MATCHCLR) 

z

Clock Representation bit in the Control register (CTRL.CLKREP) 

The following register is enable-protected:

z

Event Control register (

Any writes to these bits or registers when the RTC is enabled or being enabled (CTRL.ENABLE is one) will be discarded. 
Writes to these bits or registers while the RTC is being disabled will be completed after the disabling is complete. 

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

Before the RTC is enabled, it must be configured, as outlined by the following steps: 

z

RTC operation mode must be selected by writing the Operating Mode bit group in the Control register 
(CTRL.MODE)

z

Clock representation must be selected by writing the Clock Representation bit in the Control register 
(CTRL.CLKREP)

z

Prescaler value must be selected by writing the Prescaler bit group in the Control register (CTRL.PRESCALER)

The RTC prescaler divides down the source clock for the RTC counter. The frequency of the RTC clock 
(CLK_RTC_CNT) is given by the following formula:

The frequency of the generic clock, GCLK_RTC, is given by f

GCLK_RTC

, and f

CLK_RTC_CNT

 is the frequency of the internal 

prescaled RTC clock, CLK_RTC_CNT. 

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

The RTC

 

is enabled by writing a one to the Enable bit in the Control

 

register (CTRL.ENABLE). The RTC is disabled by 

writing a zero to CTRL.ENABLE.

The RTC should be disabled before resetting it.

The RTC

 

is reset by writing a one to the Software Reset bit in the Control register (CTRL.SWRST). All registers in the 

RTC, except DBGCTRL, will be reset to their initial state, and the RTC will be disabled. 

Refer to the 

 register for details.

CLK_RTC_CNT

GCLK_RTC

2

PRESCALER

-----------------------------

=