Datasheet

19.5.8 Register Access Protection
Registers with write-access can be optionally write-protected by the Peripheral Access Controller (PAC),
except for the following:
• Interrupt Flag Status and Clear register (INTFLAG)
• Read Request register (READREQ)
• Status register (STATUS)
• Debug register (DBGCTRL)
Note: Optional write-protection is indicated by the "PAC Write-Protection" property in the register
description.
Write-protection does not apply for accesses through an external debugger.
19.5.9 Analog Connections
A 32.768kHz crystal can be connected to the XIN32 and XOUT32 pins, along with any required load
capacitors. For details on recommended crystal characteristics and load capacitors, refer to Electrical
Characteristics for details.
Related Links
37. Electrical Characteristics
19.6 Functional Description
19.6.1 Principle of Operation
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 RTC can function in one of these modes:
• Mode 0 - COUNT32: RTC serves as 32-bit counter
• Mode 1 - COUNT16: RTC serves as 16-bit counter
• Mode 2 - CLOCK: RTC serves as clock/calendar with alarm functionality
19.6.2 Basic Operation
19.6.2.1 Initialization
The following bits are enable-protected, meaning that they can only be written when the RTC is disabled
(CTRL.ENABLE=0):
• Operating Mode bits in the Control register (CTRL.MODE)
• Prescaler bits in the Control register (CTRL.PRESCALER)
• Clear on Match bit in the Control register (CTRL.MATCHCLR)
• Clock Representation bit in the Control register (CTRL.CLKREP)
The following register is enable-protected:
• Event Control register (EVCTRL)
Any writes to these bits or registers when the RTC is enabled or being enabled (CTRL.ENABLE=1) will
be discarded. Writes to these bits or registers while the RTC is being disabled will be completed after the
disabling is complete.
SAM D21 Family
RTC – Real-Time Counter
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40001882D-page 259