Datasheet

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.
17.5.7 Analog Connections
When used, the 32.768kHz crystal must be connected between the XIN32 and XOUT32 pins, and the
0.4-32MHz crystal must be connected between the XIN and XOUT pins, along with any required load
capacitors. For details on recommended oscillator characteristics and capacitor load, refer to the
Electrical Characteristics for details.
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37. Electrical Characteristics
17.6 Functional Description
17.6.1 Principle of Operation
XOSC, XOSC32K, OSC32K, OSCULP32K, OSC8M, DFLL48M, FDPLL96M, BOD33, BOD12, and VREF
are configured through SYSCTRL control registers. Through this interface, the sub-peripherals are
enabled, disabled or have their calibration values updated.
The Power and Clocks Status register gathers different status signals coming from the sub-peripherals
controlled by the SYSCTRL. The status signals can be used to generate system interrupts, and in some
cases wake up the system from Standby mode, provided the corresponding interrupt is enabled.
The oscillator must be enabled to run. The oscillator is enabled by writing a one to the ENABLE bit in the
respective oscillator control register, and disabled by writing a zero to the oscillator control register. In Idle
mode, the default operation of the oscillator is to run only when requested by a peripheral. In Standby
mode, the default operation of the oscillator is to stop. This behavior can be changed by the user, see
below for details.
The behavior of the oscillators in the different sleep modes is shown in the table below.
Table 17-1. Behavior of the Oscillators
Oscillator Idle 0, 1, 2 Standby
XOSC Run on request Stop
XOSC32K Run on request Stop
OSC32K Run on request Stop
OSCULP32K Run Run
OSC8M Run on request Stop
DFLL48M Run on request Stop
FDPLL96M Run on request Stop
To force an oscillator to always run in Idle mode, and not only when requested by a peripheral, the
oscillator ONDEMAND bit must be written to zero. The default value of this bit is one, and thus the default
operation in Idle mode is to run only when requested by a peripheral.
SAM D21 Family
SYSCTRL – System Controller
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