Data Sheet

152
Atmel | SMART SAM D21 [DATASHEET]
Atmel-42181G–SAM-D21_Datasheet–09/2015
To force the oscillator to run in standby mode, the RUNSTDBY bit must be written to one. The oscillator will then run in
standby mode when requested by a peripheral (ONDEMAND is one). To force an oscillator to always run in standby
mode, and not only when requested by a peripheral, the ONDEMAND bit must be written to zero and RUNSTDBY must
be written to one.
Table 16-2 on page 152 shows the behavior in the different sleep modes, depending on the settings of ONDEMAND and
RUNSTDBY.
Note that this does not apply to the OSCULP32K oscillator, which is always running and cannot be disabled.
16.6.2 External Multipurpose Crystal Oscillator (XOSC) Operation
The XOSC can operate in two different modes:
z External clock, with an external clock signal connected to the XIN pin
z Crystal oscillator, with an external 0.4-32MHz crystal
The XOSC can be used as a clock source for generic clock generators, as described in the “GCLK – Generic Clock
Controller” on page 95.
At reset, the XOSC is disabled, and the XIN/XOUT pins can be used as General Purpose I/O (GPIO) pins or by other
peripherals in the system. When XOSC is enabled, the operating mode determines the GPIO usage. When in crystal
oscillator mode, the XIN and XOUT pins are controlled by the SYSCTRL, and GPIO functions are overridden on both
pins. When in external clock mode, only the XIN pin will be overridden and controlled by the SYSCTRL, while the XOUT
pin can still be used as a GPIO pin.
The XOSC is enabled by writing a one to the Enable bit in the External Multipurpose Crystal Oscillator Control register
(XOSC.ENABLE). To enable the XOSC as a crystal oscillator, a one must be written to the XTAL Enable bit
(XOSC.XTALEN). If XOSC.XTALEN is zero, external clock input will be enabled.
When in crystal oscillator mode (XOSC.XTALEN is one), the External Multipurpose Crystal Oscillator Gain (XOSC.GAIN)
must be set to match the external crystal oscillator frequency. If the External Multipurpose Crystal Oscillator Automatic
Amplitude Gain Control (XOSC.AMPGC) is one, the oscillator amplitude will be automatically adjusted, and in most
cases result in a lower power consumption.
The XOSC will behave differently in different sleep modes based on the settings of XOSC.RUNSTDBY,
XOSC.ONDEMAND and XOSC.ENABLE:
Table 16-2. Behavior in the different sleep modes
Sleep mode ONDEMAND RUNSTDBY Behavior
Idle 0, 1, 2 0 X Run
Idle 0, 1, 2 1 X Run when requested by a peripheral
Standby 0 0 Stop
Standby 0 1 Run
Standby 1 0 Stop
Standby 1 1 Run when requested by a peripheral