Data Sheet

154
Atmel | SMART SAM D21 [DATASHEET]
Atmel-42181G–SAM-D21_Datasheet–09/2015
XOSC32K can provide two clock outputs when connected to a crystal. The XOSC32K has a 32.768kHz output enabled
by writing a one to the 32kHz External Crystal Oscillator 32kHz Output Enable bit (XOSC32K.EN32K) in the 32kHz
External Crystal Oscillator Control register. XOSC32K.EN32K is only usable when XIN32 is connected to a crystal, and
not when an external digital clock is applied on XIN32.
Note: Do not enter standby mode when an oscillator is in startup:
Wait for the OSCxRDY bit in SYSCTRL.PCLKSR register to be set before going into standby mode.
16.6.4 32kHz Internal Oscillator (OSC32K) Operation
The OSC32K provides a tunable, low-speed and low-power clock source.
The OSC32K can be used as a source for the generic clock generators, as described in the “GCLK – Generic Clock
Controller” on page 95.
The OSC32K is disabled by default. The OSC32K is enabled by writing a one to the 32kHz Internal Oscillator Enable bit
(OSC32K.ENABLE) in the 32kHz Internal Oscillator Control register. It is disabled by writing a zero to OSC32K.ENABLE.
The OSC32K has a 32.768kHz output enabled by writing a one to the 32kHz Internal Oscillator 32kHz Output Enable bit
(OSC32K.EN32K).
The frequency of the OSC32K oscillator is controlled by the value in the 32kHz Internal Oscillator Calibration bits
(OSC32K.CALIB) in the 32kHz Internal Oscillator Control register. The OSC32K.CALIB value must be written by the
user. Flash Factory Calibration values are stored in the NVM Software Calibration Area (refer to “NVM Software
Calibration Area Mapping” on page 31). When writing to the Calibration bits, the user must wait for the
PCLKSR.OSC32KRDY bit to go high before the value is committed to the oscillator.
16.6.5 32kHz Ultra Low Power Internal Oscillator (OSCULP32K) Operation
The OSCULP32K provides a tunable, low-speed and ultra-low-power clock source. The OSCULP32K is factory-
calibrated under typical voltage and temperature conditions. The OSCULP32K should be preferred to the OSC32K
whenever the power requirements are prevalent over frequency stability and accuracy.
The OSCULP32K can be used as a source for the generic clock generators, as described in the “GCLK – Generic Clock
Controller” on page 95.
The OSCULP32K is enabled by default after a power-on reset (POR) and will always run except during POR. The
OSCULP32K has a 32.768kHz output and a 1.024kHz output that are always running.
The frequency of the OSCULP32K oscillator is controlled by the value in the 32kHz Ultra Low Power Internal Oscillator
Calibration bits (OSCULP32K.CALIB) in the 32kHz Ultra Low Power Internal Oscillator Control register.
OSCULP32K.CALIB is automatically loaded from Flash Factory Calibration during startup, and is used to compensate for
process variation, as described in the “Electrical Characteristics” on page 935. The calibration value can be overridden
by the user by writing to OSCULP32K.CALIB.
16.6.6 8MHz Internal Oscillator (OSC8M) Operation
OSC8M is an internal oscillator operating in open-loop mode and generating an 8MHz frequency. The OSC8M is factory-
calibrated under typical voltage and temperature conditions.
OSC8M is the default clock source that is used after a power-on reset (POR). The OSC8M can be used as a source for
the generic clock generators, as described in the “GCLK – Generic Clock Controller” on page 95.
In order to enable OSC8M, the Oscillator Enable bit in the OSC8M Control register (OSC8M.ENABLE) must be written to
one. OSC8M will not be enabled until OSC8M.ENABLE is set. In order to disable OSC8M, OSC8M.ENABLE must be
written to zero. OSC8M will not be disabled until OSC8M is cleared.
The frequency of the OSC8M oscillator is controlled by the value in the calibration bits (OSC8M.CALIB) in the OSC8M
Control register. CALIB is automatically loaded from Flash Factory Calibration during startup, and is used to compensate
for process variation, as described in the “Electrical Characteristics” on page 935.
The user can control the oscillation frequency by writing to the Frequency Range (FRANGE) and Calibration (CALIB) bit
groups in the 8MHz RC Oscillator Control register (OSC8M). It is not recommended to update the FRANGE and CALIB