•
External clock, with an external clock signal connected to the XIN pin
•
Crystal oscillator, with an external 0.4-32MHz crystal
The XOSC can be used as a clock source for generic clock generators. This is configured by the Generic
Clock Controller.
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 OSCCTRL, and
GPIO functions are overridden on both pins. When in external clock mode, only the XIN pin will be
overridden and controlled by the OSCCTRL, while the XOUT pin can still be used as a GPIO pin.
The XOSC is enabled by writing a '1' to the Enable bit in the External Multipurpose Crystal Oscillator
Control register (XOSCCTRL.ENABLE).
To enable XOSC as an external crystal oscillator, the XTAL Enable bit (XOSCCTRL.XTALEN) must
written to '1'. If XOSCCTRL.XTALEN is zero, the external clock input on XIN will be enabled.
When in crystal oscillator mode (XOSCCTRL.XTALEN=1), the External Multipurpose Crystal Oscillator
Gain (XOSCCTRL.GAIN) must be set to match the external crystal oscillator frequency. If the External
Multipurpose Crystal Oscillator Automatic Amplitude Gain Control (XOSCCTRL.AMPGC) is '1', 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
XOSCCTRL.RUNSTDBY, XOSCCTRL.ONDEMAND, and XOSCCTRL.ENABLE. If
XOSCCTRL.ENABLE=0, the XOSC will be always stopped. For XOSCCTRL.ENABLE=1, this table is
valid:
Table 21-1. XOSC Sleep Behavior
CPU Mode
XOSCCTRL.RUNST
DBY
XOSCCTRL.ONDEM
AND
Sleep Behavior
Active or Idle
-
0
Always run
Active or Idle
-
1
Run if requested by peripheral
Standby
1
0
Always run
Standby
1
1
Run if requested by peripheral
Standby
0
-
Run if requested by peripheral
After a hard reset, or when waking up from a sleep mode where the XOSC was disabled, the XOSC will
need a certain amount of time to stabilize on the correct frequency. This start-up time can be configured
by changing the Oscillator Start-Up Time bit group (XOSCCTRL.STARTUP) in the External Multipurpose
Crystal Oscillator Control register. During the start-up time, the oscillator output is masked to ensure that
no unstable clock propagates to the digital logic.
The External Multipurpose Crystal Oscillator Ready bit in the Status register (STATUS.XOSCRDY) is set
once the external clock or crystal oscillator is stable and ready to be used as a clock source. An interrupt
is generated on a zero-to-one transition on STATUS.XOSCRDY if the External Multipurpose Crystal
Oscillator Ready bit in the Interrupt Enable Set register (INTENSET.XOSCRDY) is set.
Related Links
GCLK - Generic Clock Controller
on page 121
Atmel SAM L22G / L22J / L22N [DATASHEET]
Atmel-42402E-SAM L22G / L22J / L22N_Datasheet_Complete-07/2016
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