10.5.9 32.768 kHz Crystal Oscillator Control A

The SEL and CSUT bits cannot be changed as long as the ENABLE bit is set, or the XOSC32K Stable (XOSC32KS) bit in CLKCTRL.MCLKSTATUS is high.

To change settings safely, write a '0' to the ENABLE bit and wait until XOSC32KS is '0' before re-enabling the XOSC32K with new settings.

Name:	XOSC32KCTRLA
Offset:	0x1C
Reset:	0x00
Property:	Configuration Change Protection


Bit	7	6	5	4	3	2	1	0
			CSUT[1:0]			SEL	RUNSTDBY	ENABLE
Access			R/W	R/W		R/W	R/W	R/W
Reset			0	0		0	0	0

Bits 5:4 – CSUT[1:0] Crystal Start-Up Time

This bit field selects the start-up time for the XOSC32K. It is write-protected when the oscillator is enabled (ENABLE=1).

If SEL=1, the start-up time will not be applied.

Value	Name	Description
0x0	1K	1k cycles
0x1	16K	16k cycles
0x2	32K	32k cycles
0x3	64K	64k cycles

Bit 2 – SEL Source Select

This bit selects the external source type. It is write-protected when the oscillator is enabled (ENABLE=1).

Value	Description
0	External crystal
1	External clock on TOSC1 pin

Bit 1 – RUNSTDBY Run in Standby

Writing this bit to '1' starts the crystal oscillator and forces the oscillator ON in all modes, even when unused by the system if the ENABLE bit is set. In Standby sleep mode, this can be used to ensure immediate wake-up and not waiting for oscillator start-up time. When this bit is '0', the crystal oscillator is only running when requested, and the ENABLE bit is set.

The output of XOSC32K is not sent to other peripherals unless it is requested by one or more peripherals.

When the RUNSTDBY bit is set, there will only be a delay of two to three crystal oscillator cycles after a request until the oscillator output is received, if the initial crystal start-up time has already completed.

Depending on the RUNSTDBY bit, the oscillator will be turned ON all the time if the device is in Active, Idle, or Standby sleep mode, or only be enabled when requested.

This bit is I/O protected to prevent any unintentional enabling of the oscillator.

Bit 0 – ENABLE Enable

When this bit is written to '1', the configuration of the respective input pins is overridden to TOSC1 and TOSC2. Also, the Source Select (SEL) and Crystal Start-Up Time (CSUT) bits become read-only.