12.5.8 External 32.768 kHz Crystal Oscillator Control A


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

Bit 7 – RUNSTDBY Run in Standby

This bit forces the External 32.768 kHz Crystal Oscillator (XOSC32K) to always run in all power modes except Power-Down mode when the ENABLE bit is ‘1’, even if no peripheral has requested it.

Note:

The requesting peripheral or the main clock must take the oscillator start-up time into account.
The oscillator signal is available only if requested and becomes available after a maximum of three XOSC32K cycles, assuming the initial crystal start-up time has passed.

Value	Description
0	The External 32.768 kHz Crystal Oscillator (XOSC32K) runs only when requested by a peripheral or by the main clock in Active mode and Idle sleep mode ⁽¹⁾
1	The External 32.768 kHz Crystal Oscillator (XOSC32K) always runs in Active mode, Idle sleep mode, or Standby sleep mode⁽²⁾

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

This bit field controls the External 32.768 kHz Crystal Oscillator (XOSC32K) start-up time when the Crystal Select (SEL) bit is ‘0’ (external crystal selected). If an external clock is selected, the start-up time will not be applied.

Note: This bit field is read-only when the oscillator is enabled (ENABLE bit is ‘1’) or when the XOSC32K Status (XOSCS) bit in the Main Clock Status (CLKCTRL.MCLKSTATUS) register is ‘1’.

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

Bit 3 – SEL External Source Select

This bit controls the source of the External 32.768 kHz Crystal Oscillator (XOSC32K).

Note: This bit is read-only when the oscillator is enabled (ENABLE bit is ‘1’) or when the XOSC32K Status (XOSCS) bit in the Main Clock Status (CLKCTRL.MCLKSTATUS) register is ‘1’.

Value	Name	Description
0	CRYSTAL	An external crystal is connected to the XTAL32K1 and XTAL32K2 pins
1	EXTCLK	An external clock is connected to the XTAL32K1 pin

Bits 2:1 – PMODE[1:0] Power Mode

This bit field sets the External 32.768 kHz Crystal Oscillator (XOSC32K) power mode.

Note: Selecting the power mode requires a basic understanding of crystal oscillators. Changing the power mode may alter the start-up time of a crystal.

Value	Name	Description
0x0	LOW	Low power mode. Typical crystal values: C_L = 8pF, ESR = 50 kΩ
0x1	MEDIUM	Medium power mode. Typical crystal values: C_L = 12.5pF, ESR = 70 kΩ
0x2	HIGH	High power mode. Typical crystal values: C_L = 24pF, ESR = 100 kΩ
0x3	—	Reserved

Bit 0 – ENABLE Crystal Oscillator Enable

This bit controls whether the External 32.768 kHz Crystal Oscillator (XOSC32K) is enabled.

Note: When this bit is enabled, the SEL and CSUT bits become read-only.

Value	Description
0	The External 32.768 kHz Crystal Oscillator (XOSC32K) is disabled
1	The External 32.768 kHz Crystal Oscillator (XOSC32K) is enabled and overrides ordinary Port pin operation for the respective oscillator pins (XTAL32K1 and XTAL32K2)

Name:	XOSC32KCTRLA
Offset:	0x1C
Reset:	0x0
Property:	Configuration Change Protection