12.5.12 External High-Frequency Oscillator Control A


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

Bit 7 – RUNSTDBY Run Standby

This bit controls whether the External High-Frequency Oscillator (XOSCHF) is always running when the ENABLE bit is ‘1’.

Note:

The requesting peripheral, or the main clock, must take the oscillator start-up time into account.
The oscillator signal is only available if requested and will be available after two XOSCHF cycles if the initial crystal start-up time has already ended.

Value	Description
0	The XOSCHF oscillator will only run when requested by a peripheral or by the main clock ⁽¹⁾
1	The XOSCHF oscillator will always run in Active mode, Idle sleep mode and Standby sleep mode ⁽²⁾

Bits 5:4 – CSUTHF[1:0] Crystal Start-up Time

This bit field controls the start-up time for the External High-Frequency Oscillator (XOSCHF) when the Source Select (SELHF) bit is ‘0’.

Note: This bit field is read-only when the ENABLE bit or the External Crystal/Clock Status (XOSCHFS) bit in the Main Clock Status (MCLKSTATUS) register is ‘1’.

Value	Name	Description
0x0	256CYC	256 XOSCHF cycles
0x1	1KCYC	1K XOSCHF cycles
0x2	4KCYC	4K XOSCHF cycles
0x3	-	Reserved

Bit 1 – SELHF Source Select

This bit controls the source of the External High-Frequency Oscillator (XOSCHF).

Note: This bit field is read-only when the ENABLE bit or the External Crystal/Clock Status (XOSCHFS) bit in the Main Clock Status (MCLKSTATUS) register is ‘1’.

Value	Name	Description
0	CRYSTAL	External crystal connected to the XTALHF1 and XTALHF2 pins
1	EXTCLK	External clock on the XTALHF1 pin

Bit 0 – ENABLE Enable

This bit controls whether the External High-Frequency Oscillator (XOSCHF) is enabled or not.

Value	Description
0	The XOSCHF oscillator is disabled
1	The XOSCHF oscillator is enabled and overrides normal port operation for the respective oscillator pins

Name:	XOSCHFCTRLA
Offset:	0x20
Reset:	0x00
Property:	Configuration Change Protection