20.8.8 External Multipurpose Crystal Oscillator (XOSC) Control


Bit	15	14	13	12	11	10	9	8
	STARTUP[3:0]				AMPGC	GAIN[2:0]
Access	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
Reset	0	0	0	0	0	0	0	0


Bit	7	6	5	4	3	2	1	0
	ONDEMAND	RUNSTDBY		SWBEN	CFDEN	XTALEN	ENABLE
Access	R/W	R/W		R/W	R/W	R/W	R/W
Reset	1	0		0	0	0	0

Bits 15:12 – STARTUP[3:0] Start-Up Time

These bits select start-up time for the oscillator.

The OSCULP32K oscillator is used to clock the start-up counter.

Table 20-5. Start-Up Time for External Multipurpose Crystal Oscillator
STARTUP[3:0]	Number of OSCULP32K Clock Cycles	Number of XOSC Clock Cycles	Approximate Equivalent Time [µs]
0x0	1	3	31
0x1	2	3	61
0x2	4	3	122
0x3	8	3	244
0x4	16	3	488
0x5	32	3	977
0x6	64	3	1953
0x7	128	3	3906
0x8	256	3	7813
0x9	512	3	15625
0xA	1024	3	31250
0xB	2048	3	62500µs
0xC	4096	3	125000
0xD	8192	3	250000
0xE	16384	3	500000
0xF	32768	3	1000000

Note:

Actual startup time is 1 OSCULP32K cycle + 3 XOSC cycles.
The given time neglects the three XOSC cycles before OSCULP32K cycle.

Bit 11 – AMPGC Automatic Amplitude Gain Control

Note: The configuration of the oscillator gain is mandatory even if AMPGC feature is enabled at startup.

Value	Description
0	The automatic amplitude gain control is disabled.
1	The automatic amplitude gain control is enabled. Amplitude gain will be automatically adjusted during Crystal Oscillator operation.

Bits 10:8 – GAIN[2:0] Oscillator Gain

These bits select the gain for the oscillator. The listed maximum frequencies are recommendations, and might vary based on capacitive load and crystal characteristics. The Gain bits must be properly configured even when the Automatic Amplitude Gain Control is active.


Value	Recommended Max Frequency [MHz]
0x0	2
0x1	4
0x2	8
0x3	16
0x4	30
0x5-0x7	Reserved

Bit 7 – ONDEMAND On Demand Control

The On Demand operation mode allows the oscillator to be enabled or disabled, depending on peripheral clock requests.

If the ONDEMAND bit has been previously written to '1', the oscillator will be running only when requested by a peripheral. If there is no peripheral requesting the oscillator’s clock source, the oscillator will be in a disabled state.

If On Demand is disabled, the oscillator will always be running when enabled.

In standby sleep mode, the On Demand operation is still active.

Value	Description
0	The oscillator is always on, if enabled.
1	The oscillator is enabled when a peripheral is requesting the oscillator to be used as a clock source. The oscillator is disabled if no peripheral is requesting the clock source.

Bit 6 – RUNSTDBY Run in Standby

This bit controls how the XOSC behaves during standby sleep mode, together with the ONDEMAND bit:

Value	Description
0	The XOSC is not running in Standby sleep mode if no peripheral requests the clock.
1	The XOSC is running in Standby sleep mode. If ONDEMAND=1, the XOSC will be running when a peripheral is requesting the clock. If ONDEMAND=0, the clock source will always be running in Standby sleep mode.

Bit 4 – SWBEN Clock Switch Back

This bit controls the XOSC output switch back to the external clock or crystal oscillator in case of clock recovery:

Value	Description
0	The clock switch back is disabled.
1	The clock switch back is enabled. This bit is reset once the XOSC putput clock is switched back to the external clock or crystal oscillator.

Bit 3 – CFDEN Clock Failure Detector Enable

This bit controls the clock failure detector:

Value	Description
0	The Clock Failure Detector is disabled.
1	the Clock Failure Detector is enabled.

Bit 2 – XTALEN Crystal Oscillator Enable

This bit controls the connections between the I/O pads and the external clock or crystal oscillator:

Value	Description
0	External clock connected on XIN. XOUT can be used as general-purpose I/O.
1	Crystal connected to XIN/XOUT.

Bit 1 – ENABLE Oscillator Enable

Value	Description
0	The oscillator is disabled.
1	The oscillator is enabled.

Name:	XOSCCTRL
Offset:	0x10
Reset:	0x0080
Property:	PAC Write-Protection