18.7.6 External Multipurpose Crystal Oscillator Control


Bit	31	30	29	28	27	26	25	24
					CFDPRESC[3:0]
Access					R/W	R/W	R/W	R/W
Reset					0	0	0	0


Bit	23	22	21	20	19	18	17	16
	STARTUP[3:0]						SWBEN	CFDEN
Access	R/W	R/W	R/W	R/W			R/W	R/W
Reset	0	0	0	0			0	0


Bit	15	14	13	12	11	10	9	8
	ENALC	IMULT[3:0]				IPTAT[1:0]		LOWBUFGAIN
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				XTALEN	ENABLE
Access	R/W	R/W				R/W	R/W
Reset	1	0				0	0

Bits 27:24 – CFDPRESC[3:0] Clock Failure Detector Prescaler

These bits select the prescaler for the clock failure detector.

The DFLL48 oscillator is used to clock the CFD prescaler.

The CFD safe clock frequency is the DFLL48 frequency divided by 2^CFDPRESC.

Bits 23:20 – STARTUP[3:0] Start-Up Time

These bits select start-up time for the oscillator XOSCn according to the table below.

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

Table 18-6. Start-UpTime for External Multipurpose Crystal Oscillator
STARTUP[3:0]	Number of OSCULP32K Clock Cycles	Number of XOSC Clock Cycles	Approximate Equivalent Time(
0x0	1	3	31µs
0x1	2	3	61μs
0x2	4	3	122μs
0x3	8	3	244μs
0x4	16	3	488μs
0x5	32	3	977μs
0x6	64	3	1953μs
0x7	128	3	3906μs
0x8	256	3	7813μs
0x9	512	3	15625μs
0xA	1024	3	31250μs
0xB	2048	3	62500μs
0xC	4096	3	125000μs
0xD	8192	3	250000μs
0xE	16384	3	500000μs
0xF	32768	3	1000000μs

Bit 17 – SWBEN Xosc Clock Switch Enable

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

0: The clock switch back is disabled.

1: The clock switch back is enabled. This bit is reset once the XOSCn output clock is switched back to the external clock or crystal oscillator.

Bit 16 – CFDEN Clock Failure Detector Enable

This bit controls the XOSCn clock failure detector :

0: the Clock Failure Detector is disabled.

1: the Clock Failure Detector is enabled.

Bit 15 – ENALC Automatic Loop Control Enable

This bit controls the XOSCn automatic loop control :

0: the automatic loop control is disabled.

1: the automatic loop control is enabled. Oscillator's amplitude will be automatically adjusted during Crystal Oscillator operation.

Bits 14:11 – IMULT[3:0] Oscillator Current Multiplier

These bits select the current multiplier for the oscillator XOSCn, given in table External Multipurpose Crystal Oscillator Current Settings.

Bits 10:9 – IPTAT[1:0] Oscillator Current Reference

These bits select the current reference for the oscillator XOSCn, given in table below.

Table 18-7. External Multipurpose Crystal Oscillator Current Settings
Frequency Range	Current Setting
Frequency Range	IMULT[3:0]	IPTAT[1:0]
>24MHz to 48MHz	6	3
>16MHz to 24MHz	5	3
>8MHz to 16MHz	4	3
8MHz	3	2

For relatively small CLOAD in a frequency range, the setting for the lower frequency range can be used to preserve current consumption.

Bit 8 – LOWBUFGAIN Low Buffer Gain Enable

0: The low buffer gain of oscillator XOSCn is disabled.

1: The low buffer gain of oscillator XOSCn is enabled.

When XOSCCTRLn.ENALC=0 this bit has no effect.

When XOSCCTRLn.ENALC=1, this bit is used to adjust the oscillator's amplitude in automatic loop control.

The default value of LOWBUFGAIN=0 should be used to allow operating with a low amplitude oscillator. Use this setting except to solve stability issues.

Setting LOWBUFGAIN=1 will increase the oscillator's amplitude by a factor of approximately 2. Use this setting to solve stability issues.

Bit 7 – ONDEMAND On Demand Control

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

If On Demand is set, the oscillator will be running only when requested by a peripheral and enabled (XOSCCTRLn. ENABLE=1). If there is no peripheral requesting the oscillator’s clock source, the oscillator will be in a disabled state.

If On Demand is cleared, the oscillator will always be running when enabled (XOSCCTRLn.ENABLE=1). In standby sleep mode, the On Demand operation is still active.

0: The oscillator is always on.

1: The oscillator is running when a peripheral is requesting the oscillator to be used as a clock source. The oscillator is not running if no peripheral is requesting the clock source.

Bit 6 – RUNSTDBY Run in Standby

This bit controls how the XOSCn behaves during standby sleep mode:

0: The XOSCn is not running in standby sleep mode if no peripheral requests the clock.

1: The XOSCn is running in standby sleep mode. If ONDEMAND is one, the XOSCn will be running when a peripheral is requesting the clock. If ONDEMAND is zero, the clock source will always be running in standby sleep mode.