28.8.6 External Multipurpose Crystal Oscillator Control
| Name: | XOSCCTRL |
| Offset: | 0x14 + n*0x04 [n=0..1] |
| Reset: | 0x00000080 |
| Property: | PAC Write-Protection |
| 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.
| 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.
| Frequency Range | Current Setting | |
|---|---|---|
| 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.
Bit 2 – XTALEN Crystal Oscillator Enable
This bit controls the connections between the I/O pads and the external clock or crystal oscillator XOSCn:
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
0: The oscillator XOSCn is disabled.
1: The oscillator XOSCn is enabled.