11.5.9 32.768 kHz Crystal Oscillator Control A
To change settings in a safe way: write a '0' to the ENABLE bit and wait until XOSC32KS is '0' before re-enabling the XOSC32K with new settings.
| Name: | XOSC32KCTRLA |
| Offset: | 0x1C |
| Reset: | 0x00 |
| Property: | Configuration Change Protection |
| Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |
| CSUT[1:0] | SEL | RUNSTDBY | ENABLE | ||||||
| Access | R/W | R/W | R/W | R/W | R/W | ||||
| Reset | 0 | 0 | 0 | 0 | 0 | ||||
Bits 5:4 – CSUT[1:0] Crystal Start-Up Time
These bits select the start-up time for the XOSC32K. It is write protected when the oscillator is enabled (ENABLE=1).
If SEL=1, the start-up time will not be applied.
| Value | Name | Description |
|---|---|---|
| 0x0 | 1K | 1k cycles |
| 0x1 | 16K | 16k cycles |
| 0x2 | 32K | 32k cycles |
| 0x3 | 64K | 64k cycles |
Bit 2 – SEL Source Select
This bit selects the external source type. It is write protected when the oscillator is enabled (ENABLE=1).
| Value | Description |
|---|---|
| 0 | External crystal |
| 1 | External clock on TOSC1 pin |
Bit 1 – RUNSTDBY Run Standby
Writing this bit to '1' starts the crystal oscillator and forces the oscillator ON in all modes, even when unused by the system if the ENABLE bit is set. In Standby Sleep mode this can be used to ensure immediate wake-up and not waiting for oscillator start-up time. When this bit is '0', the crystal oscillator is only running when requested and the ENABLE bit is set.
The output of XOSC32K is not sent to other peripherals unless it is requested by one or more peripherals.
When the RUNSTDBY bit is set there will only be a delay of two to three crystal oscillator cycles after a request until the oscillator output is received, if the initial crystal start-up time has already completed.
According to RUNSTBY bit, the oscillator will be turned ON all the time if the device is in Active, Idle, or Standby Sleep mode, or only be enabled when requested.
This bit is I/O protected to prevent unintentional enabling of the oscillator.
Bit 0 – ENABLE Enable
When this bit is written to '1', the configuration of the respective input pins is overridden to TOSC1 and TOSC2. Also, the Source Select bit (SEL) and Crystal Start-Up Time (CSUT) become read-only.
This bit is I/O protected to prevent unintentional enabling of the oscillator.