21.8.5 External Multipurpose Crystal Oscillator (XOSC) Control
| Name: | XOSCCTRL |
| Offset: | 0x10 |
| Reset: | 0x0080 |
| Property: | PAC Write-Protection |
| 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 | XTALEN | ENABLE | ||||||
| Access | R/W | R/W | R/W | R/W | |||||
| Reset | 1 | 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.
| 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 |
- 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
| 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.
| 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 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. |