XOSCCTRL

External Multipurpose Crystal Oscillator (XOSC) Control

  0x14 16 PAC Write-Protection 0x0080  
Parent topic: Register Description

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   SWBACK 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

Start-Up Time

This bit field selects the XOSC crystal oscillator stabilization time.

Important: This stabilization time is for guidance only. A major component of crystal start-up time is based on the second party crystal MFG parasitics that are outside the scope of this specification. If this is a major concern, the customer must characterize this based on their design choices.
Table 1. Stabilization Time for External Multipurpose Crystal Oscillator (1)
STARTUP[3:0] Number of OSCULP32K Clock Cycles Approximate Equivalent Time (2) [µs]
0x0 1 31
0x1 2 61
0x2 4 122
0x3 8 244
0x4 16 488
0x5 32 977
0x6 64 1953
0x7 128 3906
0x8 256 7813
0x9 512 15625
0xA 1024 31250
0xB 2048 62500µs
0xC 4096 125000
0xD 8192 250000
0xE 16384 500000
0xF 32768 1000000
Notes:
  1. 1.The OSCULP32K oscillator is used to clock the start-up counter.
  2. 2.Actual Start-Up time is the number of selected OSCULP32K cycles + 3 XOSC cycles.

Bit 11 – AMPGC: Automatic Amplitude Gain Control

Automatic Amplitude Gain Control

Note: The configuration of the Oscillator Gain is mandatory even if AMPGC feature is enabled at startup.
ValueDescription
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

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. Those 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

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.

ValueDescription
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

Run in Standby

This bit controls how the XOSC behaves during Standby Sleep mode, together with the ONDEMAND bit:

ValueDescription
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 – SWBACK: Clock Switch Back

Clock Switch Back

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

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

Bit 3 – CFDEN: Clock Failure Detector Enable

Clock Failure Detector Enable

This bit controls the clock failure detector:

ValueDescription
0 The Clock Failure Detector is disabled.
1

the Clock Failure Detector is enabled.

Bit 2 – XTALEN: Crystal Oscillator Enable

Crystal Oscillator Enable

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

ValueDescription
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

Oscillator Enable

ValueDescription
0 The oscillator is disabled.
1 The oscillator is enabled.