# 16/20 MHz Oscillator (OSC20M)

This oscillator can operate at multiple frequencies, selected by the value of the Frequency Select (FREQSEL) bits in the Oscillator Configuration (FUSE.OSCCFG) fuse. The center frequencies are:
• 16 MHz
• 20 MHz

After a System Reset, FUSE.OSCCFG determines the initial frequency of CLK_MAIN.

During Reset, the calibration values for the OSC20M are loaded from fuses. There are two different Calibration bit fields:
• The Calibration (CAL20M) bit field in the Calibration A (CLKCTRL.OSC20MCALIBA) register enables calibration around the current center frequency
• The Oscillator Temperature Coefficient Calibration (TEMPCAL20M) bit field in the Calibration B (CLKCTRL.OSC20MCALIBB) register enables adjustment of the slope of the temperature drift compensation

For applications requiring a more fine-tuned frequency setting than the oscillator calibration provides, factory-stored frequency error after calibrations are available.

The oscillator calibration can be locked by the Oscillator Lock (OSCLOCK) Fuse (FUSE.OSCCFG). When this fuse is ‘1’, it is not possible to change the calibration. The calibration is locked if this oscillator is used as the main clock source and the Lock Enable (LOCKEN) bit in the Control B (CLKCTRL.OSC20MCALIBB) register is ‘1’.

The Calibration bits are protected by the Configuration Change Protection Mechanism, requiring a timed write procedure for changing the main clock and prescaler settings.

The start-up time of this oscillator is the analog start-up time plus four oscillator cycles. Refer to the Electrical Characteristics section for the start-up time.

When changing the oscillator calibration value, the frequency may overshoot. If the oscillator is used as the main clock (CLK_MAIN), it is recommended to change the main clock prescaler so that the main clock frequency does not exceed ¼ of the maximum operation main clock frequency as described in the General Operating Ratings section. The system clock prescaler can be changed back after the oscillator calibration value has been updated.