# 20 MHz Oscillator (OSC20M)

This oscillator can operate at multiple frequencies, selected by the value of the Frequency Select bits (FREQSEL) in the Oscillator Configuration Fuse (FUSE.OSCCFG).

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 bit field (CAL20M) in the Calibration A register (CLKCTRL.OSC20MCALIBA) enables calibration around the current center frequency. The Oscillator Temperature Coefficient Calibration bit field (TEMPCAL20M) in the Calibration B register (CLKCTRL.OSC20MCALIBB) enables adjustment of the slope of the temperature drift compensation.

For applications requiring more fine-tuned frequency than provided by the oscillator calibration, the remaining oscillator frequency error measured during calibration is available in the Signature Row (SIGROW) for additional compensation.

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 bit (LOCKEN) in the Control B register (CLKCTRL.OSC20MCALIBB) 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.

Refer to the Electrical Characteristics section for the start-up time.