The calibrated internal oscillator provides an approximately 8 MHz clock signal. Though voltage and temperature dependent, this clock can be very accurately calibrated by the user.
This clock may be selected as the main clock by setting the Clock Main Select bits in CLKMSR (CLKMSR.CLKMS) to 0b00. Once enabled, the oscillator will operate with no external components. During reset, hardware loads the calibration byte into the OSCCAL register and thereby automatically calibrates the oscillator. The accuracy of this calibration is shown as Factory calibration in Accuracy of Calibrated Internal Oscillator of Electrical Characteristics chapter.
When this oscillator is used as the main clock, the watchdog oscillator will still be used for the watchdog timer and reset time-out. For more information on the pre-programmed calibration value, see section Calibration Section.