The calibrated internal oscillator provides an approximately 8MHz clock signal. Though voltage and temperature dependent, this clock can be very accurately calibrated by the user.
During reset, hardware loads the calibration byte into the Oscillator Calibration Register (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.
It is possible to reach higher accuracies than factory defaults, especially when the application allows temperature and voltage ranges to be narrowed. The firmware can reprogram the calibration data in OSCCAL either at start-up or during run-time. The continuous, run-time calibration method allows firmware to monitor voltage and temperature and compensate for any detected variations.
When this oscillator is used as the chip clock, it will still be used for the Watchdog Timer and for the Reset Time-out.