30.5.2 Main RC Oscillator Frequency Adjustment

The 8 MHz and 12 MHz frequencies are factory-centered to the typical values by using Flash calibration bits (refer to the “Electrical Characteristics” chapter).

The Flash calibration bits setting the Main RC oscillator frequency to 8 MHz and 12 MHz vary from device to device. To get a starting point when changing the CAL8 or CAL12 fields, it is recommended to first read their corresponding Flash calibration bits in the Flash Controller.

The user can adjust the value of the Main RC oscillator frequency by modifying the trimming values done in production on 8 MHz and 12 MHz. This may be used to compensate frequency drifts due to temperature or voltage. The values stored in the Flash cannot be erased by a Flash erase command or by the ERASE signal. Values written by the user application in the Oscillator Calibration Register (PMC_OCR) are reset after each power-up or peripheral reset.

By default, SEL4/SEL8/SEL12 are cleared, so the Main RC oscillator is driven with the factory-programmed Flash calibration bits which are programmed during chip production.

In order to calibrate the oscillator lower frequency, SEL4 must be set to ‘1’ and a valid frequency value must be configured in CAL4. Likewise, SEL8/12 must be set to ‘1’ and a trim value must be configured in CAL8/12 in order to adjust the other frequencies of the oscillator.

It is possible to adjust the oscillator frequency while operating from this oscillator. For example, when running on lowest frequency, it is possible to change the CAL4 value if SEL4 is set in PMC_OCR.

At any time, the user can measure the main RC oscillator output frequency by means of the Main Frequency Counter (refer to "Main Frequency Counter"). Once the frequency measurement is done, the main RC oscillator calibration field (CALx) can be adjusted accordingly to correct this oscillator output frequency.