How to calibrate 8MHz/1MHz RC Oscillator (RC8M) ?

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I am trying to find the functions to calibrate the "8MHz/1MHz RC oscillator" which comes integrated in my UC3C1512C MCU. I want to use it at 8 MHz and 5 V, which according to the datasheet corresponds to the callibration "RC8M_CALIB_5V" (Calibration of RC8M operating at 8MHz and at 5V).

 

However, the functions provided by ASF (read: sysclk_init()) assume 8 MHz at 3.3 V and always uses "RC8M_CALIB" (Calibration of RC8M operating at 8MHz and at 3.3V) !!!

 

This can be found in the MCU datasheet, section 8.5.11 (page 89) and section 15.10.0.1 (page 280). I checked the code of sysclk_init() and it always uses a mask which covers the first byte of the calibration data register, not letting me decide whether other byte should be used instead.

 

How come there is not a function for specifying a different calibration? Or it is there, just enough hidden that I couldn't find it?