A clock generator includes a microcontroller unit calibrated by aligning at subsequent calibration times a frequency of a first clock with respect to the frequency of a second clock having a higher frequency accuracy than the first clock, with the frequency of the first clock varying between subsequent calibration times. The frequency of the first clock is aligned to a frequency which is offset by a certain amount with respect to the frequency of the second clock to counter frequency error which may accumulate over time due to the variation in the frequency of the first clock.