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RTC calibration on sTM32f3 discovery

Question asked by arno on Feb 21, 2013
Latest reply on Oct 24, 2014 by
I meet difficulties to understand the RTC calibration protocoles presented in the UM1581, the RM0316 or the AN3371.Two distincts calibration procedures are discussed there in :
- a coarse one,
- a smooth one.
For what I understand so far from AN3371, the coarse calibration seems to be relevant to correct the offset due the 32KHz cristal inaccuracy once and for all whereas the smooth calibration allows an on the fly recalibration that could result from temperature drift for instance. Is this correct? If yes then should both calibrations be performed for a greater accuracy? I don't see any reference to the coarse calibration neither in RM0316 nor in UM1581... It seems, still from AN3381, that the coarse calib is available on f3 series (table 15) whereas no firmware driver API is available for it (Table 16)...

On an even more basic aspect, I don't understand the information given by AFO_CALIB (which seems to be refered to as RTC_CALIB in the reference manual)... How does this smooth calibration work... Do I need an external calibration source ? Do you have some illustration code to provide ?

And Finally, does the smooth calibration implies that the RTC is not available to other threads during the 32s calibration operation ?

Thanks in advance for any piece of information!