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RTC smooth digital calibration

Question asked by Burck.Steven on Feb 28, 2015
I've been called to help support a project using an STM8.  They are having problems with RTC clock drift, a couple of seconds a day, and showed me section 24.3.8/9 in the RM0031 datasheet.  I found the document AN3371, which explained it further, but I am still at a loss.

Both speak for the RTC smooth digital calibration of a 32 second period of measurement - but against what reference?  How do I measure 32 seconds without an external clock to give me the 32 second period.  The manual talks about doing the smooth calibration on the fly, but also about using the AFO_CALIB output as a feedback - which I am assuming means using something external (the signal AFO_CALIB is an output), which can't be done by a user in the field if there are crystal aging problems, or temperature problems.