Is there any documentation that explains the XT oscillator calibration procedure for the RTCs? Not for calibrating RC, but XTAL.
There is a detailed description of the XT Oscillator calibration process in section 5.9.1 of the AM18x5 data sheet. The XT Oscillator calibration circuit operation is similar to the RC calibration circuit. XTAL calibration is usually performed during board level test during manufacturing. The calibration value can be directly written to the calibration register during board manufacturing/test (if RTC battery power will be maintained continuously) or the calibration value could be stored in MCU Flash to be written to RTC register after any power cycle.