There are also some RTC chips that have a buffered version of the 32KHz oscillator available on an output pin. This permits connection of a high resolution frequency counter for calibration checks without having to probe in the crystal circuit. The PCF8563 from Philips is one such part. This part allows the particular output pin in question to be used for multiple functions and if the I²C bus has been used to change the function of the pin it would be unavailable for the calibration procedure. For this reason the PCF8563 has a POR-Override mode that can get the control registers of the part back to default values without having to access the I²C bus.

Michael Karas