PCF8523 Load Capacitor Selection

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The PCF8523 RTC integrates the load capacitor (C_L), but not the 2**15 Hz oscillator itself.

According to the PCF8523 datasheet one can chose between to internal capacitors (cf. the first control register): C_L=7 pF and C_L=12.5 pF

Looking at the Adafruit PCF8523 RTC PCB schematics I don't see any C_L value specified on Q1.

Looking at the Adafruit rtclib code I don't see the capacitor bit (in register control 1) being set anywhere. Thus, the default value of 7 pF is used.

Thus, I was inclined to believe that Adafruit's PCF8523 RTC PCB indeed uses an C_L=7pF oscillator.

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However, I calibrated an Adafruit PCF8523 RTC (STEMMA QT version, under the default C_L setting) and I measured an offset of 186 ppm.

This seems to be excessive, since almost all oscillators on common distributors (such as digikey/mouser) have a frequency stability of +- 100 ppm or less specified. And the cheapest ones even have a specification of +- 25 or 50 ppm.

Common aging specifications are +- 3 ppm per year.

Since such oscillators run faster if you use a C_L lower than the specified value I developed the suspicion that the Adafruit PCF8523 RTC PCB really uses an 32.768 kHz part with C_L=12.5 pF.

Case in point: oscillators with C_L > 7 pF also seem to be more common and easier available through distributors.
NXP agrees:

As previously pointed out crystals used for RTCs come in three versions, optimized for
three standard values for C L with 12.5 pF the most common. Generally, an RTC using a
12.5 pF crystal has a timekeeping current of about 1.6x more than an RTC using a 7 pF
crystal. [..] An oscillator using a 12.5 pF crystal will be more stable and less susceptible to noise and
parasitic capacitances. [..] Besides technical considerations there are also procurement issues. Crystals designed for a 12.5 pF load capacitance are readily available through many distributors.

I thus repeated the calibration with a C_L=12.5 pF selection (i.e. 7th Control 1 register bit set to one).
And as expected, the calibration offset is much lower: just 8 ppm

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Could you clarify what kind of oscillator the Adafruit PCF8523 PCB actually uses?

If it indeed uses a C_L=12.5 pF one it would make sense to adjust the initialization code in rtclib, of course.