Dear Hongshuai,
depending on what 32.768kHz crystal you chose, you will find in the crystal datasheet the required load capacitance CL. We recommend to chose a crystal with accuracy of 100PPM or better. For example if you chose this one: https://www.we-online.com/components/products/datasheet/830003000.pdf, the required load capacitance is CL=12.5pF. Then according to the formula in the PCA21125 datasheet, page 33. you can calculate the required external capacitor connected to the OSCI pin.
CL=12.5pF
COSCO=25pF
Cext=?
12.5pF=(Cext*25pF)/(Cext+25pF)
12.5=25Cext/(Cext+25)
12.5Cext+312.5=25Cext
312.5=25Cext-12.5Cext
312.5=12.5Cext
Cext=312.5/12.5
Cext=25pF
This is confirmed in the Table 3. in the UM10301.
Due to tolerances we recommend to use variable capacitor on the OSCI pin and tune it by measuring the signal on the CLKOUT pin, please refer to the section 14.2 in the PCA21125 datasheet.
With Best Regards,
Jozef
