Help finding capacitor for PCF8563TS/5,118

[oneisart]

I'm working with a PCF8563TS/5,118 IC, trying to get the external cap on OSCI pin right. 

I'm working with a 50k ESR, 12.5pF 32.568KHz crystal, shortest possible traces, Schematic just like in the application diagram on the datasheet.

I tried making the calculations by consulting both the datasheet and UM10301 to get an understanding on the device. I found it difficult to find out a clear way to calculate the value for the external cap because the datasheet mentions parallel and series on different pages of the same topic, OSCI and OSCO. 

I see there is an internal 25pF cap in parallel with the external OSCI and OSCO pins. Table 30 on the datasheet indicates CL is calculated in parallel, but has a note that shows the calculation for series capacitors instead.

I can't find an external cap that makes the device work. Getting no I2C response. I tried 25pF, 22.5pF, 20pF, 25pF, 10pF, 8.2pF, 6pF. All 0603 C0G/NP0 caps.

I did get it to work on a different board by not placing a capacitor at all. Which doesn't make any sense after making the calculations.

I would like to please get an example of how you calculate a value for these inputs (12.5pF 50k ESR crystal) because I haven't been able to make it work neither by calculating it nor by bruteforcing values.

[db16122]

UM10301 User Manual for PCF85x3, PCF85x63, PCA8565, PCF2123, and PCA21125

[oneisart]

Yes that's the value of the integrated internal capacitor and I mentioned that in my own post. I have read that file.
It would be way more helpful if you actually wrote what you intend to convey. Thanks for taking the time though

[JozefKozon]

Dear David,

db16122 correctly pointed you to the Table 3. in the UM10301. If your target load capacitance is 12.5pF, then you need to connect a trim capacitor on the OSCI pin with 25pF middle value. 

This 25pF value can be calculated from the formula below the Table 30. in the PCF8563 datasheet. 

Where CL is the target load capacitance, in your case 12.5pF. 

Cosco is the known internal capacitance, the 25pF. 

From the formula, the Ctrim can deduced:

CL=Ctrim*Cosco/(Ctrim+Cosco)

CL*Ctrim+CL*Cosco=Ctrim*Cosco

CL*Cosco=Ctrim*Cosco-CL*Ctrim

CL*Cosco=Ctrim*(Cosco-CL)

CL*Cosco/(Cosco-CL)=Ctrim

Ctrim=CL*Cosco/(Cosco-CL)=12.5*25/(25-12.5)=312.5/12.5

Ctrim=25pF

The variable trim capacitor is required because there is margin for the internal Cosco capacitor. Cosco may vary from 15pF to 35pF. 

With Best Regards,

Jozef