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]

db16122_0-1784461035005.png

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. 

JozefKozon_0-1784524368704.png

JozefKozon_1-1784524409356.png

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

JozefKozon_2-1784524493868.png

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

Cosco is the known internal capacitance, the 25pF. 

JozefKozon_3-1784524560452.png

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. 

JozefKozon_4-1784525019076.png

With Best Regards,

Jozef