the range of capacitors of external 32.768khz crystal

[zhang.sandy]

Hello, as the 32.768khz PPM of our product is out of the range, so i need to fine tune the C1/C2, now C1 and C2 are 22pF, is it in range? what is the range for C1 and C2, for the Cpcb1/2, how to get this  value.

[mathiaso]

Hi, Sandy.

As you may see in the nRF52810 PS, the LFXO has a maximum load capacitance, C_{L_LFXO}, of 12.5 pF. The value of the load caps, C_cap, should then be possible to calculate using the formula

C_cap = 2 * C_{L_LFXO} - 5 pF,

as explained in this guide.

Using the numbers above, a capacitor value of 22 pF indicates that the crystal has a load capacitance of 13.5 pF, which is outside of the spec (C_{L_LFXO} = (C_cap + 5 pF) / 2).

Best regards,
Mathias