RE: Choosing the value for the xtal capacitors

Håkon Alseth — Mon, 20 Jan 2014 09:59:31 GMT

Hi,

It seems that you have done some research and found the right formulas for calculating the caps for the XTAL. You have got everything correct, but to clarify: "C_xc1" is the pin input capacitance, which is 4 pF nom. for the nRF51x22 part.

C_pcb can be up to 2 pF, depending on your design and track-length. Usually it's in the 1 pF area. What I have done to verify that the caps are set correctly is to probe the XC1 pin to verify frequency and amplitude. What you need to keep in mind when doing such a measurement is to adjust for the load of the oscilloscope probe. The load capacitance of these probes are always shown on the probe.

Let's say that you have calculated C1=C2 = 12 pF. Your probe loads with 8 pF, then the C1 cap should be 4.0 pF when doing this measurement. The frequency should be 16.000 MHz ( + jitter of max 40 ppm if Bluetooth application, 60 ppm if proprietary)

The amplitude (Vpeak) should be within 0.2V to 0.4V, depending on the crystal.

Best regards Håkon