How much is the internal capacitance of the nrf52832 crystal chip?

[stanley.zhao.cn]

Hello：

From the nrf52832 data sheet can be seen in the crystal pin is the internal input capacitance, in the table (19.4.2 64 MHz crystal oscillator (HFXO))on P109 , has pointed out the value of the capacitor is 4pF.So , this value is a pin capacitance value or 2 pin sum is 4pF, because when the choice of crystal external matching capacitor need this parameter.

TKS!

[bjorn-spockeli]

Hi Stanley,

the pin capacitance is 4 pF and the values for the load capacitors should be calculated as follows

C1 = C2 = C_L * 2 - 4pF

please see this and this answer.

Best regards

Bjørn

[Brian]

Hi Bjorn,

Is it possible to remove the external loading caps completely if we were to use a 32MHz crystal with a very low load capacitance, say 4pF.  By the equation above, C1=C2= 4pF * 2 - 4F, then C1=C2=4pF.  But... isn't the capacitance of the PCB and traces to the crystal significant enough then to compensate for the 4pF?    Would it even be helpful to make the traces to the crystal longer?

Here's the crystal we were thinking of using.
www.digikey.com/.../7167748

Thanks for your advice!

Brian

[bjorn-spockeli]

Hi Brian, 

It might be possible theory be possible, but I think that this will be very hard to achieve in practice. The design will be incredibly sensitive to stray capacitance and you will probably have to go through a lot of PCB runs in order to try to tune the crystal load correctly In general we recommend to keep the trace lines to the crystals as short as possible .  

Is removing the load caps a cost-reducing measure or is this a very space constrained design?

Furthermore, the crystal you're considering has +/- 20ppm frequence stability and +/- 20ppm frequence tolerance. The +/- 40ppm requirement for BLE is for total tolerance, i.e. Frequency Tolerance (at 25 ℃) + Frequency Stability Over Operating Temperature Range + Aging x Lifetime of the device. You're on the edge with the specs for the crystal you're considering, especially if you factor in aging at +/- 2ppm per year. 

I would recommend going for a crystal with +/- 10ppm in both frequency stability and tolerance, e.g. https://www.digikey.com/product-detail/en/abracon-llc/ABM11W-32.0000MHZ-4-B1U-T3/535-13893-6-ND/7173492

If you decide to go with no load caps then my recommendation would be to lay out your PCB with space or pads for external load caps so that you have a plan B. 

There is a 1:1 relationship between the crystal frequency and the TX carrier frequency, so you can measure the carrier frequency for a given channel to see if the crystal is loaded correctly.