External low frequency crystal reference design?

RE: External low frequency crystal reference design?

Kean — Tue, 19 May 2020 02:34:12 GMT

Late answer, but presumably it is to take into account any PCB stray capacitance

Oh, and 14pF is not a standard value. The next higher standard value is 15pF which may be too much when combined with stray capactitance

RE: External low frequency crystal reference design?

MCB — Fri, 06 Sep 2019 02:06:24 GMT

Did you find the answer of this beautiful question ? because i am curious about it too.

RE: External low frequency crystal reference design?

Francisco Martinez — Thu, 30 Mar 2017 04:12:30 GMT

Øyvind,

The reference design for nrf51822-QFAC with dc/dc converter uses a 32.768 kHz crystal with a load capacitance of 9pF which, according to your comment, should use 14pF capacitors (C1 and C2)

However, the reference design uses 12 pF capacitors. Furthermore, the nrf51 DK reference design also uses 12 pF capacitors. Do you know why it is so?

Thanks in advance.

-Francisco

RE: External low frequency crystal reference design?

Øyvind Karlsen — Fri, 16 Oct 2015 07:28:25 GMT

Yes, that should work

RE: External low frequency crystal reference design?

ujwal — Fri, 09 Oct 2015 18:24:01 GMT

Hi Øyvind,

Just to be clear, the following combination should work? a crystal with load capacitance of 9pF, and C1 = C2 = 2x9 - 4 = 14pF

RE: External low frequency crystal reference design?

ujwal — Fri, 09 Oct 2015 16:20:59 GMT

And yes, I am using the nRF51822. Sorry for the omitting that key piece of information :-)

RE: External low frequency crystal reference design?

ujwal — Fri, 09 Oct 2015 16:19:57 GMT

Thank you very much!

RE: External low frequency crystal reference design?

Øyvind Karlsen — Fri, 09 Oct 2015 07:10:05 GMT

Hi,

I assume you are using the nRF51822?

Please have a look at the product specification to find the electrical specifications for the 32kHz crystal. The main restrictions are:

+/- 250 ppm or better
Load capacitance below 12.5pF
ESR lower than max 80 kΩ

C1 and C2 should be the same value and are calculated like this.

C1 = C_L * 2 - 4pF

Where 4pF is the pin capacitance.

Once you have your hardware in order you need to define in your code which accuracy your crystal has, this is seen in the post that you linked to.

Best regards,

Øyvind