External low frequency crystal reference design?

[ujwal]

Hello,

We are designing a custom board, and we would like to use an external crystal for the low frequency clock source, but we are running into disconnect issues. A similar problem is described in devzone.nordicsemi.com/.../

Are there any reference designs or recommended guidelines on the low frequency clock source and the associated caps?

Thanks,

-ujwal

[Øyvind Karlsen]

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

[ujwal]

Thank you very much!

[ujwal]

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

[ujwal]

Hi Øyvind,

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

[Øyvind Karlsen]

Yes, that should work

[Francisco Martinez]

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