Jitter requirement for external 32.768kHz oscillator for nRF52840

[Tosa]

I don't see a spec for the jitter requirement for using an external 32.768kHz oscillator for nRF52840:

Can the SIT1552AI-JF-DCC-32.768  be used?

[JONATHAN LL]

Hi Tosa,

In general as long as the low frequency crystal oscillator (LFXO) are whit in the specifications (load capacitance and series resistance) and the the layout don't stray to far from the reference design layout is should work. 

However the component you references is a MEMS oscillator and it is generally not recommended but it has been used (ref). 

Here are some similar question\answer:
here, there and here.


In this blog post advice 27:
All the examples use the external 32kHz crystal. The SoftDevice will hard-fault if your design does not have the crystal and your code tells the SoftDevice to use it. Configure the pca10040.h file or sdk_config.f ( nRF_SoftDevice / NRF_SDH_ENABLED) to use the internal low-frequency oscillator if you don’t have an external 32kHz crystal. Be sure to set the accuracy to 500PPM or you may see occasional disconnects. An external clock source may alternately be used for the 32kHz clock (SiTime or similar). An external clock must meet the phase and jitters specifications for the nRF device you are using.

The LF oscillator will be ok using a MEMS oscillator, this only requires a clock for the RTC.

Here is a link(RTC), link(radio jitter), link(PDM) to the jitter specs in the PS.



Regards,
Jonathan