NRF52840 32MHz crystal offset parameter recommendation

RE: NRF52840 32MHz crystal offset parameter recommendation

Frank Yang — Wed, 21 Dec 2022 03:18:00 GMT

Thanks for your reply, you solved my confusion!

Frank Yang

RE: NRF52840 32MHz crystal offset parameter recommendation

ketiljo — Tue, 20 Dec 2022 15:05:30 GMT

Yes, if you plan for 10 years lifetime, this will be correct.

Remember that the temperature stability is worst near the end of the temperature range. If you have a li-ion battery connected, this will limit the usable temperature range of the product.

RE: NRF52840 32MHz crystal offset parameter recommendation

ketiljo — Tue, 20 Dec 2022 15:02:55 GMT

If you're testing only in room temperature and the temperature doesn't change a lot, the majority of the frequency error comes from the crystal and the load cap tolerance. Maybe consider using load caps with tighter tolerance if the tested frequency error is outside spec.

RE: NRF52840 32MHz crystal offset parameter recommendation

Frank Yang — Fri, 16 Dec 2022 13:10:33 GMT

In addition, this is the specification of the 32MHz crystal oscillator we use, how to calculate its total tolerance？

For this crystal, you have tolerance 7 ppm, stability 12 ppm and aging 1 ppm, meaning a total of 19 ppm plus 1 ppm / year. More than good enough!

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Considering that the product has been used for 10 years, the cumulative frequency deviation of 10 years will reach 10ppm. The sum is 29ppm, which is just ±30ppm to meet the needs of long packages. Is my understanding correct?

RE: NRF52840 32MHz crystal offset parameter recommendation

Frank Yang — Fri, 16 Dec 2022 13:06:54 GMT

Hi Ketiljo

Thanks for your reply！

3. Can we set the crystal frequency at ±15ppm? What are the risks in using it?

Unless the stability over temperature is more than 10 ppm, you should be fine with this.

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How to understand this? The crystal we are using now has a frequency stability of ±12ppm. Is the condition to set the test pass parameter to exceed ±10ppm not met?

RE: NRF52840 32MHz crystal offset parameter recommendation

ketiljo — Fri, 16 Dec 2022 12:52:42 GMT

[quote user=""]1. Under what circumstances is it necessary the 32MHz crystal to achieve ±30ppm? Which applications scenarios require packet length > 200 bytes?[/quote]

This is set by the Bluetooth test spec to make sure interoperability between radios from different manufactures are handled well. If you want to increase the datarate by sending longer packets, you need to be able to send long packets.

[quote user=""]2. For general customers, what is the frequency offset requirement for the crystal? Because we also need to consider the influence of the aging of the crystal and the service life of the product.[/quote]

You have to include tolerance, stability and aging. The three parameters needs be be 30 ppm, or 40 ppm in case you only use short packets, or better.

[quote user=""]3. Can we set the crystal frequency at ±15ppm? What are the risks in using it?[/quote]

Unless the stability over temperature is more than 10 ppm, you should be fine with this.

[quote user="Frank-super"]In addition, this is the specification of the 32MHz crystal oscillator we use, how to calculate its total tolerance？[/quote]

For this crystal, you have tolerance 7 ppm, stability 12 ppm and aging 1 ppm, meaning a total of 19 ppm plus 1 ppm / year. More than good enough!

RE: NRF52840 32MHz crystal offset parameter recommendation

Frank Yang — Fri, 16 Dec 2022 06:40:42 GMT

The bill of material recommand crystal specification

RE: NRF52840 32MHz crystal offset parameter recommendation

Frank Yang — Fri, 16 Dec 2022 06:35:40 GMT

In addition, this is the specification of the 32MHz crystal oscillator we use, how to calculate its total tolerance？