understanding the accuracy/precision of LFRC on nRF52833

[mmdunc]

Hi,

We have an application where we need to predict drift between two nRF52833 systems in low power mode running from LFRC (low frequency RC oscillator).

Section 5.4.4.4 of the product specification shows that the LFRC has an accuracy of +/- 500ppm after calibration.

We're able to estimate the frequency offset between the two nRF52833s, but we'd really like to know how much that offset is likely change. Although the specification lists +/- 500ppm, we're hoping that much of this accuracy range is due to chip-to-chip manufacturing irreproducibility, and that specific chips have a much better stability / precision than this. We'd like to know how often we need to make a new estimate of the LFRC frequency difference between the two chips.

Are there any other materials, plots, or documentation that might show what kind of thermal stability / aging effects the LRFC has, independent of manufacturing irreproducibility?

[Kazi Afroza Sultana]

Hello,

There is an old case (+) What low-frequency clock sources can I use? - Nordic Q&A - Nordic DevZone - Nordic DevZone you can look at. I believe you will find a lot of relevant things here.

[mmdunc]

Thanks for forwarding this, in the post it's asserted that "The frequency drift of the RC is in fact a result of temperature change" and that "The chip have an internal RC oscillator that has an accuracy of 250 ppm when calibrated".

Interestingly It looks like the nRF52833's RC is 500ppm when calibrated, instead of 250ppm for the nRF51.

How is this accuracy spec defined?  I assume 500 ppm is with respect to the calibrating clock (HFCLK), not absolute accuracy? As far as I can tell there's no accounting for the thermal drift of the HFCLK in this process, so the true accuracy may be lower (or perhaps the 500ppm encompasses thermal inaccuracy of the HFCLK?)

[Kazi Afroza Sultana]

Hello,

For LFRC, Frequency tolerance after calibration is +/- 500ppm. With proper calibration it is achievable. I will talk to relevant team to know about the effect of temperature drift on accuracy.