How to Calibrate Timer？

RE: How to Calibrate Timer？

MDSC — Wed, 13 Sep 2017 01:52:16 GMT

Thank you for your answer. I will try your methods.

RE: How to Calibrate Timer？

AmbystomaLabs — Tue, 12 Sep 2017 15:12:26 GMT

I think I just answered your comment with the above. Look to the accuracy of your test equipment for your answer.

When you are done with that and get to tuning your xtal, keep in mind that the 12pF or whatever in the reference design is just Nordic's guidance. Your tuning will be different based on your board artwork. Values from 8pF to 12pF are equally possible.

RE: How to Calibrate Timer？

AmbystomaLabs — Tue, 12 Sep 2017 15:05:41 GMT

By the way I should point out that you need to consider the accuracy of the equipment you are using to measure the 1000Hz signal. Most O-scopes have terrible clocks and 100ppm is not unheard of. Whereas my Spectrum Analyzer has a 2.5ppm clock since that is what I use most.

Test labs often use a GPS based 10 or 13MHz reference. These have an accuracy of 0.00001 PPM. Yes that is a 1 with 4 zeros in front of it. Very, very accurate.

I think what you'll find is that between your nominal 32MHz offset and your test equipment everything is pretty normal. Likely you just have to measure each board and adjust the register settings to get them close to each other.

Also you might want to come up with load caps for the xtal that are better suited to your board.

RE: How to Calibrate Timer？

MDSC — Tue, 12 Sep 2017 14:41:33 GMT

Yes, I using 10 ppm 32mHz crystals. I will try the way of routing PClk to GPIO and check the 32MHz crystals. But how can I route PClk to GPIO, I don't find the way? I read the GPIO register,but not find the way. Do you think the offset of 1000 +/- 0.026 Hz is normal ? Because the two user board almostly have the same offset. Maybe the 32mHz is not accuracy or the 12pF capacitance is not accuracy.

RE: How to Calibrate Timer？

AmbystomaLabs — Tue, 12 Sep 2017 14:12:37 GMT

I would suggest next you verify the accuracy of your 32MHz crystals. You can't probe directly since the probe capacitance will load the crystal. Easiest way is to measure the RF channel accuracy with a spectrum analyzer. Or you can route PClk to GPIO and measure that. Pclk is 16MHz for nRF52 series.

Some of the reference designs spec 10ppm crystals with 10ppm stability. So at room temp you should see up to 10ppm error if loaded correctly. Functionally you can view this as 1000Hz +/- 0.010Hz

What is the ppm spec for the crystals you are using?

RE: How to Calibrate Timer？

MDSC — Tue, 12 Sep 2017 14:05:51 GMT

Hello, thank you for your anwser. I use sd_clock_hfclk_request() function to request the HFEXT, and using the oscilloscope to ensure the HFEXT running. When using the HFINT the frequency is unstable, than the frequency is stable but having offset after calling the sd_clock_hfclk_request() function. So I can ensure the HFEXT is running, but the frequency still have offset. So I want to know how to fix this problem.

RE: How to Calibrate Timer？

AmbystomaLabs — Tue, 12 Sep 2017 12:30:01 GMT

There is no need nor capability for calibrating the nRF timer driver. The only reason for the frequency to be off is either your 32MHz clock is incorrectly loaded or you failed to request HFEXT or essentially turn on HFEXT and instead it is running on HFINT which is not an accurate clock source. You always need to request HFEXT in your code if you require its accuracy.