Nordic Clock Drift with Internal LFCLK when trying to synchronize with an external client or another Nordic clock (Nordic -nRF52840)

RE: Nordic Clock Drift with Internal LFCLK when trying to synchronize with an external client or another Nordic clock (Nordic -nRF52840)

haakonsh — Thu, 30 Aug 2018 08:49:21 GMT

Glad to hear^^

RE: Nordic Clock Drift with Internal LFCLK when trying to synchronize with an external client or another Nordic clock (Nordic -nRF52840)

Cecylia — Sat, 25 Aug 2018 00:41:59 GMT

Hi Haakon, thanks for the reply! So, based on your inputs, and after much experiments, we decided to sync the whole system time from another Nordic which has an external clock. This shows much better results :)

RE: Nordic Clock Drift with Internal LFCLK when trying to synchronize with an external client or another Nordic clock (Nordic -nRF52840)

haakonsh — Tue, 21 Aug 2018 08:41:23 GMT

The calibration is done with the HF crystal if it is the active HF clock source at the time of calibration. I believe you can use the method your linked to.

Also Low frequency RC oscillator (LFRC), Normal mode says +/- 500ppm is after calibration, +/- 5000 without calibration.

RE: Nordic Clock Drift with Internal LFCLK when trying to synchronize with an external client or another Nordic clock (Nordic -nRF52840)

Cecylia — Mon, 20 Aug 2018 22:10:22 GMT

Hi Haakon,

For 1) & 2) Since we don't use external crystal, do you mean calibrating it with the HFINT? The spec says LFRC is already ~500ppm. Is this with or without calibration with HFINT?
And, did you mean calibrating using this method?

RE: Nordic Clock Drift with Internal LFCLK when trying to synchronize with an external client or another Nordic clock (Nordic -nRF52840)

haakonsh — Mon, 20 Aug 2018 08:14:10 GMT

If the drift is increasing with time then something is terribly wrong, the drift should be fairly constant given that you're calibrating the RC oscillator that often.
Calibrating at 4 second interval should be more than enough to keep the drift at ~500ppm
You've got two different systems that you want to synchronize the time between, one is the nRF to Client and the other is nRF to nRF.

In the first system it makes sence to use a UNIX timestamp taken from some generic timestamp server and share that with the nRF device periodically in order to update its calendar. It will have limits to its accuracy due to the fact tha the Client-to-nRF communication is non-deterministic since we cannot control the wifi stacks in the computer and module. You will always get a delay from the time your client want to update the clock of the nRF device until the nRF device has updated its clock, and this delay will have a significant variance over time.

In the second system you have nRF-to-nRF communication and this enables you to have very accurate time synchronization because you can control both radio stacks. This means the you can have fairly deterministic delays from the time one nRF device wants to update the clock of anothe nRF device and to the time when the other nRF device has updated its clock (low variance). This in turn enables you to predict this delay and add it to the timestamp used to update the second nRF device's clock, thereby reducing the two clocks time difference.

See https://devzone.nordicsemi.com/b/blog/posts/wireless-timer-synchronization-among-nrf5-devices for details.
That's ok.

RE: Nordic Clock Drift with Internal LFCLK when trying to synchronize with an external client or another Nordic clock (Nordic -nRF52840)

Akhil Bhargav Josyabhatla — Sat, 18 Aug 2018 01:06:51 GMT

Hello,
1) I am noticing a 2-3% drift and it increases gradually with time.
2) I am calibrating my LFRC at 4 Seconds interval.
3) I am confused about this and would like help with this.
4) Yes, But we have hardware limitation. Cannot add the external crystal now.

RE: Nordic Clock Drift with Internal LFCLK when trying to synchronize with an external client or another Nordic clock (Nordic -nRF52840)

haakonsh — Fri, 17 Aug 2018 12:15:58 GMT

How much drift are you experiencing?
Are you calibrating the LFRC at regular intervals?
How are you synchronizing the clocks?
An external crystal will reduce the drift by quite a lot.