nRF9160 Real Time Counter for timestamping

[bojan]

Hello guys,

We use nRF9160 and nRF Connect SDK v1.3.0. In our application, we need to keep track of time and timestamp some events. We can connect to the Cloud and get a timestamp in seconds since 1970 as an initial step. However, I wonder how we can keep track of time in our application after that? Our device is battery powered and will be most of the time in the deepest possible sleep mode periodically waking up to do some job and timestamp data taken from the sensors.

Can we use Real Time Counter with 1Hz clock for that purpose?

I came across this RTC sample that shows one use case of RTC peripheral. However, some of the threads say that the RTC counter is a 24-bit counter so it would be impossible to use it for keeping 32-bit timestamp. Also, I don't see a function that will set the value of RTC counter - I spotted only the functions for clearing RTC counter (nrfx_rtc_counter_clear()) and getting its value (nrfx_rtc_counter_get()). Am I missing something?

Thanks in advance for your time and efforts.

Sincerely,

Bojan.

[Markus Tacker (he/him)]

Hei bojan,

in Bifravst we have solved this by timestamping events using local time, and if available getting network time and GPS time and when publishing the events to the cloud we translate the local timestamps to absolute ones:

• https://bifravst.gitbook.io/bifravst/cat-tracker-firmware/protocol#timestamping
• source code: https://github.com/bifravst/firmware

Depending on your solution for example if you do not use GPS you could acquire the current time using NTP. The date-time library does that for you: https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrf/include/date_time.html

[bojan]

Hello, Markus Tacker (he/him)!

Thanks for your feedback. We discovered that we can set the Real Time Counter to count with 8Hz frequency (slowest possible). Since RTC is 24-bits wide, it will overflow after about 24 days when counting with 8Hz frequency.

Every time we connect to the Cloud, we will get an accurate timestamp (aws_timestamp) from the cloud and reset the RTC counter.  From that point on, the timestamp of an event (number of seconds since 1970) will be calculated the following way:

timestamp = aws_timestamp + rtc_counter_value/8;

We just tested the approach and it is working fine.

Cheers,

Bojan.

[Adeel]

Hi Bojan,

I had a similar task that I was working on. I needed a timsetamp to schedule a certain event, like for example I wanted my modem to start off every morning at 10:00:00 and switch off at 10:05:00.

What could be the best way to achieve this kind of functionality ?

[Markus Tacker (he/him)]

See this thread: https://devzone.nordicsemi.com/f/nordic-q-a/52151/nrf9160-calendar-mode/209915#209915

[Adeel]

Hi Markus,

Thanks for sharing this. I tried using AT+CCLK to get the time but I need to manually set the AT+CCLK time first up. Is there a way to be able to update the time using k_timer and link it to the AT+CCLK so it gets updated automatically ?

Thanks