Best Way to Keep Track of Time with the nRF52

[A Knecht]

I need to keep track of time on a device featuring the nRF52 and have an RTC run continuously in the background. The drift should not be more than a few seconds over a year. Is this possible to achieve with a moderate power consumption?

I am a bit overwhelmed with the number of of ways to keep track of time in the SDK:

• app_timer*
• Also, the changes to app_timer posted in this forum by Clem Taylor. This dates back to 2015, though and targets the nRF51?
• app_simple_timer*
• nrf_drv_timer*
• nrf_drv_rtc*
• nrf_drv_clock*

Is there anything that comprehensively explains the difference between these? Also, what should my approach be to get a system clock with as little time drift as possible?

[John]

That's a pretty strict drift requirement. I didn't do the math but that might be less than 1 ppm. You won't get that kind of performance out of the internal RC, and I haven't seen an external crystal that was that good. If you really need to hit that you will also need to calibrate the clock for is natural offset, and probably temperature compensate it periodically.

[Sigurd]

Hi,

The clock control system is basically the system that drives the whole chip, including the CPU and all peripherals. It can be divided into 2 clock systems, the low frequency clock(LFCLK) and the high frequency clock (HFCLK) . With nrf_drv_clock* you have several ways to interact with the clocks, including initialization and calibrating.

---

The nrf_drv_rtc* includes drivers for the Real-time counter (RTC) peripheral.

The nrf_drv_timer* includes drivers for the TIMER peripheral.

The main difference between TIMER and RTC (Real-time counter) is that:

• TIMER uses the high-frequency clock source (HFCLK, 16 MHz), which means better resolution (62.5 ns) and higher power consumption (typ. 5 or 70 uA depending on HFCLK source).

• RTC uses the low-frequency clock source (LFCLK, 32 KHz), which means less resolution (~30 us) and lower power consumption (typ. 0.1 uA).

The app_timer* and app_simple_timer* are libraries that builds on top the RTC1 peripheral(app_timer) and the TIMER1 peripheral(app_simple_timer). This gives more high-level functionality where you can create multiple timer instances, and you have functions to start and stop the timers. Take a look at the Application Timer Tutorial if you want to learn more.

---

For keeping track of time on the nRF52, I would recommend taking a look at the nrf-calendar-example we have on Github.

[A Knecht]

What do you reckon is an achievable drift over the course of a year with the nRF52 and an external RTC respectively?

[Sigurd]

It will depend on the ppm of the source of the RTC.

Here is an online-calculator you can use to calculate the drift.

On the nRF52-DK the 32kHz external crystal is 20 ppm, this will give you a drift at 10.51 minutes for 1 year. The lowest ppm I have seen for a 32kHz external crystal is 5 ppm (2.63 minutes drift for 1 year).

[sonalkale]

hello,

how nrf-calendar-example work?

I can create hex file using sdk14.But there is no output using printf command on uart port.

please suggest some solution.

Thank You.