nRF52840 with Zephyr RTOS: How to use RTC with struct rtc_time?

[Domi471]

Hello,
I have a custom board containing a module from Raytac with a Nordic nRF52840 microcontroller on it. The board also has a 32768kHz oscillator connected to the module to have a more accurate LFCLK. I am using VSCode with the nRF Connect SDK and the Zephyr RTOS and want to make use of Zephyrs RTC (https://docs.zephyrproject.org/latest/hardware/peripherals/rtc.html#overview) using the external oscillator to keep track of time, up to the level of months and years. I am not sure if in Zephyr it even is intended to use struct rtc_time.

Here (How to enable NRF52840 RTC using Zephyr? ) you recommend to use the kernel timer, but this is not suitable for the handling struct rtc_time.

I noticed that RTC0 still should be able to use by the user as its stated here:  Zephyr & RTC.  So I also want to use the RTC0.

I also came across https://devzone.nordicsemi.com/f/nordic-q-a/90810/how-to-rtc-with-nordic-and-zephyrbut it does not tell how to use it.

Also: I do not know if the description of the rtc in the device tree is correct. I simply enabled rtc0 in the device tree , resulting in the device tree node:

&rtc0 {

    status = "okay";

    prescaler = <4096>;

    wakeup-source;

};

Is the following define and code line the correct way to access the devicetree node and create a device?

#define RTC_TIMER DT_NODELABEL(rtc0)

const struct device *timer_dev = DEVICE_DT_GET(RTC_TIMER);

Kind regards,

Dominik

[dejans]

Hi,

Your device creation and rtc0 from devicetree seem to be without obvious errors. However, you might be missing some properties. Nordic nrf RTC properties can be found in the documentation.  As a reference, here is a relevant config from zephyr.dts file after building the Zephyr alarm sample:

rtc0: rtc@4000b000 {
			compatible = "nordic,nrf-rtc";
			reg = < 0x4000b000 0x1000 >;
			cc-num = < 0x3 >;
			interrupts = < 0xb 0x1 >;
			status = "okay";
			clock-frequency = < 0x8000 >;
			prescaler = < 0x1 >;
		};

You can find information about struct tm in Time Utilities and about rtc_time in Real Time Clock Zephyr documentation.  There is also API Reference for RTC. In addition, structure rtc_time can be found in rtc.h.

Best regards,
Dejan

[alex.heslop]

Hi dejans , 

This doesn't appear to work with an nRF52dk_52832 discovery kit. It always returns "undefined reference to '__device_dts_ord_84'" when I try to use rtc2 (and _82 for rtc0, _83 for rtc1). Is that expected behaviour due to the board I'm using?

Best Regards,

Alex

[dejans]

Hi Alex, 

I would need to look into this, but I will be out of the office until Tuesday next week. I will get back to you during next week.

Best regards,
Dejan

[Axel Schlueter]

Hey Alex, 

I had the same problem some time back. The linker error stems from the unfortunate double meaning of the "rtc" acronym. You (as did I) mistook the "rtc" in the "rtc0" device for "real-time CLOCK", when in reality it stands for the "real-time COUNTER", which can't be used with Zephyr's time APIs. 

Hope that helps (even though it doesn't solve your problem), 

Axel

[Axel Schlueter]

I realized that I should probably be a little more precise.

You can make the linker error go away if you add CONFIG_COUNTER=y to your prj.conf, this will add the necessary code for the real-time counter device driver. But if you then try to use the real-time clock APIs (e.g. rtc_set_time) with that device, you'll find that your code actually ends up calling (wrong) methods in counter_nrfx_rtc.c, the counter device driver.

So in summary, no real-time clock support for the nRF52 for now. 

[dejans]

Hi Alex,

There is no dedicated support for Real-Time Clock in nrf52832. As Axel already mentioned, there is only Real-Time Counter which does not preserve time during power outages and resets. More information can be found in this ticket.

Best regards,
Dejan