The Real-Time Clock (RTC) is a hardware component that tracks wall clock time and is battery backed so it works even with system power off. Such clocks will normally not track the local time zone or daylight saving time but will instead be set to Coordinated Universal Time.

On the ConnectCore MP15:

  • The STM32MP15 SoC provides one RTC

On the ConnectCore MP15 Development Kit:

  • One RV3028 RTC chip is connected to I2C6

Kernel configuration

You can manage the real-time clock driver support through the following kernel configuration options:

  • STM32 On-Chip Real Time Clock (CONFIG_RTC_DRV_STM32)

  • Micro Crystal RV3028 (CONFIG_RTC_DRV_RV3028)

These options are enabled as built-in on the default ConnectCore MP15 Development Kit kernel configuration file.

Kernel driver

File Description


Driver for the STM32 RTC device


Driver for the RV3028 RTC device

Device tree bindings and customization

Internal RTC

The STM32MP15 RTC interface device tree binding is documented at Documentation/devicetree/bindings/rtc/st,stm32-rtc.yaml.

The device tree for the STM32MP15 RTC is defined in the ConnectCore MP15.

ConnectCore MP15 device tree
&rtc {
	st,lsco = <RTC_OUT2_RMP>;
	pinctrl-0 = <&rtc_out2_rmp_pins_a>;
	pinctrl-names = "default";
	status = "okay";
Hardware version 1 of the ConnectCore MP15 has a spurious capacitance on the 32 kHz oscillator lines that causes the internal RTC to drift by approximately 6.8 seconds per day (if not connected to the Internet). To compensate for this drift, a SOM hardware version 1 specific device tree overlay is applied by the Digi Embedded Yocto default U-Boot boot script.

External RTC

The Micro Crystal RV3028 I2C interface device tree binding is documented at Documentation/devicetree/bindings/rtc/trivial-rtc.yaml.

The device tree for the RV3028 RTC is defined in the ConnectCore MP15 Development Kit.

ConnectCore MP15 Development Kit device tree
ext_rtc: ext_rtc@52 {
	compatible = "microcrystal,rv3028";
	reg = <0x52>;
	interrupt-parent = <&gpioa>;
	interrupts = <4 IRQ_TYPE_EDGE_FALLING>;

RTC user space usage

The different RTCs are accessible via the following file descriptors:

  • /dev/rtc0 (for the STM32MP15 RTC)

  • /dev/rtc1 (for the external RV3028 RTC)

The /dev/rtc device is a symbolic link to /dev/rtc0.

For more information, refer to the Linux kernel documentation at Documentation/devicetree/bindings/rtc/rtc.yaml.

Initializing the RTC

When connected to the Internet, Digi Embedded Yocto uses an NTP (Network Time Protocol) daemon to set the RTC time and keep the system time up to date. You must initialize the RTC the first time you power the board after unpacking the kit and whenever power is completely lost (including RTC backup battery). When not initialized, or if the date is set to a value before the year 1970, the Linux system reports the following error message:

system time... hwclock: settimeofday() failed: Invalid
Linux cannot handle dates before the year 1970.

To initialize the RTC from the Linux shell, set a correct system time using the date command and then issue hwclock -w to write the system time into the RTC.

RTC test application

Digi Embedded Yocto provides a basic RTC test application. Build the package dey-examples-rtc in your Yocto project to install the test application rtc_test.

The RTC test application allows you to read the current time from the RTC; set it using the system time; and read, set, and test the alarm interrupt.


To display the application syntax, run:

# rtc_test -h


Test the alarm interrupt with the specified seconds:

# rtc_test -e -s 20

In this case, the test sets the RTC alarm to 20 seconds from the current time and then waits for the alarm interrupt to occur.