DESK-MX8M-AN-0003: Add an external RTC to SBC ORCA

Info Box Applies to ORCA Application Notes Applies to ORCA SBC Application Notes

This application note has been validated using the kit version in the History table.

History

Version	Date	Development Kit version
4.0.0	Mar 2024	DESK-MX8M-L 4.0.0

Introduction

An external RTC device can be connected to ORCA SBC to keep date & time and synchronizing the system clock at boot time.

As an example, an easy - just ready - RTC small board can be bought from Amazon with the DS3231 RTC device.

DS3231 small board with CR2032 battery

Linux driver

DESK-MX8M-L 4.0.0 kernel has the related rtc-ds1307 driver module already built and available in the root file system image.

It is enough to connect the RTC board to an I2C: for the purposes of this AN, it is possible to use the GROOVE connector which has the four required signals available: SCL, SDA, VDD, GND (see the SBC schematics page).

Then, the driver has to be enabled in the device tree using the i2c6 interface available in the GROOVE connector:

/* groove connector */
&i2c6 {
        clock-frequency = <100000>;
        pinctrl-names = "default";
        pinctrl-0 = <&pinctrl_i2c6>;
        status = "okay";

        ds3231: ds3231@68 {
                compatible = "maxim,ds3231";
                reg = <0x68>;
                status = "okay";
        };
};

Accessing the rtc device

Setting the RTC device as rtc0 lets the system clock be synchronized. The related section can be added to the device tree as an alias:

    aliases {
            /* ds3231 is our default rtc dev (rtc0) */
            rtc0 = &ds3231;
            rtc1 = &snvs_rtc;
    };

The RTC device is available as /dev/rtc0 device and the related time can be read using the hwclock command:

root@123456789:~# dmesg | grep rtc0
[    6.949783] rtc-ds1307 5-0068: registered as rtc0
root@desk-mx8mp:~# hwclock -f /dev/rtc0
2024-03-06 16:18:28.182912+01:00
root@desk-mx8mp:~#

Now the date command shows the system clock synchronized at any reboot.

root@desk-mx8mp:~# date
Wed Mar  6 16:40:44 CET 2024
root@desk-mx8mp:~#