Real-Time Clock

Linux keeps the time in the system clock while the board runs. To keep the time while the board is off, Cargt designs use a battery-backed real-time clock (RTC).

The examples on this page are from i.MX 93 00363 SOMs on 00428 carriers. The RTC differs between SOMs:

SOM	Battery-backed RTC
i.MX 93 00363, i.MX 91 00363, i.MX 93 00324, i.MX 8M Plus 00377	Epson RX8111 at I²C address 0x32, as `rtc0`. On the 00377 the processor’s own RTC is `rtc1` (`snvs_rtc`).
i.MX 93 00359	Micro Crystal RV3032 at I²C address 0x51, as `rtc0`
i.MX 95 00414	Epson RX8111, but on an I²C bus that belongs to the i.MX 95’s System Manager, so Linux cannot reach it in current images. Until it can, do not rely on the 00414 to keep the time while it is off; use network time.

RTC Devices

The i.MX 93 00363 has two RTCs:

Device	Hardware	Notes
`rtc0`	Epson RX8111 at I²C address 0x32	Battery-backed. Keeps the time while the board is off, if a backup battery is fitted. `/dev/rtc` links to it, and Linux reads and writes it by default.
`rtc1`	i.MX 93 internal RTC (BBNSM)	Runs only while the board has power, so it starts at 1970 after every power-up. It has a wake alarm; see Waking from Suspend.

The two RTCs are there because of power. Keeping the i.MX’s internal RTC running while the board is off would mean powering the processor’s battery-backed domain from the backup battery, which draws far more than a dedicated RTC chip. The Epson RX8111 keeps time on about 100 nA (typical, from its datasheet), so a small backup battery lasts for years. These boards therefore keep the time in the RX8111 while the board is off, and run the internal RTC only while the board has power, where its wake alarm is still useful.

root@imx93-cargt-00363-00428:~# for r in /sys/class/rtc/rtc*; do echo "$(basename $r): $(cat $r/name)"; done
rtc0: rtc-rx8111 0-0032
rtc1: bbnsm_rtc 44440000.bbnsm:rtc

Reading and Setting the Time

root@imx93-cargt-00363-00428:~# timedatectl
               Local time: Tue 2026-10-27 18:47:57 UTC
           Universal time: Tue 2026-10-27 18:47:57 UTC
                 RTC time: Tue 2026-10-27 18:47:57
                Time zone: UTC (UTC, +0000)
System clock synchronized: yes
              NTP service: active
          RTC in local TZ: no

root@imx93-cargt-00363-00428:~# hwclock -r
2026-10-27 18:47:57.218413+00:00

root@imx93-cargt-00363-00428:~# hwclock -r -f /dev/rtc1
1970-01-01 00:07:00.424777+00:00

Keep the RTC in UTC (RTC in local TZ: no), and set the time zone with timedatectl set-timezone.

With network time (the default): systemd-timesyncd sets the system clock from NTP, and the kernel copies the system clock to rtc0 about every 11 minutes while it is synchronized (CONFIG_RTC_SYSTOHC). There is nothing to configure.

Without network access, turn NTP off, set the system clock, and write it to the RTC:

timedatectl set-ntp false
timedatectl set-time "2026-10-27 18:50:00"
hwclock --systohc       # copy the system clock to rtc0
hwclock -r

To go back to network time, turn NTP on again. systemd-timesyncd may take a while to correct the clock; restarting it makes it synchronize immediately:

timedatectl set-ntp true
systemctl restart systemd-timesyncd
timedatectl              # wait for "System clock synchronized: yes"

At Boot

Early in boot, the kernel sets the system clock from rtc0 (CONFIG_RTC_HCTOSYS). On a board with a backup battery, after the power had been off:

root@imx93-cargt-00363-00428:~# dmesg | grep rtc
[    1.324561] bbnsm_rtc 44440000.bbnsm:rtc: registered as rtc1
[    2.801999] rtc-rx8111 0-0032: registered as rtc0
[    2.807241] rtc-rx8111 0-0032: setting system clock to 2026-10-27T18:40:59 UTC (1793126459)

On a board without a battery, the RX8111 flags that it lost power, and the kernel leaves the system clock alone until NTP sets it:

[    2.777646] rtc-rx8111 0-0032: registered as rtc0
[    2.782502] rtc-rx8111 0-0032: hctosys: unable to read the hardware clock

If a board with a battery shows this message, check the battery.

Waking from Suspend

The RX8111 alarm is not used on these boards (rtc0 has no wakealarm), but the internal RTC can wake the board from suspend:

rtcwake -d rtc1 -m freeze -s 10   # suspend-to-idle, wake after 10 seconds

Use suspend-to-idle (-m freeze) on i.MX 93 00363 boards. cat /sys/power/pm_wakeup_irq shows the interrupt that last woke the board.