High Accuracy Pi RTC (DS3231) installation in Ubuntu 19.10 in Raspberry Pi 4 Model B Rev 1.1

rtc-ds3231-ubuntu-19.10-rpi4.txt

Base instructions are here:

http://wiki.seeedstudio.com/High_Accuracy_Pi_RTC-DS3231/

But I've diverted my setup procedure somewhat because Ubuntu boot setup is different than Raspbian and setup does some things not needed.

To use RTC early in the boot add "fixrtc" to kernel boot command line. Correct file is "/boot/firmware/nobtcmd.txt".

This is implemented in "/usr/share/initramfs-tools/scripts/local-premount/fixrtc".

To add the at at device tree, add "dtoverlay=i2c-rtc,ds3231" to "/boot/firmware/usercfg.txt".

Uninstall possible "fake-hwclock".

There should not be need for configuring any kernel modules or udev/systemd and other initscript changes.

If changes work correctly you should see correct time at start of the boot when looking messages "journalctl -b0". If not, date is "Apr 11" (mine was 2019-04-11T16:28:37.384186+0000).

There should be no (minimal) change in time when your time server starts.

Some lower level debugging helpers, install i2c-tools. You should get UU at location 68 in i2cdetect.

$ ls -l /dev/i2c*
crw-rw---- 1 root i2c 89, 1 Jan 19 20:47 /dev/i2c-1
$ sudo i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- UU -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --                         

Syslog says:
kernel: hctosys: unable to open rtc device (rtc0)
kernel: rtc-ds1307 1-0068: registered as rtc0

The module manages many models: modinfo rtc-ds1307

$ sudo hwclock -v
hwclock from util-linux 2.34
System Time: 1579463067.337580
Trying to open: /dev/rtc0
Using the rtc interface to the clock.
Assuming hardware clock is kept in UTC time.
Waiting for clock tick...
ioctl(4, RTC_UIE_ON, 0): Invalid argument
Waiting in loop for time from /dev/rtc0 to change
...got clock tick
Time read from Hardware Clock: 2020/01/19 19:44:28
Hw clock time : 2020/01/19 19:44:28 = 1579463068 seconds since 1969
Time since last adjustment is 1579463068 seconds
Calculated Hardware Clock drift is 0.000000 seconds
2020-01-19 21:44:27.819024+02:00

If you use systemd-timesyncd,
$ timedatectl status
               Local time: Sun 2020-01-19 21:45:54 EET
           Universal time: Sun 2020-01-19 19:45:54 UTC
                 RTC time: Sun 2020-01-19 19:45:54
                Time zone: Europe/Helsinki (EET, +0200)
System clock synchronized: yes
              NTP service: inactive
          RTC in local TZ: no

If you use chrony, add to /etc/chrony/chrony.conf:
# This directive enables kernel synchronisation (every 11 minutes) of the
# real-time clock. Note that it can’t be used along with the 'rtcfile' directive.
rtcsync