Setting up hardware RTC in Raspbian

My Raspberries are expanded with DS3231 I2C realtime clock. It’s a very simple solution, as only required external component is a backup battery (crystal is integrated into the chip!). And it’s also claimed to be very accurate 🙂

By default, Raspbian installation uses “fake hwclock” to read previously stored date/time from file just before clock is synced by NTP client. We’re going to switch to “full” hwclock, first ensuring that our RTC is seen by kernel.

First, modify /etc/init.d/ to initialize clock’s i2c device.
Create RTC initialization function:

  [ -e /dev/rtc0 ] && return 0;

  # load i2c and RTC kernel modules
  modprobe i2c-dev
  modprobe rtc-ds1307

  # iterate over every i2c bus as we're supporting Raspberry Pi rev. 1 and 2
  # (different I2C busses on GPIO header!)
  for bus in $(ls -d /sys/bus/i2c/devices/i2c-*);
    echo ds1307 0x68 >> $bus/new_device;
    if [ -e /dev/rtc0 ];
      log_action_msg "RTC found on bus `cat $bus/name`";
      break; # RTC found, bail out of the loop
      echo 0x68 >> $bus/delete_device

Call it from the “start” section.
We also have to comment out/remove udev check as our RTC won’t be handled by it.


# Raspberry Pi doesn't have udev detectable RTC
# if [ -d /run/udev ] || [ -d /dev/.udev ]; then
# return 0
# fi

Next, reboot and check if /dev/rtc0 is present and/or “hwclock -r” successfully gets time from RTC.
If everything seems working OK, our RasPi will get proper time even without network connectivity or with ntp client disabled.

We can now disable fake-hwclock from running:

# update-rc.d fake-hwclock remove

17 thoughts on “Setting up hardware RTC in Raspbian

  1. Anthony

    I can’t thank you enough for this! Everyone tells me, “no, put it in rc.local..”
    Each reboot we go back to the when Hendrix was alive and sometimes the RTC doesnt kick in until after data logging daemon has started. But now its real time all the way. I even reread syslog four times to make sure i wasn’t dreaming. Thank you!

    I’m using rdate to synch clock with an NTP box every fortnight even though the ds3231 is more accurate than most RTCs. I was wondering what you rthoughts are?

  2. andres

    Hi I’m so newbie with this. I’m trying but I don’t know what is the “start” section. Please I need help. Whenever, thanks for this post.

    1. elevendroids Post author

      In /etc/init.d/ file search for hwclocksh() procedure:


      Few lines further, there’s a “case” clause with “start” block:

      case "$1" in
      # If the admin deleted the hwclock config, create a blank
      # template with the defaults.
      if [ -w /etc ] && [ ! -f /etc/adjtime ] && [ ! -e /etc/adjtime ]; then
      printf "0.0 0 0.0n0nUTC" > /etc/adjtime

      Insert “init_rtc_device” call right after above fragment.

      1. andres

        Thanks for your response. Unfortunately it doesn’t works. I tried this with the raspbian last release and one ds1307 but is useless . I know ds1307 have 0x68 address and I did everything about configuration and the system recognizes all. I save the commands in /etc/rc.local and I review the exec permissions (chmod)., but every time than I do a system reboot, the time is wrong on system but is right on rtc(ds1307). Then I must to write “sudo hwclock -s” to update the system date. It’s so frustrating. Anyway, thanks for your generosity triyng help to me.

        1. elevendroids Post author

          You have to put this (see post) code into /etc/init.d/ not /etc/rc.local as RTC should be enabled as early as possible to avoid problems with services starting with invalid system time. rc.local is executed at the end of system initialization (it’s too late)…

          1. andres

            I’m sorry. I made a mistake saying etc/rc.local. It was /etc/init.d/ where I did it. To the end doesn’t work. Thanks again.

          2. elevendroids Post author

            Could you send me your file? If you can access your RTC manually, then it has to work this way too.
            Use my email from Contact tab 🙂

  3. Hormiguero

    I was having the same problem as Andres. The script apparently needs to be activated in the /etc/rcS.d/ directory by renaming the symlink from to The update-rc.d should be run. Now the system clock is set from the rtc on boot.

    My problem is that when I change the system date/time from the command line, the hardware clock isn’t changed on shutdown to preserve the new setting… How do I do that?? I have fiddled with the in the other runlevels, but it still doesn’t change the hwclock on shutdown…

  4. regregex

    Excellent page, thank you! However I found the I2C kernel module is blacklisted by default, which can be remedied by commenting out the relevant line in /etc/modprobe.d/raspi-blacklist.conf . This was taken from a page on enabling full I2C support on the Pi (see the link on my name; no affiliation) and after following those instructions as well, I have a working real-time clock.

    Also the init_rtc_device() function can be improved by adding the line:
    ln -s rtc0 /dev/rtc
    just above log_action_message. This lets hwclock find the RTC with the default settings.

    1. regregex

      One more thing – as hormiguero suggested, I had to do
      sudo update-rc.d enable
      sudo update-rc.d fake-hwclock disable

    2. David Eisenberg

      Hello, Thank you elevendroids for this tutorial and to everyone who has chimed in. I am fairly new to Raspberry Pi and I am trying to install the RS2321 RTC and am struggling.

      I updated the /etc/init.d/ file as these instructions tell me to. I also tried adding in regregex’s suggested line “ln -s rtc0 /dev/rtc”. When that didn’t work (along with the update-rc.d commands), I also tried following Homiguero’s advice about renaming the file to However, I only have a file, which I renamed to

      Unfortunately, none of this has worked. When I reboot, I still don’t have a dev/rtc0 file in my directory, and when i type the command hwclock -r into the terminal, it still tells me that it cannot access the hardware clock via any known method.

      Does anyone have any suggestions on what I might be doing wrong or how to fix it?


  5. Mark

    Thank you for this great tutorial, i did things a little different but it works 🙂
    i put my modules (i2c-dev and i2c:mcp7941x) in /etc/modules
    and just added the line:
    echo mcp7941x 0x6f > /sys/class/i2c-adapter/i2c-1/new_device
    at the top from in /etc/init.d
    Edited the symlinks and used update-rc.d to enable everything.

    When i reboot i see that the time is set from the RTC, but when i change the
    time at the command line and do another reboot, the time isn’t saved to the
    When i do manually ./ stop the time is saved to the RTC, any ideas
    why this doesn’t happen when i reboot?

    Grtzz Mark

  6. Pingback: Real Time Clock (Software – I2C) | MyRaspberryAndMe

  7. John L. Males


    I too have a DS3231 based RTC for my RPi which I am trying to configure to set the system time from the RTC at boot. The distribution I use is systemd based and so far despite all the searchng, reading and attempts what I have read and varied in hopes of being able to cause the system time to be set from the DS3231 RTC have not ben successful.

    The RPi distribution I am using had no fakeclock package installed from the start. It was easy to enable the DS3231 RTC such that every time the RPi boots it has access to the RTC. This means if I use “hwclock -s” at the command line the system time that is 1970 based is set to the time of the RTC. So the only challenge is how to have systemd execute the “hwclock -s” command during boot.

    I have read at that systemd has some method to set the RTC at boot that bypasses drift calculations, such as hwclock calculates, which I really do nto want to bypass. The RPi will be run without being connected to the internet almost all the time. I intend to connect the RPi a few times every few weeks on purpose to allow RTC drift calculations to be made by hwclock/ntpd so the clock is accutate like my laptop is when I do not connect to the internet for days or weeks at a time.

    Any help would be greatly appreciated.

  8. dontjinxit123

    After adding init_rtc_device() i get this error

    hwclock: Cannot access the Hardware Clock via any known method.
    hwclock: Use the –debug option to see the details of our search for an access method.


  9. Frank387

    I have similar rtc problems as others have, and so far am stumped for a solution.

    I have an rpi (B+ as I remember) working as an infra red (IR) camera trap. I added a rtc some time ago which worked fine until the battery failed; the hardwired battery was replaced with a cr2032 and now runs correctly on any one of several older rpis but totally loses its time, when installed in the camera, every time the camera is shutdown. Effectively the rtc time reverts to whatever the fake clock is showing at the time, and it it may or may not get corrected if the camera has internet access (totally unpredictable). The rtc is essential for correct time stamping of any videos taken whilst away from it’s home internet. Access to the camera’s content is via the home internet, or another rpi with rtc (that works) when away from home.

    Is this a problem with later versions of rpis running Jessie, since swapping clocks around doesn’t help; all 3 rpis were setup in the same way. Hwclock setting is done in rc.local using “sudo hwclock -r” in all cases, however the camera autoruns from a script also called in rc.local using the & suffix. Could this be interfering the time keeping in any way.

    Any suggestions would be most welcome.


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