time-y wimey... stuff

it's more like a big ball of wibbly wobbly... time-y wimey... stuff

In which we get a Dallas Semiconductor real time clock (RTC) working on the Beagle Bone Black over the I2C bus under Arch Linux Arm.

The Beagle Bone Black has a real time clock (RTC from now on); however, the RTC does not have a battery to allow it to maintain the time when the board is powered off.  This will generally not pose a problem if your board is always connected to the Internet over the wired interface, but may be a stumbling block otherwise.  Luckily for you, it is easy enough to add a RTC.

Arch Linux Arm has the I2C bus enabled by default, and Arch Linux Arm has the driver for the DS1307 compiled into the kernel as can be seen below in the screenshot of the config file.  This may not be absolutely critical but should prevent any timing issues others have experienced when loading kernel modules at boot.

One great thing about the DS1307 driver is that the driver actually works with several different RTCs.  The below screenshot shows the RTCs listed in the driver source.  This tutorial uses the ChronoDot based on the DS3231; however, you should be able to use any listed.

More information on the ChronoDot can be found at [1], and it was purchased from [2].  It is essentially a break out board for the DS3231, an extremely accurate real time clock.

Connecting the ChronoDot to the Beagle Bone Black requires just four wires.  All you need to do is connect GND, Vcc, SCL, and SDA.  By default you can find all the pins needed on expansion header P9.

GND    pin 1 or 2
Vcc      pin 3 or 4
SCL      pin 19 (I2C2)
SDA     pin 20 (I2C2)

You will want to use the I2C2 bus as I2C1 is used by the board itself.

Power off your board and connect the corresponding pins on the ChronoDot to the pins on the Beagle Bone Black as listed above.  You can safely ignore the other pins on the ChronoDot as they are not needed for adding a RTC to the Beagle Bone Black.

Power on the Beagle Bone Black and you are almost done.

If you would like to verify that your Beagle Bone Black can see the ChronoDot, you can install the I2C-tools package with
pacman -S i2c-tools
Once installed, you can issue the following command
i2cdetect –y –r  1
This will scan I2C bus 1 (bus 1 in software corresponds to I2C2 on the hardware) for connected devices.  If you have connected the ChronoDot correctly, you should see an entry for 68 as in the image below.  68 is the bus address of the DS3231.  If you do not see a 68, verify your connections and/or the location of the I2C2 bus pins as they can be changed in software.


In order to register the new I2C RTC, you need to issue the command

echo ds3231 0x68 >/sys/bus/i2c/devices/i2c-1/new_device

After this you should see an entry for /dev/rtc1 as well as /sys/class/rtc/rtc1 – both point to the same device.

To verify you have the right rtc, check the name with

cat /sys/class/rtc/rtc1/name

and to check the time

cat /sys/class/rtc/rtc1/time

Most likely the time is wrong, and you should correct it before proceeding.  The easiest way to do this is to update the system time using ntpd and then synchronize the RTC.

If ntpd is running, you can either wait for it to eventually update the system time or stop it and force an update with

systemctl stop ntpd
ntpd –dqg

Once the system time is correct, synchronize the RTC with

hwclock -f /dev/rtc1 –w

Notice we are specifying the hwclock to use in the command above.

Now that the RTC has been set to the correct time, we can use it at boot to set our system time.

The first thing you need to do is create a script to recreate the I2C RTC device at boot and synchronize the system clock with it.  Once that is done, you can configure the system to run the script at boot.

Create the script with

nano /usr/lib/systemd/scripts/rtc

and type or copy and paste the following into the script

echo ds3231 0x68 >/sys/bus/i2c/devices/i2c-1/new_device
hwclock -f /dev/rtc1 –s

As promised, this will recreate the I2C RTC and synchronize the system clock to it.

Next you will need to make the script executable with

chmod 755 /usr/lib/systemd/scripts/rtc

Now to make it run at boot.

Create the systemd file with

nano /etc/systemd/system/rtc.service

and type or copy and paste the following into the script

Description=RTC clock



Test it out by issuing

systemctl start rtc

If you receive no errors, you can enable this to run at boot with

systemctl enable rtc

Otherwise review the error messages to determine the issue.

You might have noticed the line Before=netctl-auto@wlan0.service in the systemd file. This instructs systemd  to bring up the RTC before starting the wireless LAN adapter.  This should prevent any wpasupplicant issues concerning timestamp validation.

You should now have a working battery backed RTC on your Beagle Bone Black.

[1] docs.macetech.com/doku.php/chronodot_v2.0
[2] www.evilmadscientist.com