Raspberry Pi Real Time Clock Module DS1338
Raspberry Pi Real Time Clock
The Raspberry Pi does not have a real time clock. This is ok if your Raspberry Pi is connected to the internet as it will automatically sync the clock to one of the NTP servers. If you use the RPi standalone and require a real time clock then you can add the functionality toe have the RPi real the date and time from an RTC module.
The RTC module we are going to use is our own RTC module based on the DS1338 real time clock chip. It runs at 3.3V so can be connected directly to the RPi I2C port pins and 3.3V power
Raspbian Wheezy kernel version 3.2 and above already include the required drivers for the DS1338/DS1307 RTC chip
Enable I2C and Install I2C Tools
If you don't already have I2C enabled and i2c-tools installed, then we must install i2c-tools. If you already do, you can skip this stage.
By default the Raspberry Pi Raspbian config doesn’t load the I2C modules at boot. To change this you need to comment out or delete the I2C entry from the following
file: /etc/modprobe.d/raspi-blacklist.conf
Add a # character to the beginning of each line and save the file
#blacklist spi-bcm2708 #blacklist i2c-bcm2708
Next, install ic2-tools
sudo apt-get install i2c-tools
Then add the I2C functionality into the kernel with
sudo modprobe i2c-dev sudo modprobe i2c-bcm2708
Enable RTC Driver
This specific RTC is a DS1338, which is supported by the DS1307 driver. We add support with:
sudo modprobe rtc-ds1307
Next we can check to make sure the RTC module can be seen. Run i2cdetect with port 0 for RPi V1 or port 1 for RPi V2
sudo i2cdetect -y 0
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- 68 -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --
The RTC module should appear at address 68
Add RTC Device at boot
To make the RTC module load at boot time we need to add a new device
sudo bash echo ds1307 0x68 > /sys/class/i2c-adapter/i2c-0/new_device exit
Test the RTC module
We should now be in a position to test the RTC module. Run
sudo hwclock -r hwclock: Warning: unrecognized third line in adjtime file (Expected: `UTC' or `LOCAL' or nothing.) Fri 11 Apr 2014 16:22:23 UTC -0.686251 seconds
Ok, we have a date and time returned, but you may see a Warning error (you may not).
If you get the error it is due to a bug with the creation of the adjtime file. There should be a carriage return after the UTC line. Edit the file and add a carriage return.
sudo vi /etc/adjtime 0.0 0 0.0 0 UTC
Load the RTC module at boot
Finally, once we get everything working, we must ensure that all the modules are loaded at boot. We edit /etc/modules:
sudo vi /etc/modules
we add the following lines:
i2c-bcm2708 rtc-ds1307
sudo vi /etc/rc.local
and add the following lines before ‘exit 0’ (change /i2c-0/new_device to i2c-1/new_device if you’re using rev2):
echo ds1307 0x68 > /sys/class/i2c-adapter/i2c-0/new_device hwclock -s
The next time you boot, it should have the correct time.
Setting the Date and Time
We need to set the right time set on the RPi. This is done automaticallyif you connect it up to Ethernet or Wifi - it will automatically set the time from the network.
But you can set it manually using the 'date' command, e.g.
sudo date -s "11 APR 2014 16:51:00"
Check that the date is set correctly
date Fri Apr 11 16:51:02 UTC 2014
Once the date and time is correct you can write it to the RTC module with
sudo hwclock -w
You can then verify it with
sudo hwclock -r