The Linux-based credit card sized single board computer, the Raspberry Pi or the RBPi is designed to be low-cost and of small form factor. As such, many features that are available on normal computers, but considered superfluous here, have been left out. The real time clock is one among them. That makes the RBPi unable to keep time when its power supply has been removed.
Typically, the RBPi is expected to be connected to the Internet via the Wi-Fi or the Ethernet and to update its time automatically from the Network Time Protocol servers available globally. In the absence of an on-board RTC, when there is no Internet connection or when the power to the board is removed, the RBPi is unable to keep time. However, that can be easily rectified by adding a small RTC module running on DS1307 and a tiny coin battery. This allows the RTC to continue to keep time even when the RBPi does not have power supplied to it.
To make things easy, use Adafruit’s Breakout Board kit for the DS1307 RTC. This kit already has all the parts required, including the coin battery. Although the components can be purchased separately and assembled on a breadboard, the coin battery holder can pose a problem, as it is not breadboard-friendly. The kit on the other hand, has a dedicated place for the battery holder, making it more convenient to use.
To allow the RTC chip to communicate effectively with the RBPi, the two 2.2KΩ resistors on the kit must be left out. There is no need for these resistors since the RBPI already has two 1.8KΩ resistors on-board and they are connected to the 3.3V supply, as the RBPi needs them to be. Therefore, either do not solder the two resistors to the breakout board, or, if you have already soldered them in, remove or clip them out. The breakout board needs 5V, so connect the VCC on the board to the 5V pin of the RBPi.
Now, you will need to set up the I2C interface on the RBPi. For this, your RBPi must be running a kernel that includes the RTC and DS1307 modules. The latest version of the Raspbian OS already has the modules included, but older versions may not have them. Adafruit has a wonderful tutorial that will guide you for setting up and testing I2C on the RBPi, check it out here.
At the command line, you can run the command “sudo i2cdetect -y 0” to check your wiring. If you have a rev2 RBPi, enter the command “sudo i2cdetect -y 1”. Once you see ID #68 being displayed, you know that your wiring is correct, as this is the address of the DS1307. Once you get the kernel driver running, i2cdetect will show UU instead of 0x68, further confirming that everything is good.
The next step is to load up the RTC module and set it up as root. Follow the tutorial for doing that and you can check the time with the command line “sudo hwclock -r”. If you are using the module for the first time, the date will be Jan 1 2000 – set it to the current time, and you are done.