Question 11.13: You want your Raspberry Pi to remember the time, even when i......

Use an RTC (real-time clock) module.

A very common RTC chip is the DS1307. It has an I2C interface and can be bought as a ready-to-use module that includes the chip itself, a quartz crystal for it to maintain accurate timekeeping, and a battery holder for a 3V lithium cell.

To make this recipe, you will need:

• A DS1307 or compatible RTC module (see “Modules” on page 381). You can also use an aLaMode board (Recipe 14.13).

• Female-to-female jumper wires (see “Prototyping Equipment” on page 380)

The RTC module that you are using must be 3.3V compatible. That means that its I2C interface should either have no pull-up resistors at all, or they should pull up to 3.3V and NOT 5V. When using the Adafruit model here, simply don’t include the two resistors when soldering together the module. If you have a ready-made module, carefully remove any pull-up resistors.

Assemble the RTC module if it is in kit form, remembering to omit pull-up resistors and then connect the module to the Raspberry Pi, as shown in Figure 11-17.

The DS1307 is an I2C module, so your Raspberry Pi must be set up to work with I2C (see Recipe 8.4). You can check that the device is visible using I2C tools (Recipe 8.5).

The 68 in the table indicates that the RTC module is connected to the I2C bus at hex address 68.

If you are using one of the original Raspbery Pi model B, revision 1, boards, then use 0 after the y option in the preceding line. The revision 1 boards were distinctive for having a black audio socket.

You now need to run the following commands so that the RTC can be used with a program called hwclock.

Once again, if you are using a revision 1 board, change i2c-1 to i2c-0.

You can now access the RTC using the following command:

As you can see, the clock is not currently set.

To set the time on the RTC module, you first need to make sure that your Raspberry Pi has the right time. If your Pi is connected to the Internet, this should happen automatically. You can check this using the date command:

If the time is wrong, you can also set it manually using date (Recipe 3.33). To transfer the Raspbery Pi system time onto the RTC module, use the following command:

You can then read the time using the -r option:

The RTC having the correct time is pointless unless it is used to set the correct system time in Linux when it reboots. To arrange for this to happen, you need to make a few configuration changes.

First, edit /etc/modules (using sudo nano /etc/modules) and add rtc-ds1307 to the end of the list of modules. If you have already added some modules while setting up I2C, SPI, and other options, your file might look something like this:

Next, you need two commands to run automatically during startup to set the system time. So edit the file /etc/rc.local using the command sudo nano /etc/rc.local, and just before the final exit 0 line, insert the following two lines. Remember to change i2c-1 to i2c-0 if you are using a revision 1 Pi.

When you finish, the file should look something like this:

Now, when you reboot, your Raspbery Pi should set its system time from the RTC. However, if there is an Internet connection available, this will take precedence for setting the time.

Discussion

An RTC is not essential for a Raspberry Pi by any means, since a Raspberry Pi that is connected to the Internet will automatically connect to a network time server and set its own clock. However, you may not always be using the Pi when connected to a network, in which case a hardware RTC is a good option.

The aLaMode (Recipe 14.13) Arduino-based interface board for Raspberry Pi also includes an RTC module that will work with this arrangement.

See Also

AB Electronics have a neat RTC that plugs directly into the GPIO socket. This is shown in Figure 11-18.

This recipe is based on a tutorial from Adafruit.