Page 102 - Programming the Raspberry Pi Getting Started with Python

a “slave” device and hence only listens on the SDA and SCL lines. Other I2C devices may act as a
master device, and if they are 5V, there is a good chance this could damage your Pi. Therefore,
before you connect any I2C device to your Raspberry Pi, make sure you understand what you are
doing.
We can now link the Cobbler to the Raspberry Pi using the ribbon cable supplied with the Cobbler.
This should be done with the Raspberry Pi powered down. The cable will only fit one way into the
Cobbler, but no such protection is provided on the Raspberry Pi. Therefore, make sure the red line on
the cable is to the outside of the Raspberry Pi, as shown in Figure 10-1.
Turn on the Raspberry Pi. If the usual LEDs do not light, turn it off immediately and check all the
wiring.

Software
Everything is connected, and the Raspberry Pi has booted up. However, the display is still blank
because we have not yet written any software to use it. We are going to start with a simple clock that
just displays the Raspberry Pi’s system time. The Raspberry Pi does not have a real-time clock to tell
it the time. However, it will automatically pick up the time from a network time server if it is
connected to the Internet.
The Raspberry Pi displays the time in the bottom-right corner of the screen. If the Pi is not
connected to the Internet, you can set the time manually using the following command:

However, you will have to do this every time you reboot. Therefore, it is far better to have your
Raspberry Pi connected to the Internet.
If you are using the network time, you may find that the minutes are correct but that the hour is
wrong. This probably means that your Raspberry Pi does now know which time zone it is in. This can
be fixed by using the following command, which opens up a window where you can select your
continent and then the city for the time zone you require:

At the time of writing, in order to use the I2C bus that the display uses, the Raspbian Wheezy
distribution requires that you issue a few commands to make the I2C bus accessible to the Python
program we are going to write. It is likely that later releases of Raspbian (and other distributions) will
have the port already configured so that the following commands are not necessary. However, for the
moment, here is what you need to do:

NOTE You may find that you have to issue the last two of these commands each time you reboot the
Raspberry Pi.
So now that the Raspberry Pi knows the correct time and the I2C bus is available, we can write a
Python program that sends the time to the display. To help simplify this process, I have produced a
Python library module specifically for this kind of display. It can be downloaded from
http://code.google.com/p/i2c7segment/downloads/list.

As with other modules you have installed, you need to fetch the file, extract it into some convenient
location (using tar -xzf), and then issue the following command to install it under Python 2:

The actual clock program itself is contained in the file bundle that accompanies this book (see
www.raspberrypibook.com); it is called 10_01_clock.py and is listed here: