Raspberry Pi - Smart Home Automation with Linux and Raspberry Pi - page 274

The Drawbacks
Of course, if the Raspberry Pi were perfect we wouldn't need to invent any more computers. Ever. However, aside from
its lack of power to do modern tasks (even speech synthesis is limited to a much simplified, and less aurally pleasing,
algorithm), there are some issues that limit the places in which a Raspberry Pi can be used. Whether or not these are
important issues will depend on your use case.
The first point to raise is that you cannot boot across the network with PXE or similar technologies. Therefore, you
will always need an SD card or (less usually) a hard drive, connected. So, if you're planning on putting the Raspberry
Pi in a confined space, bear in mind that the SD will stick out further than the case.
Another caution when physically mounting the device is that the power supply is connected via a USB lead, which
adds another few millimeters to the total dimension, as does the composite cable, Ethernet plug, and so on. The more
usual dimensions of a Raspberry Pi is actually 175.0 x 85.0 x 25.0mm, as seen in Figure 8-3 .
Figure 8-3. The effective size of a Raspberry Pi, without bending the cables excessively
Also, talking of mounting, remember to buy version 2 boards because these are the first ones to have mounting
holes so they can be fixed into cases and attached to arbitrary devices, using the two holes. 2 Previous revisions relied
on being tightly wedged into a case, and laid horizontal, which limited their location.
As you might notice from looking at the board, there is no on/off switch. Given the amount of current it draws,
this is probably unimportant for most people. Indeed, the hacks necessary to make an NSLU2 or Mini-ITX work
around the issues of the power switch might mean this was a sensible choice. However, if you think you'll need such a
button, you'll have to insert one into the USB cable, manually.
Another missing element is a real-time clock. Without one, you have to manually set the clock every time you
plug it in. This means that log files and file stamps can get into a mess if you don't reprogram the time, or reprogram
it incorrectly. Normally, the clock circuit would consist of a resonator or crystal and backup battery, to retain
the time and date when the machine is switched off. However, this was considered too much extra circuitry and
therefore omitted from the design. Therefore, any solution requiring knowledge of the time or date (which includes
anything that might log data) will either need to synchronize the clock with a networked time server (NTP), keep it
permanently remain on, or buy/build an external USB clock such as the one at http://ahsoftware.de/usb-rtc .
For a software-based solution, you could write a short script that looked for the most recent date in the log files, and
use that. The time would be wrong from a human's perspective but, with the date stamps being sequentially, the
machine wouldn't cause problems.
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