Operation Manual

Chapter 3: Troubleshooting
Sometimes, things don’t go entirely smoothly. The more complex the device, the more complex the problems that can occur—
and the Pi is an extremely complex device indeed.
Thankfully, many of the most common problems are straightforward to diagnose and fix. In this chapter, we’ll look at some of
the most common reasons for the Pi to misbehave and how to fix them.
Keyboard and Mouse Diagnostics
Perhaps the most common problem that users experience with the Raspberry Pi is when the keyboard repeats certain characters.
For example, if the command startx appears onscreen as sttttttttttartxxxxxxxxxxxx, it will, understandably, fail to
work when the Enter key is pressed.
There are typically two reasons why a USB keyboard fails to operate correctly when connected to the Raspberry Pi: it’s
drawing too much power, or its internal chipset is conflicting with the USB circuitry on the Pi.
Check the documentation for your keyboard, or the label on its underside, to see if it has a power rating given in milliamps (mA).
This is how much power the keyboard attempts to draw from the USB port when it’s in use.
The Pi’s USB ports have a component called a polyfuse connected to them, which protects the Pi in the event that a device
attempts to draw too much power. When this polyfuse is tripped, it causes the USB port to shut off, at around 150 mA. If your
keyboard draws anywhere around that much power, it may operate strangely—or not at all. This can be a problem for
keyboards that have built-in LED lighting, which require far more power to operate than a standard keyboard.
If you find that your USB keyboard may be drawing too much power, try connecting it to a powered USB hub instead of
directly to the Pi. This will allow the keyboard to draw its power from the hub’s power supply unit, instead of from the Pi itself.
Alternatively, swap the keyboard out for a model with lower power demands. The repeating-letter problem may also be traced
to an inadequate power supply for the Pi itself, which is addressed in the next section, “Power Diagnostics”.
The issue of compatibility, sadly, is harder to diagnose. While the overwhelming majority of keyboards work just fine with the Pi,
a small number exhibit strange symptoms. These range from intermittent response, the repeating-letter syndrome or even crashes
that prevent the Pi from operating. Sometimes, these issues don’t appear until other USB devices are connected to the Pi. If your
keyboard was working fine until another USB device, in particular a USB wireless adapter, was connected, you may have an
issue of incompatibility.
If possible, try swapping the keyboard out for another model. If the new keyboard works, your old one may be incompatible
with the Pi. For a list of known-incompatible keyboards, visit the eLinux wiki:
http://elinux.org/RPi_VerifiedPeripherals#Problem_USB_Keyboards
The same advice on checking compatibility in advance applies to problems with the mouse: the majority of USB mice and
trackballs work fine, but some exhibit incompatibility with the Pi’s own USB circuitry. This usually results in symptoms like a
jerky or unresponsive mouse pointer, but it can sometimes lead to the Pi failing to load or crashing at random intervals. If you’re
looking to buy a new mouse, an up-to-date list of models known to work with the Pi is available at the eLinux wiki site:
http://elinux.org/RPi_VerifiedPeripherals#Working_USB_Mouse_Devices
Power Diagnostics
Many problems with the Raspberry Pi can be traced to an inadequate power supply. The Model A requires a 5 V supply
capable of providing a 500 mA current, while the Model B’s extra components bump up the current requirement to 700 mA.
Not all USB power adapters are designed to offer this much power, even if their labelling claims otherwise.
The formal USB standard states that devices should draw no more than 500 mA, with even that level of power only available to the device following a
process called negotiation. Because the Pi doesn’t negotiate for power, it’s unlikely that it will work if you connect it to the USB ports on a desktop
or laptop computer.
If you’re having intermittent problems with your Pi—particularly if it works until you connect something to a USB port or start a
processor-intensive operation like playing video—the chances are that the power supply in use is inadequate. The Pi provides a