Hacking the Xbox

Appendix E - Debugging: Hints and Tips 249
never, ever ignore an inexplicable or inconsistent behavior, even if it is
intermittent. For example, sometimes a system will work properly or
break if you touch a certain location on the circuit board or wave your
hand near a certain area; sometimes a system will demonstrate different
behavior for a brief moment after power-on. It is tempting to write off
such observations as anomalies or trivial occurrences, but the fact is that
they did happen and there must be an explanation. One specific example
is touching a circuit board and observing a change in the state of the
system. Where did you touch? How did you touch it? Are your hands
sweaty or dry? When you touch a circuit board, your body acts like a
small capacitance and a large resistance. This can slightly slow down
signals or discharge high-impedance nodes such as an unconnected digital
input. If you pressed firmly on the board, you could be flexing the board
in such a way that changes the electrical properties of a cracked trace or a
bad solder joint.
There are some symptoms that are often times incorrectly interpreted as
causes. A burned-out trace or a damaged component is usually a symptom
and not a cause of the problem. In other words, a malfunction elsewhere in
the circuit is usually responsible for the failure of a component. Spontaneous
component failure is a relatively rare occurrence. Suppose you are
debugging a broken stereo. You smell something burning coming from the
stereo, and you see a large resistor that is blackened from overheating.
Chances are that if you just replace that resistor, the replacement will just
burn out again. The real cause might be a shorted transistor or a damaged
power supply circuit, but these do not manifest themselves as obviously as
the burned out resistor.
Another potent observation technique is comparison against a known good
system. If you are trying to debug a broken device, find a working one and
compare voltages and other operational characteristics between the two. If
you are trying to debug your own home-brew system, construct a simulation
of the circuit if possible, or find a circuit with a similar design. You can
use these known good systems to quickly isolate anomalous behavior.
Furthermore, you can induce failures in the known good sample in a
controlled fashion to check if you have really found the root cause of the
problem. This technique is particularly applicable to simulated systems.
Common Bugs
The most common source of hardware bugs in home-brew projects are
poor solder joints and improperly installed polarized components, such as
capacitors, diodes, ICs, and connectors. Also, connectors are particularly
notorious sources of failures because they are subjected to the most
physical abuse and it is typically difficult to determine if a connector is in
good condition through visual inspection alone. The following is a list of
common bugs, ranked loosely in descending order of popularity.