Hacking the Xbox

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Hacking the Xbox: An Introduction to Reverse Engineering
more specifics you have up front, the easier you will find the design
process. You should have an idea of how big you want the final board to
be, how much it should cost, and of course what it should do. I always
find it helpful to draw sketches and, in the case of large projects, to write
design documents that help me organize and record my thoughts.
For the first few designs, one of the most difficult steps will be nailing
down an idea, because you won’t know what kinds of components are
available to implement your idea, and what sorts of real-world constraints
you will have to design around. The best way to get started is to find an
existing idea that is very similar to yours and to model your idea after it.
(Many chip manufacturers offer free application notes and design samples
that form a great starting point.) Another way to define your ideas is to learn
from existing products: If you want to build an alarm clock, take an existing
clock apart to see how it was built.
Schematic Capture
Once you have your idea, you need to create a schematic diagram. A schematic
is a symbolic representation of your idea, expressed as a collection of part
symbols and virtual wires.
Most schematic capture software comes with a library of parts to help
accelerate the schematic capture process. However, if you do not find the
part you need in the library, you will have to build your own schematic
symbol. All schematic symbols are linked to a PCB component footprint,
which is a pattern of copper on a PC board that mates to a component.
One common source of errors stems from not checking the link between
the symbol and the footprint: A 16-pin DIP footprint does not fit a 16-pin
in-line surface mount connector, and most design tools cannot tell the
difference between the two. Therefore, check to make sure that your
footprint assignments are all correct, and double and triple check the
schematic symbol. In particular, always check and recheck power pins, since
they can cause the most difficult and destructive kinds of errors. Consider
having a friend double check the symbols as well, to avoid repeating errors
or allowing errors to pass unknowingly. (This amount of redundancy may
sound silly for simple parts, but it becomes absolutely essential for hundred
and thousand-pin parts where your brain becomes mush halfway through
the checking process.)
Attention to detail from the very beginning is the most important skill for
capturing a schematic, and can save you from the headache of frustrating
bugs later on. Every pin on every component is there for a reason, and if
any pin is left unconnected you should understand why that is or is not
okay. To this end, read the product data sheets, including every single page
and footnote. Don’t ignore the fine print that requires a pull-up resistor to
set startup conditions, or a capacitor to filter noise or stabilize the system or
you’re likely to end up with more frustrating bugs.