Hacking the Xbox

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Hacking the Xbox: An Introduction to Reverse Engineering
The proper sizing of a power trace depends upon the thickness of the
copper. Typical boards use “1-oz copper” that is 1.35 mils thick (one square
foot of 1.35 mil thick copper foil weights one ounce). An exterior 12 mil
wide trace in 1 oz copper is required to pass 1 ampere of current with a 10
degree Celsius temperature rise. Thicker traces are required for buried layers
for a similar current handling capacity.
When routing power traces between layers, remember that vias have
resistance as well. A single via is insufficient to connect critical power traces
between layers. Critical power traces should have multiple vias connecting
them between layers to keep parasitic resistances and inductances down.
Distributed power planes on multiple layers should also have vias generously
distributed throughout to ensure that a common potential is
preserved.
Note
In high performance or low noise applications, placing a
via between a decoupling capacitor and the power pin
may carry too high an electrical integrity price for the
routing convenience. Vias disrupt the propagation of highspeed
(hundreds of megahertz) electrical waves. Thus,
the optimal location of a decoupling capacitor in these
applications is between the component pin and the power
via.
Timing reference signals include clocks and strobes. Many memory devices
require asynchronous control strobes that have sensitive timing requirements.
These signals should be properly terminated and routed in a manner
consistent with the termination strategy, typically a “daisy chain” route. Daisy
chain routes have no branches, so there is only one path for the wavefront
of the signal to travel.
Electric signals travel at about 1/4 the speed of light on a circuit board, or
about three inches in a nanosecond. Thus, high speed traces must have
matched lengths, or signals can arrive significantly out of phase with respect
to the timing reference. Trace lengths are matched by extending shorter traces
to the length of the longest trace. Trace length extension is accomplished
using serpentine traces that meander and increase the effective length of a
trace without changing the placement of the trace’s endpoints.
Analog and mixed-signal routing is well beyond the scope of this appendix.
In an average hobbyist’s digital design, most of the analog circuitry will be
isolated to the power supplies. Any special layout requirements for a
particular power supply component is typically well-documented in the
component’s datasheet.
Keep in mind that electrical signals are lazy and promiscuous: signal
current will always follow the path of least resistance, and signals will
couple into adjacent traces. Furthermore, current must be conserved, so
every signal current path must have a return current path, whether it is
explicit or not. Keep these simple rules in mind as you layout any analog
sections on your circuit board.