Hacking the Xbox

Appendix C - Getting Into PCB Layout 229
decoupling capacitor can share the power via used by the component’s
power pins. The view in the lower left hand corner of Figure C-2
provides a clear illustration of this technique. (There are some special
cases where you may not want to do this, as noted in the next section.)
fanout regions
Know Your Special Traces
component pads
component body
Figure C-3: Fan-out regions around an SMD component’s footprint.
The good news about laying out digital circuit boards is that most traces
require little thought, unlike a typical analog board. The bad news is that if
you don’t do the rest of the traces correctly, your board will exhibit strange
and frustrating behavior that will be hard to debug. As a result, routing
these special traces is a little bit of a black art. This section gives just a few
guidelines for coping with these special traces, but I encourage interested
readers to find a text dedicated to board layout to really learn and appreciate
these techniques. Two texts that I recommend are Digital Systems Engineering
by William J. Dally and John W. Poulton (Cambridge University Press), and
High Speed Digital Design: A Handbook of Black Magic by Howard W.
Johnson and Martin Graham (Prentice-Hall PTR).
Typically, the kinds of traces that require special attention when routing a
circuit board are:
• Power traces
• Timing reference (clock) traces
• High speed traces
• Analog/mixed signal traces
As a general rule, power traces should be thicker than your average signal
trace, especially if you are using one of the higher-end fabrication processes
that offer narrow (~ 5 mil) trace widths. Power traces need to be thickened
to counter both resistive heating and parasitic inductance. Narrow power
traces, especially near the key power distribution points, will act like resistors
and heat up, dropping the supply voltage to a level that causes indeterminate
malfunctions in your circuits.