Intel Embedded Intel486, Hardware Reference Manual

Page: 311 / 334

10-27
PHYSICAL DESIGN AND SYSTEM DEBUGGING
The impedances Z
2
through Z
4
depend upon thickness of copper printed circuit board foil, the
circuit switching speeds and the effective lengths of the traces. The current flowing through these
common impedance paths radiates more noise as its value increases. The amount of voltage gen-
erated by these switching currents and multiplied by the impedance is difficult to predict.
An effective way to reduce EMI is to decouple the power supply by adding bypass capacitors be-
tween V
CC
and Ground. This technique is similar to the general technique discussed earlier. (The
goal of the previous technique was to maintain correct logic levels.)
The design of effective coupling and bypass schemes centers on maximizing the charge stored in
the circuit bypass loops while minimizing the inductances in these loops. Some other precautions
that can minimize the EMI are as follows:
•
Running a ground line between two adjacent lines. The lines should be grounded at both
ends.
•
The address and data busses can be separated by a ground line. This technique may be
expensive due to large number of address and data lines.
•
Removing closed loop signal paths, which create inductive noise as shown in Figure 10-23 .