Intel Embedded Intel486, Hardware Reference Manual

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10-3
PHYSICAL DESIGN AND SYSTEM DEBUGGING
electromagnetic interference (EMI). It is very important for a designer to have a controlled im-
pedance design where high speed signals are involved. The formula for impedance is as follows:
Impedance = (L/C)
1/2
The total characteristic impedance for the power supply can be reduced by adding more lines. The
effect of adding more lines to reduce impedance is illustrated in
Figure 10-1 which shows that
two lines in parallel has half the impedance of a single line. To reduce impedance even further,
more lines should be added. To lower the impedance, the number of lines or planes should be in-
creased.
Figure 10-1. Reduction in Impedance
For multi-layer boards, power and ground planes must be used in the Intel486 processor family
designs. The ground plane allows best performance at high speeds. It serves two purposes. First
it provides a constant characteristic impedance to signal interconnections. Second, it provides a
low impedance path for ground currents on the V supply. The advantage of a power plane is to
reduce EMI. For example, when adjacent signal lines are switching, EMI may occur. The power
plane is used to separate adjacent layers of signal lines, which reduces EMI.
All power and ground pins must be connected to their respective planes. Ideally, the Intel486 pro-
cessor should be placed at the center of the board to take full advantage of these planes. Although
Intel486 processors generally demand less power than conventional devices, the possibility of
power surges is increased due to the processor’s higher operating frequency and its wide address
and data buses. Peak-to-peak noise relative to V should be maintained at no more than 200 mV.
Although power and ground planes are preferable to power and ground traces, double-layer
boards present a need for routing of the power and ground traces.
A5284-01
C
0
C
0
L
0
Z
0 =
L
0
C
0
L
0
C
0
2
= 1/2
C
0
L
0
Z
0 =
L
0
C
0
L
0
C
0