Intel Embedded Intel486, Hardware Reference Manual

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EMBEDDED Intel486™ PROCESSOR HARDWARE REFERENCE MANUAL
10-10
sponds to the linear ramp amplitude from 0% to 100%. Normally the rise times are specified be-
tween 10% to 90% or 20% to 80% amplitude points. The respective values are multiplied by 1.25
or 1.67 to obtain the linear-ramp duration from 0% to 100% amplitude.
For example in a PCB using G-10 and polymide (the two main dielectric systems available for
printed circuit boards) signals travel at approximately 5 to 6 inches per nanosecond (ns).
When T
r
/l x v
≥
8, the signal path is not a transmission line but it is a lumped element,
where:
T
r
= rise time 0% - 100%;
V = speed of propagation (5 to 6 inches/sec); and
L = length of interconnection (one-way only).
The calculation is given by:
T
r
/L x 6
≤
8, so
L
≥
(
T
r
x 6)/8
≥
(1.25 x 4 x 6)/8
≥
3.75 inches
This calculation is based on the fact that the maximum rise time of the signals for the Intel486
processor is 4 ns. For L
≥
3.75 inches, interconnections act as transmission lines.
Every conductor that carries an AC signal and acts as a transmission line has a distributed resis-
tance, an inductance and a capacitance which combine to produce the characteristic impedance
(Z). The value of Z depends upon physical attributes such as cross-sectional area, the distance
between the conductors and other ground or signal conductors, and the dielectric constant of the
material between them. Because the characteristic impedance is reactive, its effect increases with
frequency.
10.3.1.1 Transmission Line Types
Although many different types of transmission lines exist, those most commonly used on the
printed circuit boards are micro-strip lines, strip lines, printed circuit traces, side-by-side conduc-
tors and flat conductors.
10.3.1.2 Micro-Strip Lines
The micro-strip trace consists of a signal plane that is separated from a ground plane by a dielec-
tric as shown in
Figure 10-7 . G-10 fiberglass epoxy, which is common, has an e
r
= 5,
where:
e
r
is the dielectric constant of the insulation;
w is the width of signal line (inches);
t is the thickness of copper (.0015 inches for 1 oz. Cu/.003 inches for 2 oz. Cu);
h is the height of dielectric for controlled impedance (inches).
The characteristic impedance Z
0
, is a function of dielectric constant and the geometry of the
board. This is theoretically given by the following formula:
Z
0
= ln (5.98h/.8w + t) ohms 87 e
r
1.41
+
( ) ⁄ [ ]