Intel 386 manual Propagation Delay

PHYSICAL DESIGN AND DEBUGGING

11.4.3.3 ELECTROSTATIC INTERFERENCE

We have discussed two types of coupling, namely inductive and radiative coupling which are responsible for creating electromagnetic interference. A third, known as capacitive coupling, occurs when two equipotential parallel traces are separated by a dielectric and act as a, capacitor. According to the standard capacitor equation, the electric field between the two capacitor surfaces varies with the permitivity of the dielectric and with the area of the parallel conductors.

Electrostatic interference (ESI) is caused by this type of coupling. The charge built on one plate of the capacitor induces opposite charge on the other. To minimize the ESI, the following steps should be taken.

•Separate the signal lines so that' the effect of capacitive coupling is negated.

•Run a ground line in between the two lines to cancel the electrostatic fields.

For high-frequency designs, a rule of thumb is to include ground planes under each signal layer. ,Ground planes limit the cross-talk caused by a capacitive coupling between small sections of adjacent layers that are at equipotentials. Additionally, when the width and the thickness of signal lines and their distance from the ground is constant, the effect of capacitive coupling upon impedance remains uniform within ( + -)5 percent across the board. Using fixed impedance does not reduce capacitive coupling, but it does simplify the modeling of propagation delays and coupling effects. In addition, capacitive coupling can cause interference between layers so the wires sho,uld be routed orthogo- nally on neighboring board layers;

11.4.4 Propagation Delay

The propagation delay of a circuit is a function of the loads on the line, the impedance,

,and the length of the line segments. The term propagation delay means the propagation delay in the entire circuit, including the delay in the transmission line (which is a func- ' tion of its dielectric constant).

Also, the printed-circuit interconnections add to the propagation delay of the signal on a wire. These interconnections not only decrease the operating speed of the circuits, but.

also cause reflections which produce undershoot and'overshoot. .

When the propagation delays in a circuit are significant, the design must compensate for signal skew. Signal skew occurs when the wire lengths (and thus the propagation delays) between each source and each corresponding load are unequal.

Another negative aspect of propagation delay is that it may generate a race condition. This condition occurs when two signals must reach the same destination within one clock pulse of one another. To avoid race conditions, it is necessary to have the signals travel through same-length traces. If one route is shorter, then the signals will arrive at differ- ent times, causing race conditions.

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