Intel Xeon Processor and Intel E7500/E7501Chipset Compatible Platform Design Guide

Intel
®
Xeon™ Processor and Intel
®
E7500/E7501 Chipset Compatible Platform Design Guide 207
High-Speed Design Concerns
12.3 Serpentine Routing
A serpentine net is a transmission line that is routed in such a manner that sections of the net double
back and couple to other segments of the same net.
Serpentining a transmission line is sometimes necessary to properly match lengths between nets. It
is important to properly control the serpentine to avoid signal integrity and timing problems. The
primary impact of a serpentined trace is an observed decrease in the flight time when compared to
a straight trace of equal length. This decrease in the flight time is a result of the crosstalk between
parallel sections of the serpentined net. As the signal travels down the transmission line, a
component of the signal follows the transmission line and behaves as though it were a straight line
with no serpentine. However, another portion of the energy propagates perpendicular to the parallel
routed portions of the serpentined net via the mutual capacitance and mutual inductance. This
creates an extra mode that arrives at the receiver significantly earlier than the other component of
the signal. If the coupling between parallel sections is high, significant timing skew can occur
when attempting to match trace lengths on a bus. Furthermore, if the coupling is very high,
significant signal integrity problems can result.
Serpentine routing requirements are defined using two parameters, as depicted in Figure 12-3.
Parameter ‘S’ is the distance between the two segments of the serpentined trace. Parameter ‘H’ is
the distance between the signal and the referenced plane. The ratio is specified as S/H.
Figure 12-2. Serpentine Routing
Figure 12-3. Serpentine Spacing - Spacing to Reference Plane Height Ratio
S
H