Intel 82562EX, Руководство по конструкции

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82540EP/82541(PI/GI/EI) & 82562EZ(EX) Dual Footprint Design Guide
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Figure 6. Trace Routing
•
Traces should be routed away from board edges by a distance greater than the trace height
above the ground plane. This allows the field around the trace to couple more easily to the
ground plane rather than to adjacent wires or boards.
•
Do not route traces and vias under crystals or oscillators. This will prevent coupling to or from
the clock. And as a general rule, place traces from clocks and drives at a minimum distance
from apertures by a distance that is greater than the largest aperture dimension.
•
The reference plane for the differential pairs should be continuous and low impedance. It is
recommended that the reference plane be either ground or 1.8 V (the voltage used by the
PHY). This provides an adequate return path for any high frequency noise currents.
•
Do not route differential pairs over splits in the associated reference plane.
•
Differential termination components should be placed as close as possible to the LAN silicon.
4.1.5 Signal Trace Geometry
The key factors in controlling trace EMI radiation are the trace length and the ratio of trace-width
to trace-height above the ground plane. To minimize trace inductance, high-speed signals and
signal layers that are close to a ground or power plane should be as short and wide as practical.
Ideally, this trace width to height above the ground plane ratio is between 1:1 and 3:1. To maintain
trace impedance, the width of the trace should be modified when changing from one board layer to
another if the two layers are not equidistant from the power or ground plane.
Each pair of signals should have a differential impedance of 100
Ω
±20%. If a particular tool
cannot design differential traces, it is permissible to specify 55 to 65
Ω
single-ended traces as long
as the spacing between the two traces is minimized. As an example, consider a differential trace
pair on Layer 1 that is eight mils (0.2 mm) wide and two mils (0.05 mm) thick, with a spacing of
eight mils (0.2 mm). If the fiberglass layer is eight mils (0.2 mm) thick with a dielectric constant,
E
R
, of 4.7, the calculated single-ended impedance would be approximately 61
Ω
and the calculated
differential impedance would be approximately 100
Ω
..
45°
45°