Data Sheet

Design Considerations—Ethernet Controller I210
821
12.5.6.4 Differential Pair Trace Routing for 10/100/1000 Designs
Trace routing considerations are important to minimize the effects of crosstalk and propagation delays
on sections of the board where high-speed signals exist. Signal traces should be kept as short as
possible to decrease interference from other signals, including those propagated through power and
ground planes. Observe the following suggestions to help optimize board performance:
• Maintain constant symmetry and spacing between the traces within a differential pair.
• Minimize the difference in signal trace lengths of a differential pair.
• Keep the total length of each differential pair under four inches. Although possible, designs with
differential traces longer than five inches are much more likely to have degraded receive Bit Error
Rate (BER) performance, IEEE PHY conformance failures, and/or excessive Electromagnetic
Interference (EMI) radiation.
— Consider using a second I210 instead of a LAN switch and long MDI traces. It is difficult to
achieve excellent performance with long traces and analog LAN switches. An optimization effort
is required to tune the system and magnetics modules.
• Keep differential pairs more than seven times the dielectric thickness away from each other and
other traces, including Flash traces and parallel digital traces or other disturbing traces.
• Keep in-pair trace separation to 7 mils to maintain highly-coupled signaling.
• For high-speed signals, the number of corners and vias should be kept to a minimum. If a 90° bend
is required, it is recommended to use two 45° bends instead. Refer to Figure 12-15.
Note: In manufacturing, vias are required for testing and troubleshooting purposes. The via size
should be a 17-mil (±2 mils for manufacturing variance) finished hole size (FHS).
• Traces should be routed away from board edges by a distance greater than the trace height above
the reference 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 prevents 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.
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Figure 12-15.Trace Routing
• The reference plane for the differential pairs should be continuous and low impedance. It is
recommended that the reference plane be either ground or 0.9 Vdc (the voltage used by the PHY).
This provides an adequate return path for and high frequency noise currents.
• Do not route differential pairs over splits in the associated reference plane as it might cause
discontinuity in impedances.
45°
45°