- Keep the trace length difference between TX+ and TX- (or RX+ and RX-) in 700 mils (17 mm).
- Keep RX+/- signal on the top layer, the RX+/- signal should avoid any vias, if possible. Avoid right angle signal trace.
- The crystal/oscillator clock and the switching noise from digital signals should be far away from TX+/-, RX+/- pairs.
- Keep TX, RX differential signals running symmetric, equal length, and closely. The trace spacing between TX+ and TX- or between RX+ and RX- pair should be in 8 ~ 10 mils. The better spacing between TX+/- and RX+/- pairs should be larger than 200 mils.
- The trace length from LAN87xx to the transformer should not be longer than 5 inches (127 mm), keep the trace as straight as possible, and keep it parallel for differential pairs.
- The termination resistors 49.9O and capacitors of TX± and RX± pairs should be placed near the transformer side and should be shorter than 400 mils (10 mm).
- When physically placing the LAN8710 in an RMII application, the designer should be aware of the relative trace lengths determined by the relationship of the Phy and the MAC in the application. By placing the MAC too far away from the LAN8710, this may result in operational problems associated with excessive trace lengths.
- The designer has some latitude in the placement of the LAN8710 with respect to the magnetics. This should allow for some adjustment on the Phy-to-MAC RMII interface, should this be necessary. By moving the Phy farther away from the magnetics, the RMII interface can be shortened.
- SMSC recommends the final RMII interface trace lengths to remain under 6” (152.4 mm) long. When compared to the MII interface, due to the higher operational frequencies involved, the trace lengths in the RMII interface are much more critical.
- It is strongly recommended to place the Ethernet PHY on the Carrier Board close to the pins of the COM's Ethernet interface to achieve a trace length of less than 6 inch (152.4 mm).
The clock trace's length on the KARO TX28 baseboard should be about 35 mm.
