PCB traces are lossy and long traces can degrade signal quality. Keep all traces as short as possible. Unless mentioned otherwise, all signal traces must be 50-Ω single-ended impedance. Differential traces must be 100-Ω differential. Take care to ensure impedance is controlled throughout. Impedance discontinuities causes reflections leading to emissions and signal integrity issues. Stubs should be avoided on all signal traces, especially differential signal pairs.

Figure 12-1 Differential Signal Traces

Within the differential pairs, trace lengths must be run parallel to each other and be matched in length. Matched lengths minimize delay differences, avoiding an increase in common mode noise and emissions. Length matching is also important for MAC interface connections. All MII and RMII transmit signal traces should be length matched to each other and all MII and RMII receive signal traces should be length matched to each other.

Ideally, there should be no crossover or vias on signal path traces. Vias present impedance discontinuities and should be minimized when possible. Route trace pairs on the same layer. Signals on different layers should not cross each other without at least one return path plane between them. Differential pairs should always have a constant coupling distance between them. For convenience and efficiency, TI recommends routing critical signals first (that is, MDI differential pairs, reference clock, and MAC IF traces).