27 Auxiliary Signal AC Match Termination

Unterminated pins create resonance and propagate cross talk. These resonances can couple to the high-speed lanes. Care needs be taken to AC terminate these signals. All auxiliary or unused signals must be terminated though a nominal 42.9 Ω Resistor in series with a 1.0 pF ± 20% Capacitor. This provides AC short preventing propagation of the resonance.

Notes:

1. Each higher bifurcation requires previous settings. For example, X16 AIC, requires X8, X4, and X2 settings.
2. Logic to drive auxiliary signals can drive 1.0 pF and 42.9 Ω passive termination, refer to Figure 27-1. The combination of these R&C can produce close to 50 Ω termination at the 32 Gbps Nyquist frequency.
3. The trace impedance to drive the logic needs to be 50-Ω single ended.

Figure 27-1 Auxiliary Pin Logic