intent/property-ac-coupled

Remedy ¶

Put a series capacitor in the net, sized for the lowest frequency the link has to pass.

What it checks ¶

When the design intent declares a net AC-coupled, a series capacitor must actually carry the net. If no series capacitor is on it, the net is DC-connected and the declaration is unmet.

For hardware engineers ¶

AC coupling puts a capacitor in the signal path so the alternating signal passes while any steady DC level does not. It is standard on high-speed serial links (PCIe, SGMII, USB SuperSpeed) because the two ends often sit at different common-mode voltages by design. The transmitter idles at one level, the receiver expects another, and the capacitor lets each keep its own without fighting.

Leave it out and the two ends are tied together at DC. The receiver’s input sits wherever the transmitter’s output holds it rather than at its own bias point. That shows up as a link that either does not train or trains and then errors under load. It is also a part-count-of-one omission, easy to lose in a respin.

How the check tells a coupling cap from a decoupling cap ¶

Both are “a capacitor on the net”, and they differ in what the far side connects to.

• A decoupling cap’s far side is ground (or a rail). It shunts noise, and the signal does not pass through it.
• A coupling cap’s far side is another signal net, and the signal passes through it.

So the rule looks for a capacitor on the declared net whose other net is neither ground nor a power rail.

What a pass means here ¶

This property is decidable from the netlist, since a series capacitor is either present or it is not. So a pass means the declaration is met, not merely “no contradiction found”.

Its sibling property-reset-polarity is different, so read that card before assuming the two behave alike.

Declaring it ¶

intent:
  nets:
    PCIE_TX0_P: {ac_coupled: true}

No level to state, since the fact is the whole assertion. The rule does not check a net you do not declare. It iterates the declaration, never the design, so it has no opinion where your intent is silent.

Fixing a finding ¶

Either the coupling capacitor is missing from the schematic, or the declaration names the wrong net. Check which before adding a part, since on a differential pair it is easy to declare _P and wire _N.