Each transmit channel of a PCIe lane must be AC coupled to allow link detection.
For non-PCIe applications, the SmartFusion2/IGLOO2 device requires the receive inputs to be AC coupled to prevent common-mode mismatches between devices. Suitable values (for example, 0.1 µF) for AC-coupling capacitors must be used to maximize link signal quality and must conform to DS0128: IGLOO2 and SmartFusion2 Datasheet electrical specifications.