Clamp diodes are not automatically enabled in Virtex and Spartan-II devices that are using the TTL I/O standard. For CardBus- compliance, clamp diodes are required. How do I enable clamping diodes?
This device selection recommendation is based on electrical compliance. CardBus is 3.3V, and requires clamp diodes and a particular output slew rate (this is achieved by picking a certain TTL Select I/O mode). In Virtex and Spartan-II, that TTL mode does not have the clamp diodes enabled. To use CardBus with Virtex and Spartan-II devices, contact your local Xilinx FAE for assistance on enabling the clamp diodes.
In Virtex-E, and Spartan-IIE, those diodes are turned on with the TTL Select I/O mode for device protection reasons. A "side effect" of this is full CardBus electrical compliance.