When using the XAUI Core by itself (not connected to a MAC), the XAUI Core inserts error codes into the Received XGMII data stream, RXD, when there are three or fewer IDLE characters (0x07) between frames. The error code (0xFE) precedes the frame's "Terminate" delimiter (0xFD).
The IEEE 802.3ae-2002 specification (Section 46.2.1) requires a minimum Inter-frame Gap of five octets on the receive side. This includes the preceding frame's Terminate control character and all Idles up to and immediately preceding the following frame's Start control character. Since three (or fewer) Idles and one Terminate character are less than the required five octets, this would not meet the specification; therefore, the XAUI Core is expected to signal an error in this manner if the received frame does not meet the specification.
This behavior should only be seen when the XAUI Core is not connected to a MAC. This is because the MAC is normally responsible for inserting the minimum Inter-frame Gap required on the transmitted XGMII data stream, and so the receiving XAUI would never see this situation; therefore, it is up to the user to provide appropriate simulation stimulus on the XGMII interface side of the XAUI Core that meets the IEEE specification.