Порадуйтесь! О невозможности завести CLK2X на CLKFB в DCM-ках у третих спартанов! Только что нарвался!

Hardware Errata Details

This section provides a detailed description of each hardware issue known at the release time of this document.

DCM Using CLK2X Feedback May Lose Lock

This issue only affects applications that use the DCM CLK2X output signal as the CLKFB feedback input to the DCM.
This issue only affects the XC3S200 FPGAs shown in Table 4................

Table 4: Spartan-3 XC3S200 FPGAs Affected by the CLK2X Feedback Issue

Device Types: XC3S200, XC3S400,...................................
Mask Revision Codes: A only
Fabrication/Process Codes: FQ only
Packages: All
Speed Grades: All

Они IMHO уже не встречаются.