Will the LOCKED signal assert if CLKFB is not used on the DCM?
The LOCKED signal will assert when CLKFB is not connected to a feedback signal when the only DCM outputs used are CLKFX or CLKFX180.
If any other DCM outputs are used, CLKFB should be connected to the CLK0 (or CLK2X for Virtex/-E and Virtex-II) signal via BUFG/BUFGMUX for internal feedback, or via IBUFG for external feedback.
When CLKFB is not used, the LOCKED signal indicates that DCM is locked to a valid frequency. When CLKFB is used, the LOCKED signal indicates that DCM is locked to a valid frequency with a proper deskew to CLKIN.
If CLKFB is not used, the attribute CLK_FEEDBACK must be set to "NONE."
For more information on the CLK_FEEDBACK attribute, see the DCM primitive description in the Xilinx Software Manuals at Manuals -> Libraries Guide:
NOTE: Beginning in version 5.1i of the ISE design tools, the implementation tool will report an error if DCM is configured with DLL outputs(CLK0/CLK90/CLK180/CLK270/CLK2X/CLK2X180/CLKDV), but the CLKFB is not connected.