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AR# 13642

Virtex-II/Pro, Virtex-4, Virtex-5 DCM - Will the LOCKED signal assert if CLKFB is not used on the DCM?

Description

Will the LOCKED signal assert if CLKFB is not used on the DCM?

Solution

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:

http://www.xilinx.com/support/software_manuals.htm

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.

AR# 13642
Date Created 08/29/2007
Last Updated 12/15/2012
Status Active
Type General Article