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AR# 29374

10.1 Timing Analyzer - "WARNING:Timing:3158 - The DCM, %s, has the attribute CLK_FEEDBACK set to NONE."

Description

I have not set the FEEDBACK attribute of the DCM. The warning below is reported: 

 

"WARNING:Timing:3158 - The DCM, %s, has the attribute CLK_FEEDBACK set to NONE. No phase relationship exists between the input and output clocks of this DCM. Data paths between these clock domains must be constrained using FROM/TO constraints."

Solution

The feedback clock (CLKFB) input pin provides a reference or feedback signal to the DCM to delay-compensate the clock outputs, and align them with the clock input. When the CLKFB pin is connected, all clock outputs will be deskewed to CLKIN. When the CLKFB pin is not connected, DCM clock outputs are not deskewed to CLKIN. However, the relative phase relationship between all output clocks is preserved. So if there is any data path which goes across the domains of the CLKIN and any of the DCM output clocks, a FROM TO constraint is needed to be added to cover that path.

AR# 29374
Date Created 03/24/2008
Last Updated 05/22/2014
Status Archive
Type General Article