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AR# 29375

10.1 Timing Analyzer - "WARNING:Timing:3262 - Feedback on DCM or PLL top/DCM_ADV_INST forms an incomplete loop"

Description

Keywords: feedback, clkin, analysis

The following warning is reported in the timing report:

"WARNING:Timing:3262 - Feedback on DCM or PLL top/DCM_ADV_INST forms an incomplete loop. The Tdcmino calculation will be invalid. If the DCM or PLL uses external feedback, please apply the FEEDBACK constraint to indicate the external board delay. Consult the Constraints Guide for further information on how to apply the FEEDBACK constraint."

Solution

When external feedback mode is used, you need to specify the FEEDBACK constraint within the UCF or the HDL file. This can help the timing tool correctly count the compensated timing value in the DCM or PLL.
AR# 29375
Date Created 09/18/2007
Last Updated 03/24/2008
Status Active
Type General Article