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AR# 31269

11.1 Constraints System - Cannot prevent a PERIOD constraint from propagating through a DCM, PLL, or DLL

Description

If a NET/TIMESPEC PERIOD constraint is set on the input clock of a DCM and a TIMESPEC PERIOD is set on an output of the DCM, the NET PERIOD and initial TIMESPEC PERIOD overrides the second TIMESPEC PERIOD.

How do I prevent the constraints on the output from being overridden?

Solution

The work-around is to only place a PERIOD constraint on the input or only the outputs of the PLL/DCM/MMCM, but not both.

Using PRIORITY Keyword

To work around this problem, add the PRIORITY keyword to the TIMESPEC PERIOD which propogates through DCM/PLL/PMCD components, so the derived clocks can be overridden by the user's TIMESPEC PERIOD with the PRIORITY keywords with a higher value.

AR# 31269
Date Created 11/07/2008
Last Updated 12/15/2012
Status Active
Type General Article