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?
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.