Constrained clock appears as unconstrained input

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In the "good" constraints, you've created a set_output_delay targeted to sdram*. Thus, this output delay constraint is covering the data output timing of this output clock, which as I mentioned, should not be constrained because you want it analyzed as a clock path, not a data path. Thus, you don't get the unconstrained path.

In the "bad" constraints, you're not doing this, so DRAM_CLK appears unconstrained because it's still considered a data path.

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Indeed, it's the case. Thanks a lot !