priority period

	period	setup/hold	uncertainty	applied to	add
create_clock	√	√		node	second assignment will be ignored
CREATE_GENERATED_CLOCK	√	√		node	ignore the new constraint and issue a warning
DERIVE_PLL_CLOCKS	√	√		node

If a create_clock or create_generated_clock apply a clock to a node that already has a clock on it from a previous call of these commands,

then the second clock will not be added.  and a warning would be issued.

use the -add option for the second constraint

 The command derive_pll_clocks runs when it is immediately met, executing create_generated_clock assignments for each of the PLL output as if the user had them directly in their .sdc.

create_generated_clock assigned earlier, derive_pll_clocks will not add a new assignment.

derive_pll_clocks assigned earlier, create_generated_clock is ignored with a warning, unless it has the -add option.