You must use the
-add_delay option if:
- A max output delay constraint already exists, and
- You want to specify a second max output delay constraint on the same port.
The same is true for a min output delay constraint. This option is commonly used to constrain an output port relative to more than one clock edge, as, for example, rising and falling edges in DDR interfaces, or when the output port is connected to several devices that use different clocks.