Need Help to Verify Timing Constraints On Output Pins to External Device

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0ns external delay doesn't mean no IO requirement, just that the external device isn't chewing up any of the requirement. .

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it is not 0ns external delay. it is 0ns set_output_delay which according to sdc standards is defines as follows:

max value = tSU of external device + board delay

min value = negative of tH + external delay

so if tSU and tH are zero, what does it mean to you? If that isn't the case then why sdc gives such definitions and equations