How do you constrain a port that receives paths from multiple clocks, and why is -add_delay needed?

Short Answer

Specify each clock's delay on the port with -add_delay (SDC). Without it, a new set_input_delay/set_output_delay on a port removes the port's existing delay data and replaces it, so a second command without the flag would silently wipe out the first clock's budget instead of adding to it.

Technical Explanation

When paths from more than one clock, or more than one edge of one clock, arrive at the same port, each launching condition needs its own budget on that port — and -add_delay (SDC) is the mechanism for accumulating them.

• Why the flag is mandatory, not stylistic: by default, a new set_input_delay or set_output_delay on a port removes whatever delay data already exists there and replaces it entirely.
• What happens without it: writing a second clock's delay on the same port with no -add_delay doesn't add a second budget — it silently discards the first one, leaving that port constrained relative to only the most recent clock.
• What happens with it: the new delay sits alongside the existing ones, so the port ends up constrained relative to every relevant clock at once.
• A worked pair: set_input_delay 4.5 -clock PHI1 {IN1} followed by set_input_delay 2.3 -clock PHI2 -add_delay {IN1} gives IN1 delays relative to both PHI1 and PHI2; the same second command without -add_delay would have erased the PHI1 delay entirely.
• The same rule applies within one clock: set_input_delay 4.5 -clock PHI1 -max -rise -add_delay {IN1} and set_input_delay 4.0 -clock PHI1 -max -fall -add_delay {IN1} accumulate the rise and fall budgets rather than one overwriting the other.
• When this actually matters: whenever a port genuinely receives data launched by different clocks, different edges, or different modes — each distinct launching condition needs its own preserved budget.

Follow-up Question & Model Response

How would you verify, on an existing SDC file, that a port meant to carry delays from two clocks actually has both, rather than one that got silently overwritten?

Candidate Model Response: Run report_port -input_delay [get_ports IN1] (PT), which lists every currently active input delay on that port along with the clock each one is relative to. If only one clock's delay shows up where two were intended, that's the signature of a missing -add_delay on the second command. Grepping the SDC source for every set_input_delay targeting that port and confirming every command after the first one carries -add_delay is the faster static check before even running the tool.