How do you tell the tools which supply drives or receives a top-level port?

Short Answer

Use set_port_attributes (UPF) with -driver_supply on inputs and -receiver_supply on outputs to say which supply set drives or reads a top-level port; set_related_supply_net (UPF) does the same job with supply nets. The tools compare that supply with the domain on the inside to decide whether isolation or a level shifter is needed. Get it wrong and you get missing cells on real crossings, or extra cells on crossings that do not exist.

Technical Explanation

• Default: with nothing set, a top-level port is treated as powered by the primary supply of the top domain.
• Driver vs receiver: -driver_supply describes the logic that drives the port; -receiver_supply describes the logic that reads it.
• PT precedence: -ports attributes, then -elements attributes, then set_related_supply_net (UPF), then the top domain primary supply.
• ICC2 black box input: HighConn -receiver_supply, then HighConn related supply net, then the same two at LowConn, then the domain primary.
• ICC2 hard macro input: HighConn -receiver_supply, then HighConn related supply net, then the Liberty related_power_port of the pin.
• Last wins: repeating set_port_attributes (UPF) on the same object overrides the earlier setting.
• What breaks: an input really driven at 0.9 V but left on the 1.0 V default gets no level shifter, so its 1.0 V load sees a weak high.

# [UPF]  design.upf
create_supply_set SS_SNS -function {power VDD0p9} -function {ground VSS}
set_port_attributes -ports {sensor_in} -driver_supply SS_SNS
set_port_attributes -ports {irq_out} -receiver_supply SS_AON
# [ICC2]  icc2_shell
get_related_supply_set [get_ports sensor_in]
check_mv_design

What To Check

• Every top-level input has a driver supply that matches its real source.
• Outputs read by switchable or lower-voltage logic have a receiver supply.
• get_related_supply_set (ICC2) returns the intended supply for each boundary port.
• No port attribute is overridden by a later line on the same port.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): sensor_in resolves to SS_SNS at 0.9 V, and one low-to-high level shifter sits between the port and U_PC.
• Suspicious (illustrative): Several inputs resolve to the top primary supply by default; confirm each one really is driven at 1.0 V.
• Hard stop: sensor_in resolves to SS_AON at 1.0 V while the sensor drives 0.9 V: no level shifter, and the load sees a weak high.

What The Interviewer Is Testing

• Do you know what supply a port gets when you say nothing?
• Can you keep driver and receiver supply straight?

Follow-up Question & Model Response

"When would you use set_related_supply_net instead of set_port_attributes?"

Candidate Model Response: Both give a port a related supply, but set_related_supply_net names supply nets while set_port_attributes names a supply set. Older UPF files and PT scripts often use set_related_supply_net, and the PT guide says set_port_attributes can replace it. When both exist, the port attribute wins in the precedence order. Pick one style per design so a reader finds the supply in one place.