How do you create a power domain in UPF, and what does the create_power_domain command actually do?

Short Answer

create_power_domain (UPF) names a group of instances that share one primary supply and one set of isolation, level-shifter and retention strategies. It is bookkeeping for the tools, not a netlist cell: nothing gets inserted until strategies act on it. In MYCHIP you create the always-on top domain, then one child domain per block, and confirm the result with report_power_domains (ICC2).

Technical Explanation

• Top domain: -elements {.} puts the current scope, and everything no other domain claims, into PD_MYCHIP. ICC2 also accepts the non-1801 -include_scope (ICC2).
• Child domains: -elements {U_CPU} claims an instance and its descendants for PD_CPU. Names resolve relative to the current scope.
• Primary supply: the IEEE 1801-2015 form is -supply {primary SS_CPU}, which binds the domain to a supply set created earlier in the file.
• Legacy form (still accepted by ICC2/PT): set_domain_supply_net (UPF) with -primary_power_net and -primary_ground_net.
• -atomic must be given when the domain is first defined; ICC2 will not turn an existing domain atomic later.
• Before commit_upf (ICC2), every instance in the design must belong to some domain, or the intent is incomplete.

# [UPF]  mychip.upf
create_power_domain PD_MYCHIP -elements {.} -supply {primary SS_AON}
create_power_domain PD_CPU -elements {U_CPU} -supply {primary SS_CPU}
create_power_domain PD_DSP -elements {U_DSP} -supply {primary SS_DSP}
create_power_domain PD_COP -elements {U_COP} -supply {primary SS_COP}
# [ICC2]  icc2_shell
load_upf mychip.upf
commit_upf
report_power_domains

What To Check

• U_CPU, U_DSP and U_COP each sit in exactly one child domain.
• Top-level glue such as the power controller U_PC lands in PD_MYCHIP, so it stays on.
• Each domain shows a primary supply set bound to real nets.
• Element names were resolved in the scope you intended.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): report_power_domains lists four domains: PD_MYCHIP with the top scope, and PD_CPU, PD_DSP and PD_COP with one instance each and a bound primary.
• Suspicious (illustrative): A child domain holds the right instance, but its primary is still an unbound handle.
• Hard stop: load_upf errors on an element that does not exist in the current scope, or an instance belongs to no domain.

What The Interviewer Is Testing

• Do you know a domain is a logical grouping, not a cell or a floorplan region?
• Can you write the 1801-2015 supply binding and still read the legacy form?
• Do you verify with a report instead of trusting the UPF file?

Follow-up Question & Model Response

"What does report_power_domains tell you that reading the UPF does not?"

Candidate Model Response: It shows what the tool actually resolved, not what you meant to write. You see the real element list after scope resolution and the primary supply each domain ended up with. A typo in an instance path or a wrong scope shows up as an empty or unexpected element list. Reading the UPF only shows intent, and intent is where the bug lives.