What does the create_power_domain command do in UPF?

Short Answer

create_power_domain (UPF) names a power domain and lists the instances that belong to it, so every cell in those instances shares one primary supply and one set of strategies. The domain is created in the current scope, and every instance in the design must end up in exactly one domain.

Technical Explanation

• Syntax order: domain name first, then options: create_power_domain PD_CPU -elements {U_CPU} (UPF). No trailing semicolon.
• -elements lists instances rooted in the current scope. Their descendants join too, unless they already belong to another domain.
• -elements {.} puts the current scope itself in the domain. ICC2 examples also use -include_scope, which is not in the 1801-2015 syntax.
• -exclude_elements removes listed instances from the element list; each excluded instance still needs a home in some other domain.
• -supply {primary SS_CPU} is the IEEE 1801-2015 way to give the domain its primary supply set.
• Legacy form (still accepted by ICC2/PT): set_domain_supply_net (UPF) with -primary_power_net and -primary_ground_net.
• Every instance must belong to a domain before commit_upf (ICC2); one left out has no defined supply and ICC2 reports an error.

# [UPF]  design.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 design.upf
commit_upf
report_power_domains

What To Check

• Every top-level instance, including the power controller U_PC, lands in exactly one domain.
• Each domain has a primary supply set.
• Element names are rooted in the scope where the command runs.
• No domain is left with an empty extent by mistake.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): report_power_domains shows four domains, each with its instances and a primary supply.
• Suspicious (illustrative): A domain shows no primary supply set yet, so its cells have no defined rail until one is added.
• Hard stop: An instance appears in two domains, or load_upf errors on an element name that does not exist in that scope.

What The Interviewer Is Testing

• Can you explain scope versus extent without mixing them up?
• Do you know the modern -supply form and the legacy supply-net form?

Follow-up Question & Model Response

"What happens if you create PD_CPU while the scope is set to U_SOC instead of the top?"

Candidate Model Response: The domain is created in scope U_SOC, and its element names are resolved relative to that scope. So -elements {U_CPU} now means U_SOC/U_CPU. If that instance does not exist, the command fails; if it does, the domain lives in the wrong place in the hierarchy. Always check the current scope before creating domains.