What is a supply set in UPF, and how is it different from a plain supply net?

Short Answer

A supply net is one wire carrying one function, such as VDD1p0 or GND. A supply set is a named bundle of functions (power, ground, and optionally nwell and pwell) that you bind to real nets, so a strategy can say "power this from SS_AON" without listing nets. IEEE 1801-2015 builds domain primaries, isolation supplies and retention supplies on supply sets, created with create_supply_set (UPF).

Technical Explanation

• Supply net: a single conductor from create_supply_net (UPF). It has one voltage and one job.
• Supply set: create_supply_set SS_AON -function {power VDD1p0} -function {ground GND} (UPF) groups functions under one name.
• Late binding: create a set with no functions, then fill it with -update -function once nets exist. ICC2 requires both options together.
• Handles: each domain has a primary handle, referenced as PD_COP.primary. ICC2 also accepts default_isolation and default_retention handles.
• Why it matters: -isolation_supply and -retention_supply take sets, so one name carries matched power and ground.
• ICC2 updates a set only with domain-independent nets, and the set voltage must match the net it is bound to.
• Use associate_supply_set (UPF) to make two sets electrically equivalent instead of re-listing nets at each level.

# [UPF]  mychip.upf
create_supply_net VDD1p0
create_supply_net VDD1p0_SW
create_supply_net GND
create_supply_set SS_AON -function {power VDD1p0} -function {ground GND}
create_supply_set SS_COP
create_supply_set SS_COP -update -function {power VDD1p0_SW} -function {ground GND}
create_power_domain PD_COP -elements {U_COP} -supply {primary SS_COP}
associate_supply_set SS_AON -handle PD_COP.default_isolation
# [ICC2]  icc2_shell
report_supply_sets

What To Check

• Each set has both power and ground bound.
• SS_COP power resolves to the switched net VDD1p0_SW, not VDD1p0.
• Isolation and retention use a set that stays on, here SS_AON.
• No option that expects a set is given a bare net name.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): report_supply_sets shows SS_AON on VDD1p0 and GND, and SS_COP on VDD1p0_SW and GND.
• Suspicious (illustrative): A set shows power bound but ground empty, or a net you did not expect.
• Hard stop: SS_COP power resolves to VDD1p0, so PD_COP can never switch off, or the isolation supply is SS_COP.

What The Interviewer Is Testing

• Can you explain function binding and why it can happen late?
• Do you know which strategy options take a set rather than a net?

Follow-up Question & Model Response

"Why does IEEE 1801-2015 prefer supply sets over set_domain_supply_net?"

Candidate Model Response: A set keeps power, ground and bias together, so a strategy cannot pair the wrong ground with a power net. It also supports refinement: you write intent with abstract sets early and bind nets later with -update. Power states are written on sets with add_power_state, which is how simulators decide when cells are corrupted. The standard lists set_domain_supply_net as legacy and recommends the -supply form on create_power_domain.