How does a "virtual rail" get created in practice -walk through the switched supply net in the Mychip example.

Short Answer

No single UPF command creates a virtual rail. You declare a separate supply net, make it the output of a power switch fed by the always-on rail, and bind it as the primary of the shutdown domain. In MYCHIP that net is VDD1p0_SW: the switch PD_COP_SW drives it from VDD1p0, and PD_COP runs on it through the supply set SS_COP.

Technical Explanation

• Step 1: create_supply_net VDD1p0_SW (UPF) declares a net that is separate from VDD1p0.
• Step 2: create_power_switch (UPF) with -output_supply_port {VDD VDD1p0_SW} makes that net switched.
• Step 3: a supply set SS_COP bundles VDD1p0_SW with GND and becomes the PD_COP primary.
• Step 4: add_power_state (UPF) on SS_COP defines COP_ON at 1.0 V and COP_OFF, so tools know the rail can be off.
• Legacy form (still accepted by ICC2/PT): set_domain_supply_net (UPF) for the primary and add_port_state (UPF) on the switch output.
• VDD1p0 stays on for isolation, retention and always-on buffers inside PD_COP.

# [UPF]  mychip.upf
create_supply_net VDD1p0
create_supply_net VDD1p0_SW
create_supply_net GND
create_power_switch PD_COP_SW -domain PD_COP -input_supply_port {VDDG VDD1p0} -output_supply_port {VDD VDD1p0_SW} -control_port {EN U_PC/PSE} -on_state {SW_on VDDG {EN}} -off_state {SW_off {!EN}}
create_supply_set SS_COP -function {power VDD1p0_SW} -function {ground GND}
create_power_domain PD_COP -elements {U_COP} -supply {primary SS_COP}
add_power_state SS_COP -state {COP_ON -supply_expr {power == {FULL_ON 1.0} && ground == {FULL_ON 0.0}}}
add_power_state SS_COP -state {COP_OFF -supply_expr {power == {OFF}}}
# [ICC2]  icc2_shell
report_supply_sets
report_pst -derived

What To Check

• VDD1p0_SW is a distinct net, not an alias of VDD1p0.
• The switch output and the PD_COP primary name the same net.
• SS_COP has both an on and an off state.
• Always-on supplies for isolation and retention are still available in PD_COP.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): report_supply_sets shows SS_COP on VDD1p0_SW, and report_pst -derived shows COP_ON and COP_OFF.
• Suspicious (illustrative): VDD1p0_SW has only an on state, so the tools treat PD_COP as always on.
• Hard stop: PD_COP primary resolves to VDD1p0: there is no virtual rail at all.

What The Interviewer Is Testing

• Can you build a switched rail from primitives rather than name one command?
• Do you know the off state must be declared for the tools to see it?

Follow-up Question & Model Response

"Why does the virtual rail need its own power states when the switch already has on and off expressions?"

Candidate Model Response: The switch expressions say how the rail follows the control signal. The power states say which rail values are legal and at what voltage, which is what isolation, level-shifter and PST checks read. Without COP_OFF, the tools see only a 1.0 V rail. With it, they check every crossing out of PD_COP for the off case.