What is a "virtual rail," and how is it different from the primary supply?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
A virtual rail is the switchable (switched) power or ground net delivered to a shutdown domain through a power switch, as opposed to the always-on primary supply net that feeds the switch itself.
Mentor Explanation
The header-switch pin naming makes the distinction concrete: VDDG is the primary, always-on power feeding the switch, while VDD is the switched/virtual power that the domain actually sees, controlled by NSLEEPIN. A common worked example shows the same idea by name: the switchable supply downstream of the switch is called 'VDD1g,' distinct from the always-on VDD1 feeding the switch. There's also a related mechanism for purely logical, non-physical virtual nets: 'create_supply_port -direction internal' specifies an internal port used to connect virtual nets in the design — if a UPF supply net only connects to leaf ports with the internal direction, the tool recognizes that this net should not exist in the physical implementation, which is used for example to connect block supplies known to switch together.
Example
A block's internal cells are all tied to a net called 'VDD_SW' rather than directly to the chip-level 'VDD' — VDD_SW is the virtual Daughter switch turns ON first (weaker) Mother switch turns ON second (stronger) Domain rail (in-rush current staged, not a single spike) rail that goes dead when the block's power switch turns off, while VDD itself never stops being driven.
Why It Matters
I ask this to see if candidates can separate 'the supply that is always alive' from 'the supply the logic actually experiences,' since confusing the two leads to wrong assumptions about what stays powered during shutdown.
Command
create_supply_port port_name -direction internalCommon Beginner Mistake
Assuming the virtual/switched rail and the primary rail are the same net just because they're both called 'VDD' informally — inside the domain the switched rail is a genuinely separate net that can go to zero volts.
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