What is the difference between primary power and backup/retention power?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
Primary power is the main supply that gets switched off when a domain shuts down; backup (retention) power is a separate always-on supply that keeps specific logic — like a retention register's save-latch, or isolation/level-shifter control logic — alive even while the domain's primary supply is off, so state or valid outputs can be preserved.
Mentor Explanation
A beginner often assumes retention just means 'don't turn the power off,' but the whole point is that only the primary logic power is cut, while a small, separate backup rail (often labeled VDDG) stays live to hold onto the bits that matter. Think of it this way: the Liberty pg_type attribute literally distinguishes primary_power from backup_power as different pin roles on the same cell.
Example
A DRFFQX0 retention flip-flop cell has both a VDD pin tagged pg_type: primary_power and a VDDG pin tagged pg_type: backup_power — the main flop uses VDD, but its save-latch uses VDDG so state survives when VDD drops.
Why It Matters
This separation is what makes state retention possible at all — without a distinct backup supply, there would be nothing left powered to hold the saved state once the domain shuts down.
Command
pg_pin(VDD) { voltage_name : VDD ; pg_type : primary_power ; }
pg_pin(VDDG) { voltage_name : VDDG ; pg_type : backup_power ; }Common Beginner Mistake
A beginner mistake is assuming backup power always runs at the same voltage as primary power — backup power (VDDG) can be operated at a reduced voltage (e.g., 0.6V vs. a 1.0V primary) purely to save additional power during retention mode.
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