What does "power-up" and "power-down" mean for a domain?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
Power-down means turning off a domain's primary supply (via a power switch controlled by a sleep signal), while power-up means re-enabling that supply and bringing the domain back to normal operation — often through a controlled, staged sequence rather than an instantaneous switch, to manage in-rush ('rush') current and, if retention is used, to restore saved state.
Mentor Explanation
A beginner often assumes power-up/power-down is just flipping a switch, but the sequencing problem deserves real attention: turning on many parallel power-switch cells at once can cause a current spike large enough to sag the supply voltage for other still-on domains. Think of it this way — power-up is closer to gradually opening a valve than flipping a light switch.
Example
Consider 5,000 power switches in a single daisy chain, each with a 30 ps propagation delay from SLEEPIN to SLEEPOUT — that gives roughly 150 ns of total turn-on propagation delay if staged one after another, versus turning them all on in parallel, which is almost never employed due to rush-current risk.
Why It Matters
Getting power-up/power-down sequencing right avoids brownouts on neighboring always-on logic and, when retention is involved, ensures SAVE happens cleanly before power-down and RESTORE happens cleanly after power-up — SAVE and RESTORE being asserted simultaneously is an illegal condition.
Equation
5,000 switches x 30 ps propagation delay per switch = ~150 ns total
turn-on delay (single daisy chain)Common Beginner Mistake
A beginner mistake is assuming power-up is 'free' once the switch is flipped — real designs must budget wake-up time (which can span many clock cycles, especially if memory contents must be restored) before functional operation is safe.
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