What is an isolation cell and why is it needed?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
An isolation cell is a special standard cell placed on a signal that leaves a power domain, whose job is to hold that signal at a known, valid logic value whenever the driving domain is powered down.
Mentor Explanation
Think of it this way: when a domain shuts off, every net inside it stops being driven — it floats, or settles at some intermediate voltage. If a receiver in an always-on domain is still reading that net, it may see garbage instead of a clean 0 or 1. Isolation is needed whenever the driving domain shuts down but the receiving domain stays on, because a valid signal must be logic-0 or logic-1, whereas an unpowered driver leaves the net floating or at an intermediate voltage. The isolation cell sits at that boundary and clamps its output to a safe, predetermined value the moment the source domain is shut off.
Example
Picture a shutdown block whose output feeds an always-on controller. Without isolation, when the block powers down, the controller's input pin could float to a mid-rail voltage and cause both the pullup and pull-down transistors in the next gate to conduct at once.
Why It Matters
I ask this early because it's the single most common low-power physical design concept interviewers probe first — if you can't explain why a floating signal is dangerous, everything else about power gating won't make sense.
Common Beginner Mistake
A frequent beginner mistake is assuming isolation is only about 'blocking' a signal; really it's about substituting a known, controlled value for whatever the shut-off domain would otherwise present.
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