What is a "valid" vs an "invalid" signal, and why does this matter when a domain shuts down?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
A valid signal is one driven to a proper logic-0 or logic-1 level; an invalid signal is one that is floating or sitting at some intermediate voltage — which is exactly what happens to the output of a logic block when its power domain is shut down and its driver can no longer actively drive a clean logic level.
Mentor Explanation
A beginner often assumes an 'off' block's outputs simply read as logic-0 — but in reality a shut-down driver produces an invalid, floating/intermediate signal, not a clean 0. Think of it this way: without intervention, an always-on receiver reading that signal could interpret garbage as a real logic value, potentially causing incorrect behavior or even extra power draw in the receiving gate.
Example
This is precisely why isolation cells exist: an AND-type (clampto-0) isolation cell forces its output to a clean logic-0, and an OR-type (clamp-to-1) isolation cell forces its output to a clean logic-1, whenever the driving domain has been shut down — converting an invalid floating signal back into a valid, known logic level for the always-on receiver.
Why It Matters
This matters directly at the shutdown boundary: any signal crossing from a domain that can power down into a domain that stays on needs isolation, precisely to guarantee the receiver never sees an invalid (floating) value.
Common Beginner Mistake
A beginner mistake is assuming isolation is only about voltage-level mismatches (that's what level shifters are for) — isolation is specifically about validity: making sure a signal from a powered-down block reads as a defined 0 or 1 rather than floating.
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