What is a voltage crossing between power domains?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
A voltage crossing is any signal path where the driving side and the receiving side belong to power domains operating at different voltages, requiring a level shifter (and possibly isolation, if one side can also be shut off) to be inserted along that path.
Mentor Explanation
The rule is plain: a level shifter is required for any signal crossing voltage domains. It's worth noting, though, that not every domain-to-domain crossing needs a level shifter — when a net's source and target domains happen to share the same voltage (even though they are physically distinct domains), no level shifter is needed, unless buffers are still required along the path to satisfy electrical design rules, in which case a shifter effectively becomes necessary anyway. This same logic is implemented automatically: if a voltage violation exists at a domain boundary, the tool inserts level shifters even without a user-defined strategy, and the default location and direction ('-location', '-rule') options let a designer refine exactly where and for which direction crossings occur.
Example
A 0.9V APB peripheral block feeding a signal into the 1.0V top-level domain represents a voltage crossing of about 0.1V — small, but still requiring a level-shifter strategy at that boundary per the multivoltage topology described above.
Why It Matters
I want candidates to connect 'voltage crossing' as a structural concept (a path between domains of differing voltage) to the concrete tool behavior it triggers (automatic or strategy-driven level-shifter insertion).
Common Beginner Mistake
Assuming every boundary between two named power domains is automatically a voltage crossing — two domains can be electrically distinct constructs yet share the exact same voltage, in which case no shifter is functionally required.
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