What is a voltage crossing between power domains?
From PDVerse Low-Power Physical Design Mentor Guide, part of the pdVerse Mentor Guide
Short Answer
A voltage crossing is a net whose driver and receiver sit in power domains at different voltages. Every domain crossing raises two questions: does the signal need a level shifter, and does it need isolation because the driver can be off while the receiver is on?
Technical Explanation
- Level shifter question: do the driver and receiver voltages differ in any legal power state? Low to high always needs one.
- High to low depends on the library and the voltage gap.
set_level_shifter(UPF)-thresholdlimits the strategy to crossings whose voltage difference exceeds it (default 0). - Isolation question: can the driver domain be off while the receiver is on? Then the net needs an isolation cell, whatever the voltages.
- If both answers are yes, one enable level shifter can do both jobs.
- Two domains at the same voltage with no shutdown difference are a domain crossing but not a voltage crossing, so neither cell is needed.
- By default ICC2 inserts level shifters at a violating boundary even without a strategy;
mv.upf.insert_ls_on_user_cstr_only(ICC2) restricts it to your strategies. - A missed low-to-high crossing leaves the receiving PMOS partly on: leakage, slow edges, or a stuck value.
Common Mistake
The Trap: Assuming every boundary between two named domains needs a level shifter.
- Same-voltage domains then get needless shifters that add delay, area and routing, and a real shutdown crossing may still miss its isolation.
Follow-up Question & Model Response
"How does the tool know the voltages on each side?"
Candidate Model Response: It reads them from the power state definitions and the voltages set on the supplies. The tool uses the power states to find the driver and receiver voltage in every legal state, then applies the level-shifter rules and thresholds. A crossing that is equal in one state but different in another still needs a shifter. That is why a missing or wrong power state can hide a crossing.
Practical Example
Design Scenario: (illustrative) In a sensor hub, the switchable filter domain PD_SNS at 0.75 V drives an interrupt into PD_MYCHIP at 1.0 V. The voltages differ and it is low to high, so it needs a level shifter. PD_SNS can also be switched off while PD_MYCHIP stays on, so the net also needs isolation. One enable level shifter with its enable driven by the always-on power controller covers both. A second net, from PD_CPU at 0.9 V into PD_SNS at 0.75 V, runs high to low: it gets a shifter only if the library or the -threshold asks for one, and isolation only if PD_CPU can be off while PD_SNS is on.
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