What is the difference between a dangling connection and a no-load net?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
A dangling connection is a hard disconnect: a pin or port that simply isn't wired to anything on one side — think of an unplugged cable end. It usually shows up as an unconnected pin in report_design or a floating net in LVS. A no-load net is more subtle: the net has a real driver and is properly connected at the netlist level, but nothing downstream actually consumes the signal in a meaningful way — like a wire that's plugged in but runs to a dead outlet.
Technical Explanation
- A dangling connection is a hard disconnect.
- Both can be completely legitimate: a spare gate's output can be intentionally left as a no-load net for future ECO use, and a black-boxed macro's unused pin can be a deliberate dangling port.
- The danger is treating either as automatically wrong. A dangling connection found on a signal that should be driving logic is a real bug; a no-load net on an intentional spare cell is expected and fine.
- ) rather than reflexively "fixing" it.
- Practically:
check_design,report_design -nets, and LVS/connectivity reports are where these first surface, and the fix (if it is a bug) usually means tracing back to synthesis or RTL to see why the connection was dropped.
Visual Verification
A dangling net is an unconnected wire stub indicating a broken connection defect, whereas a no-load net may legitimately connect to spare gate pins reserved for post-silicon metal ECOs.
What To Check
- Warning sign: Unused test logic is mixed with an accidentally disconnected functional path.
- Inspect: Start with one named object in the report and trace it to the netlist, library, or SDC statement that created it.
- Correct: Correlate instance pins, hierarchy, and synthesis reports, then fix or waive each named object with an owner.
Command Checks & Actions
- check_netlist: reports structural connectivity problems
Run the commands in order. Each line answers a separate part of the check.
Healthy, Suspicious & Hard-stop Results
- Expected: A dangling pin or port is disconnected at an interface, while a no-load net has a driver but no meaningful sink; both require context.
- Stop before floorplanning when required logic or timing coverage is missing or unexplained.
Common Mistake
The Trap: Do not assume the check passed just because ICC2 continued. Fix or narrowly justify the named objects, then rerun the same command.
What The Interviewer Is Testing
Be ready to explain why this matters before floorplanning.
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