Why are multiply driven nets a hard structural concern?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Two active drivers fighting over one ordinary net is a real structural problem, not a style nitpick — they can demand different logic values on the same wire at the same time. That's exactly why a "multiple driver" message from the tool deserves immediate review rather than being waived through.
Technical Explanation
- The one legitimate exception is when the design genuinely intends a bus or tri-state structure, where multiple drivers are expected to be mutually exclusive by design (enable logic guarantees only one drives at a time) — but that intent needs to be explicitly confirmed, not assumed.
Visual Verification
Driving a standard net with multiple active push-pull outputs causes logic contention, short-circuit crowbar current, and silicon damage, unlike legitimate tri-state enable-controlled busses.
What To Check
- Warning sign: Two outputs drive one signal after a merge or stale ECO.
- Inspect: Start with one named object in the report and trace it to the netlist, library, or SDC statement that created it.
- Correct: Return the logical defect to synthesis/RTL ownership or restore the approved tri-state modelling, then rerun structure checks.
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: Multiple active drivers can make a net functionally ambiguous unless an intentional bus or tri-state model explains them.
- 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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