ExpertQuestion 75 of 161

What real problem does topological_ndr solve, and what's the actual sort order it enforces?

From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide

By Tabish Iqbal, who has 9+ years of VLSI Physical Design experience

Short Answer

Without the level-based rule, post-CTS NDR application at different tree levels can be discontinuous -- an internal net at one level might get a heavier NDR than the root net feeding it, which is physically backwards. With cts.compile.topological_ndr enabled, NDRs are topologically sorted: Root NDR > Internal NDR > Sink NDR, ensuring the NDR weight actually decreases as you move from source toward leaves, matching real current/EM demand.

Technical Reference DiagramWhat real problem does topological_ndr solve, and what's the actual sort order it enforces?

Technical Explanation

  • Without the level-based rule, post-CTS NDR application at different tree levels can be discontinuous -- an internal net could end up with a heavier NDR than the root net feeding it, which is physically backwards for current/EM demand.
  • cts.compile.topological_ndr, when enabled, sorts NDRs topologically: Root NDR > Internal NDR > Sink NDR -- ensuring NDR weight decreases monotonically from source toward leaves.
  • This is implemented only during initial CTS, not as a general post-hoc fix -- it shapes how NDRs get assigned as the tree is first built, not something you apply after the fact to a tree that already has discontinuous NDRs.
  • The underlying assignment rules feeding this: nets connected to a pure sink get sink NDR, nets connected to both a sink and intermediate cells get internal NDR, nets at root level until the branch point get root NDR.

Formula Or Decision Rule

Enforced order with topological_ndr enabled: Root NDR > Internal NDR > Sink NDR (heaviest near the source, lightest near the leaves).

What To Check

  • Warning sign: post-CTS NDR inspection shows an internal net with a heavier NDR than the root net feeding it.
  • Inspect: check whether cts.compile.topological_ndr was enabled BEFORE the initial CTS run, not applied afterward.
  • Correct: if the tree already exists with discontinuous NDR assignment, this option won't retroactively fix it -- the tree likely needs to be rebuilt with the option enabled from the start.

Command Checks & Actions

ICC2set_app_options -list {cts.compile.topological_ndr true}

Enforces Root > Internal > Sink NDR ordering during initial CTS.

ICC2report_clock_qor -type drc_violators

Can surface issues stemming from NDR discontinuity indirectly via DRC violations.

Healthy, Suspicious & Hard-stop Results

  • Expected: NDR weight decreases monotonically from root nets through internal nets to sink nets across the whole tree.
  • Investigate: an internal net shows a heavier NDR than its feeding root net -- check whether topological_ndr was ever enabled for this tree's initial CTS run.
  • Stop: topological_ndr is enabled on an already-built tree expecting retroactive correction -- it doesn't work that way; the tree needs rebuilding with the option set beforehand.

Common Mistake

The Trap: Applying cts.compile.topological_ndr after CTS has already built a tree with discontinuous NDR assignment, expecting it to retroactively fix the existing assignment -- it's implemented only during initial CTS, not as a post-hoc correction.

What The Interviewer Is Testing

Whether you know topological_ndr is an initial-CTS-only mechanism with a specific enforced order (Root > Internal > Sink), not a general-purpose NDR-consistency fixer you can apply at any stage.

Practical Example

Debug Scenario: Post-CTS NDR inspection on a tree finds an internal net with a heavier NDR assignment than the root net feeding it -- physically backwards. Checking the build script, cts.compile.topological_ndr was never enabled before the initial CTS run that built this tree, so the discontinuity was never prevented in the first place.

PnR Flow Mentor Guide

Read the complete 8-chapter PnR Flow Mentor Guide free on the web — library setup through placement, clock tree synthesis, routing, chip finishing, hierarchical implementation, and ECO, all the way to stream-out.

Explore PnR Flow Mentor Guide →

Offline PDF Bundle

Want all 1109 questions offline?

Get the complete 4-book PDF bundle (PnR, STA, MMMC, Low Power) with a clickable table of contents - no ads, no internet needed.

See what's inside the bundle
PnR Flow Physical Design Mentor Guide — eight chaptersPnR Flow Mentor GuideEight chapters, library setup through to stream-out.

Continue practising

Related PnR (Place & Route) questions

ExpertWhat problem do via ladders actually solve on a clock tree, and where does insert_via_ladders fit in the clock_opt flow?BeginnerWhat is a floorplan, and what problem does it solve?IntermediateWhat problem does split_fanout actually solve during a timing ECO, and how do you tell it what to split on?ExpertWhat does -check_prerequisites on place_group_repeaters actually validate before attempting placement, and what do a couple of its real error codes tell you?