What is Live DRC in ICC2, and when is it useful?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Live DRC runs IC Validator with the foundry runset on what is displayed in the ICC2 layout window, so you can check a hand edit or ECO area in seconds. It needs IC Validator P-2019.06 or later, a runset in signoff.check_drc_live.runset (ICC2) and a layer map in signoff.physical.layer_map_file (ICC2). It is a local debug tool, not a replacement for full signoff DRC.
Technical Explanation
- It checks the displayed area plus a 1 micron extension by default.
signoff.check_drc_live.halo(ICC2) changes the extension. - By default it keeps only violations fully inside the checking region.
set_app_options -name signoff.check_drc_live.keep_enclosing_results -value true(ICC2) keeps those inside the region plus half the halo. - Density and connectivity rules are excluded by default through the Live DRC exclude_command_class application option. Those need a larger view than a window, so they belong to full runs.
- It uses frame views by default. For more detail, merge real GDSII or OASIS through
signoff.physical.merge_stream_files(ICC2); that also enables the error environment overlay showing child shapes that contribute to an error. - The first run caches the runset and processes merged stream files, which can take a couple of minutes.
signoff_init_live_drc(ICC2) does this ahead of time, and later runs are quick unless layers, rules or stream files change. - You can run it from the DRC toolbar or with
signoff_check_live_drc -rect(ICC2) for a given area. Results go to the error browser. - Use it for manual edits near macros, ECO cleanup, pin-access fixes and checking a Zroute fix before a full run.
What To Check
- That the runset and layer map are both set before the first run.
- The displayed layers and hierarchy levels, since the check covers what is shown.
- Whether the area needs density or connectivity rules that Live DRC leaves out.
- A full
signoff_check_drc(ICC2) after the edits are done.
Command Checks & Actions
set_app_options -name signoff.check_drc_live.runset -value drc_runset.rsSets the foundry runset for Live DRC, which is required.
set_app_options -name signoff.physical.layer_map_file -value icv_layer.mapSets the layer map, which Live DRC requires.
signoff_init_live_drcPre-processes the runset, layer map and merged stream files so the first check is fast.
signoff_check_live_drc -rect {{100 200} {140 240}}Runs Live DRC on one rectangle from the command line.
save_blockSaves the edits, since signoff DRC reads the on-disk block.
signoff_check_drcFull signoff check once local edits are complete.
Healthy, Suspicious & Hard-stop Results
- Healthy (illustrative): Each edited window checks in under a minute and shows 0 violations, and the full run afterwards agrees.
- Suspicious (illustrative): Live DRC is clean but the full run finds density errors in the same region, which Live DRC excluded by default.
- Hard stop: Edits approved on Live DRC alone with no full run. Rules outside the window were never checked.
Common Mistake
The Trap: Using Live DRC with frame views to clear a manual reroute beside a PHY. The window was clean, but the frame view had no internal PHY metal, and the full run on merged GDS found 18 spacing errors against it. Merging the PHY GDS for Live DRC would have shown them at edit time.
What The Interviewer Is Testing
- Knowledge of the two required settings and the version needed.
- Understanding what Live DRC leaves out: density, connectivity and anything outside the window.
- A strong answer places it in the edit loop, not at the signoff gate.
Follow-up Question & Model Response
"Why does Live DRC exclude density rules by default?"
Candidate Model Response: Density rules measure metal over windows that are often larger than the area on screen. Checking them on a small region gives meaningless results at the edges. Connectivity rules similarly need the whole net. The default keeps Live DRC fast and local, and the full signoff run covers those rule classes.
Practical Example
Tapeout Scenario: An engineer fixes 12 pin-access DRCs by hand near an SRAM (illustrative). After signoff_init_live_drc (ICC2), each Live DRC check takes about 20 seconds against 6 hours for a full run. Two fixes create new end-of-line errors that Live DRC flags at once, and both are redone. The full signoff run that night shows 0 errors in that region.
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