What do routing-track pitch and offset mean?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
A track grid is just a simple arithmetic sequence: coordinate = offset + k × pitch, where k is an integer within the track count — pitch sets the spacing, offset sets where the very first line sits. That means a coordinate can be arbitrarily close to a track without actually landing on one — e.g. a value like 100 µm isn't automatically centered on a track just because it "looks round."
Technical Explanation
- A track grid is just a simple arithmetic sequence:
coordinate = offset + k × pitch, where k is an integer within the track count — pitch sets the spacing, offset sets where the very first line sits. - That means a coordinate can be arbitrarily close to a track without actually landing on one — e.g. a value like 100 µm isn't automatically centered on a track just because it "looks round."
- DEF's naming is a classic beginner trap:
TRACKS Xenumerates x-coordinates, which describe vertical track lines, andTRACKS Yenumerates y-coordinates, describing horizontal lines — many people instinctively assume "X" means a horizontal wire, and it's the opposite. - Track alignment alone doesn't guarantee pin access — a pin rectangle can overlap a usable track even with its center off-track, but via size, enclosure, spacing rules, and allowed access patterns still decide whether the router can actually connect it. Never move a macro purely to align one single pin without checking all its other pins, the legal placement grid, and nearby routing resources too.
Command Checks & Actions
set_technology -node 7Applies the tech-file-defined track pitch and offset for the node before placement or routing runs.
set_technology -report_onlyConfirms which track definition is actually in effect without changing it -- useful when a routing or DRC result looks inconsistent with what you expect the track grid to be.
Common Mistake
The Trap: Reading STEP as edge-to-edge spacing, or forgetting that an otherwise correct pitch can have the wrong offset.
Follow-up Question & Model Response
"Why can matching pitch still leave two grids misaligned?"
Candidate Model Response: Their offsets may differ by a value that is not an integer multiple of the pitch.
Practical Example
Tapeout Scenario: A DEF using 2000 DBU/um can encode the example as TRACKS X 48 DO 5 STEP 96 LAYER M2. The starting x coordinate is 48/2000 = 0.024 um, and the step is 96/2000 = 0.048 um.
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