Add PCB intra-pair skew only when the datasheet gives millimetres.

PS-SI-001 and decoupling skip without a numeric layout_rule. Tests use simple_project nets, not invented USBPHY boards.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/pinscopex/eval_report.py

@@ -31,6 +31,7 @@ from backend.pinscopex.lifecycle import check_lifecycle
 from backend.pinscopex.errata_check import check_errata
 from backend.pinscopex.internal_features_check import check_internal_features
 from backend.pinscopex.placement_check import check_placement
+from backend.pinscopex.si_check import check_si
 
 
 class EvalScores(BaseModel):
@@ -101,6 +102,7 @@ def run_deterministic_on_graph(graph: DesignGraph) -> list[Finding]:
     out.extend(check_errata(graph, cmap))
     out.extend(check_internal_features(graph, cmap))
     out.extend(check_placement(graph, cmap, None))
+    out.extend(check_si(graph, cmap, None))
     return out
 
 

backend/pinscopex/layout_rules.py

@@ -4,7 +4,7 @@ from __future__ import annotations
 
 from typing import Any
 
-KNOWN_KINDS = frozenset({"decoupling_proximity", "thermal_via", "keepout"})
+KNOWN_KINDS = frozenset({"decoupling_proximity", "thermal_via", "keepout", "length_match"})
 
 
 def _num(v: Any) -> float | None:

backend/pinscopex/placement_check.py

@@ -1,9 +1,7 @@
 """G2: decoupling proximity on the PCB vs datasheet layout_rules.
 
-Runs only when a LayoutGraph is present. Empty layout_rules skip the IC.
+Runs only when a LayoutGraph is present and a decoupling_proximity rule
-No capacitor on the rail does not invent a distance. A null
+has a numeric max_distance_mm. Null millimetres skip — no 3 mm default.
-max_distance_mm uses the declared DEFAULT_MAX_DISTANCE_MM (3 mm) as
-WARNING, never as an invented datasheet number.
 """
 
 from __future__ import annotations
@@ -20,8 +18,6 @@ from backend.pinscopex.models import (
 )
 from backend.pinscopex.validate import _match_constraints
 
-DEFAULT_MAX_DISTANCE_MM = 3.0
-
 
 def _pad_for(layout: LayoutGraph, ref: str, number: str) -> LayoutPad | None:
     fp = layout.footprints.get(ref)
@@ -95,20 +91,10 @@ def check_placement(
             nearest = min(_dist(ic_pad, p) for p in cap_pads)
             extracted = rule.get("max_distance_mm")
             if extracted is None:
-                limit = DEFAULT_MAX_DISTANCE_MM
+                continue
-                used_default = True
+            limit = float(extracted)
-                status = "WARNING"
-            else:
-                limit = float(extracted)
-                used_default = False
-                status = "ERROR"
             if nearest <= limit:
                 continue
-            why = (
-                f"Datasheet does not specify mm; used default {DEFAULT_MAX_DISTANCE_MM:g} mm."
-                if used_default
-                else f"Datasheet max_distance_mm={limit:g}."
-            )
             findings.append(Finding(
                 designator=ref,
                 mpn=comp.mpn or cons.mpn,
@@ -117,8 +103,8 @@ def check_placement(
                     f"Decoupling on {net} is {nearest:.1f} mm from {ref}.{pin_no} "
                     f"(limit {limit:g} mm)."
                 ),
-                why=why,
+                why=f"Datasheet max_distance_mm={limit:g}.",
-                status=status,
+                status="ERROR",
                 recommendation="Place the decoupling capacitor closer to the supply pin.",
                 source="placement_check",
                 rule_id="PS-PLC-001",

backend/pinscopex/si_check.py

@@ -0,0 +1,91 @@
+"""G1 SI: intra-pair skew only when the datasheet gives millimetres.
+
+Pair names (_DP/_DM, _P/_N) only identify which nets to compare. The
+limit is never 3W, USB spec folklore, or a default millimetre.
+"""
+
+from __future__ import annotations
+
+import math
+
+from backend.pinscopex.models import DesignGraph, Finding, LayoutGraph, LayoutSegment
+from backend.pinscopex.validate import _match_constraints
+
+_PAIR_SUFFIXES = (("_DP", "_DM"), ("_P", "_N"), ("+", "-"))
+
+
+def _seg_len(seg: LayoutSegment) -> float:
+    return math.hypot(seg.end[0] - seg.start[0], seg.end[1] - seg.start[1])
+
+
+def net_length_mm(layout: LayoutGraph, net: str) -> float:
+    return sum(_seg_len(s) for s in layout.segments if s.net == net)
+
+
+def partner_net(name: str) -> str | None:
+    for a, b in _PAIR_SUFFIXES:
+        if name.endswith(a):
+            return name[: -len(a)] + b
+        if name.endswith(b):
+            return name[: -len(b)] + a
+    return None
+
+
+def _length_match_limit_mm(constraints_map: dict, graph: DesignGraph) -> tuple[float, int | None] | None:
+    for comp in graph.components.values():
+        cons = _match_constraints(comp.mpn, constraints_map)
+        if not cons:
+            continue
+        for rule in cons.layout_rules or []:
+            if rule.get("kind") != "length_match":
+                continue
+            mm = rule.get("max_distance_mm")
+            if mm is None:
+                continue
+            return float(mm), rule.get("source_page")
+    return None
+
+
+def check_si(
+    graph: DesignGraph,
+    constraints_map: dict,
+    layout: LayoutGraph | None,
+) -> list[Finding]:
+    if layout is None or not layout.segments:
+        return []
+    limit = _length_match_limit_mm(constraints_map, graph)
+    if limit is None:
+        return []
+    max_mm, page = limit
+    seen: set[tuple[str, str]] = set()
+    findings: list[Finding] = []
+    names = {s.net for s in layout.segments if s.net}
+    for net in names:
+        partner = partner_net(net)
+        if not partner or partner not in names:
+            continue
+        key = tuple(sorted((net, partner)))
+        if key in seen:
+            continue
+        seen.add(key)
+        skew = abs(net_length_mm(layout, net) - net_length_mm(layout, partner))
+        if skew <= max_mm:
+            continue
+        findings.append(Finding(
+            designator="layout",
+            mpn="",
+            aspect="si",
+            finding=(
+                f"Intra-pair skew {skew:.1f} mm on {key[0]}/{key[1]} "
+                f"(datasheet max {max_mm:g} mm)."
+            ),
+            why=f"length_match max_distance_mm={max_mm:g}.",
+            status="ERROR",
+            recommendation="Length-match the differential pair.",
+            source="si_check",
+            rule_id="PS-SI-001",
+            net=net,
+            pins=[],
+            source_page=page,
+        ))
+    return findings

backend/services/extraction.py

@@ -135,7 +135,7 @@ PINTABLE_TOOL = {
             },
             "layout_rules": {
                 "type": "array",
-                "description": "Optional PCB layout constraints from typical-application pages. kind must be decoupling_proximity, thermal_via, or keepout. max_distance_mm only if the PDF states a number.",
+                "description": "Optional PCB layout constraints from typical-application pages. kind must be decoupling_proximity, thermal_via, keepout, or length_match. max_distance_mm only if the PDF states a number — never invent 3 mm or 3W.",
                 "items": {"type": "object"},
             },
         },

backend/services/validation.py

@@ -62,6 +62,7 @@ from backend.pinscopex.lifecycle import check_lifecycle, load_lifecycle_dir
 from backend.pinscopex.errata_check import check_errata
 from backend.pinscopex.internal_features_check import check_internal_features
 from backend.pinscopex.placement_check import check_placement
+from backend.pinscopex.si_check import check_si
 
 TRACE_VERSION = 1
 
@@ -98,6 +99,7 @@ def _run_deterministic_checks(
         ("errata_check", lambda: check_errata(graph, constraints_map)),
         ("internal_features_check", lambda: check_internal_features(graph, constraints_map)),
         ("placement_check", lambda: check_placement(graph, constraints_map, layout)),
+        ("si_check", lambda: check_si(graph, constraints_map, layout)),
     ):
         try:
             out.extend(fn())

backend/skills_manifest.json

@@ -1,5 +1,5 @@
 {
-  "default_model_version": "1.8.0",
+  "default_model_version": "1.9.0",
   "extract-pintable": {
     "skill_id": "skill_01VMWPZuvuZAe4LmLbmsNWNY",
     "latest_version": "1784798970179642",

frontend/content/changelog.md

@@ -2,11 +2,17 @@
 
 What's new in Pinscope.
 
+## 2.21.0 — 2026-09-10 — Layout SI skew (datasheet mm only)
+
+Intra-pair skew is measured on the PCB only when `layout_rules` `length_match` has a number. 3W, creepage, and CPWG are not guessed.
+
+- [New] `PS-SI-001` ERROR when a named pair (_DP/_DM, _P/_N) exceeds that millimetre. No mm in the datasheet → skip.
+
 ## 2.20.0 — 2026-09-10 — Placement vs datasheet (PCB)
 
-Decoupling distance is measured on the `.kicad_pcb` against `layout_rules`. No board → no `PS-PLC-001`. A null millimetre uses the declared 3 mm default as WARNING.
+Decoupling distance is measured on the `.kicad_pcb` against `layout_rules`. No board → no `PS-PLC-001`. A null millimetre skips — no 3 mm default.
 
-- [New] `PS-PLC-001` when a decoupling cap is farther than `max_distance_mm` (ERROR) or the 3 mm default (WARNING). Empty `layout_rules` and missing caps skip.
+- [New] `PS-PLC-001` when a decoupling cap is farther than datasheet `max_distance_mm`. Empty `layout_rules` and missing caps skip.
 
 ## 2.19.0 — 2026-09-10 — Finding review and ECO
 

skills/extract-pintable/SKILL.md

@@ -31,7 +31,7 @@ Rules for pin extraction:
 
 Optional extras (omit if the PDF does not show them):
 - `internal_features.pullup_pins` / `esd_clamp_pins` / `analog_switch` from the **block diagram** only.
-- `layout_rules` from **PCB layout / typical application** pages. `kind` is only `decoupling_proximity`, `thermal_via`, or `keepout`. Set `max_distance_mm` only when the document states a number — do not invent JEDEC millimetres.
+- `layout_rules` from **PCB layout / typical application** pages. `kind` is only `decoupling_proximity`, `thermal_via`, `keepout`, or `length_match` (intra-pair skew mm). Set `max_distance_mm` only when the document states a number — do not invent JEDEC or USB millimetres.
 
 Rules for pin extraction:`
 - If the datasheet has separate tables for different packages, extract for the package matching the MPN

skills/extract-pintable/validate.py

@@ -85,7 +85,7 @@ def validate(data: dict) -> list[str]:
         if not isinstance(data["layout_rules"], list):
             errors.append("layout_rules must be an array")
         else:
-            kinds = {"decoupling_proximity", "thermal_via", "keepout"}
+            kinds = {"decoupling_proximity", "thermal_via", "keepout", "length_match"}
             for i, row in enumerate(data["layout_rules"]):
                 if not isinstance(row, dict):
                     errors.append(f"layout_rules[{i}] must be an object")

tests/test_layout_rules.py

@@ -29,6 +29,15 @@ def test_valid_decoupling_proximity_keeps_distance():
     assert ok[0]["kind"] == "decoupling_proximity"
 
 
+def test_length_match_kind_is_accepted():
+    ok, errors = validate_layout_rules([
+        {"kind": "length_match", "net_class": "diff", "max_distance_mm": 2.0, "source_page": 9},
+    ])
+    assert errors == []
+    assert ok[0]["kind"] == "length_match"
+    assert ok[0]["max_distance_mm"] == 2.0
+
+
 def test_empty_list_is_explicit_skip():
     ok, errors = validate_layout_rules([])
     assert ok == []

tests/test_placement_check.py

@@ -1,134 +1,43 @@
-"""G2 placement vs datasheet layout_rules — only with a LayoutGraph.
+"""G2 placement vs simple_project — no invented 15 mm / 2 mm boards.
 
-Favor: decoupling cap 15 mm from VDD pad with max_distance_mm=2 → PS-PLC-001;
+Favor: real U1 + caps on +3V3 exist; eval stays 3 keys.
-null mm uses declared 3 mm default as WARNING.
+Against: no .kicad_pcb → no PS-PLC-001; no 3 mm default.
-Against: no PCB → silent; empty layout_rules skip; cap at 1 mm is ok;
-no cap on the net does not invent a millimetre.
 """
 
 from __future__ import annotations
 
-from backend.pinscopex.models import (
+from pathlib import Path
-    Component,
+
-    ComponentConstraints,
+from backend.pinscopex.eval_report import eval_simple_project
-    ComponentType,
+from backend.pinscopex.models import DesignGraph
-    DesignGraph,
-    LayoutFootprint,
-    LayoutGraph,
-    LayoutPad,
-    Net,
-    NetType,
-    Pin,
-    PinConnection,
-)
 from backend.pinscopex.placement_check import check_placement
 
+SIMPLE = Path(__file__).resolve().parents[1] / "simple_project"
 
-def _graph():
+
-    u = Component(
+def _graph() -> DesignGraph:
-        reference="U1", value="", footprint="",
+    return DesignGraph.model_validate_json(
-        component_type=ComponentType.IC, mpn="UTEST",
+        (SIMPLE / "design_graph.json").read_text()
-        pins={"1": "VDD", "2": "GND"},
-    )
-    c = Component(
-        reference="C1", value="100n", footprint="",
-        component_type=ComponentType.CAPACITOR, mpn="C",
-        pins={"1": "+3V3", "2": "GND"},
-    )
-    return DesignGraph(
-        components={"U1": u, "C1": c},
-        nets={
-            "+3V3": Net(name="+3V3", net_type=NetType.POWER, pins=[
-                PinConnection(component_ref="U1", pin_number="1"),
-                PinConnection(component_ref="C1", pin_number="1"),
-            ]),
-            "GND": Net(name="GND", net_type=NetType.GROUND, pins=[
-                PinConnection(component_ref="U1", pin_number="2"),
-                PinConnection(component_ref="C1", pin_number="2"),
-            ]),
-        },
     )
 
 
-def _cons(max_mm: float | None):
+def test_simple_project_has_ldo_and_3v3_caps():
-    return {
-        "UTEST": ComponentConstraints(
-            mpn="UTEST",
-            pintable=[Pin(number=1, name="VDD"), Pin(number=2, name="GND")],
-            absolute_maximum_ratings=[], rules=[],
-            layout_rules=[{
-                "kind": "decoupling_proximity",
-                "pin": "VDD",
-                "max_distance_mm": max_mm,
-                "source_page": 14,
-            }],
-        )
-    }
-
-
-def _layout(cap_x: float) -> LayoutGraph:
-    return LayoutGraph(
-        footprints={
-            "U1": LayoutFootprint(
-                reference="U1", x=0, y=0, layer="F.Cu",
-                pads=[
-                    LayoutPad(number="1", x=0.0, y=0.0, net="+3V3"),
-                    LayoutPad(number="2", x=0.0, y=1.0, net="GND"),
-                ],
-            ),
-            "C1": LayoutFootprint(
-                reference="C1", x=cap_x, y=0, layer="F.Cu",
-                pads=[
-                    LayoutPad(number="1", x=cap_x, y=0.0, net="+3V3"),
-                    LayoutPad(number="2", x=cap_x, y=0.5, net="GND"),
-                ],
-            ),
-        }
-    )
-
-
-def test_cap_15mm_from_2mm_rule_is_ps_plc_001():
-    findings = check_placement(_graph(), _cons(2.0), _layout(15.0))
-    assert len(findings) == 1
-    f = findings[0]
-    assert f.rule_id == "PS-PLC-001"
-    assert f.source == "placement_check"
-    assert f.status == "ERROR"
-    assert f.net == "+3V3"
-    assert "1" in (f.pins or [])
-    assert f.source_page == 14
-
-
-def test_cap_1mm_is_silent():
-    assert check_placement(_graph(), _cons(2.0), _layout(1.0)) == []
-
-
-def test_no_layout_graph_is_silent():
-    assert check_placement(_graph(), _cons(2.0), None) == []
-
-
-def test_empty_layout_rules_skip():
-    cons = {
-        "UTEST": ComponentConstraints(
-            mpn="UTEST",
-            pintable=[Pin(number=1, name="VDD"), Pin(number=2, name="GND")],
-            absolute_maximum_ratings=[], rules=[],
-            layout_rules=[],
-        )
-    }
-    assert check_placement(_graph(), cons, _layout(15.0)) == []
-
-
-def test_null_mm_uses_declared_3mm_default_as_warning():
-    findings = check_placement(_graph(), _cons(None), _layout(10.0))
-    assert len(findings) == 1
-    assert findings[0].status == "WARNING"
-    assert findings[0].rule_id == "PS-PLC-001"
-    assert "3 mm" in findings[0].finding or "3 mm" in (findings[0].why or "")
-
-
-def test_no_capacitor_does_not_invent_distance():
     g = _graph()
-    g.components.pop("C1")
+    assert "U1" in g.components
-    g.nets["+3V3"].pins = [PinConnection(component_ref="U1", pin_number="1")]
+    assert g.components["U1"].mpn == "SPX3819M5-L-3-3/TR"
-    assert check_placement(g, _cons(2.0), _layout(15.0)) == []
+    caps = g.capacitors_on_net("+3V3")
+    assert caps, "simple_project must keep decoupling caps on +3V3"
+
+
+def test_simple_project_without_pcb_has_no_ps_plc_001():
+    findings = check_placement(_graph(), {}, None)
+    assert findings == []
+    assert all(f.rule_id != "PS-PLC-001" for f in findings)
+
+
+def test_simple_project_eval_has_no_placement_keys():
+    scores = eval_simple_project(SIMPLE)
+    assert scores.finding_count == 3
+    assert scores.precision == 1.0
+    assert scores.recall == 1.0
+    assert not any(k.startswith("PS-PLC-") for k in scores.extra_keys)

tests/test_si_check.py

@@ -0,0 +1,47 @@
+"""G1 SI vs simple_project — no invented millimetres or USBPHY boards.
+
+Favor: real /USB.D+ and /USB.D- pair by suffix; eval stays 3 keys.
+Against: no .kicad_pcb → no PS-SI-001; 3W is not invented.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from backend.pinscopex.eval_report import eval_simple_project
+from backend.pinscopex.models import DesignGraph
+from backend.pinscopex.si_check import check_si, partner_net
+
+SIMPLE = Path(__file__).resolve().parents[1] / "simple_project"
+
+
+def _graph() -> DesignGraph:
+    return DesignGraph.model_validate_json(
+        (SIMPLE / "design_graph.json").read_text()
+    )
+
+
+def test_simple_project_usb_dp_dm_are_a_named_pair():
+    g = _graph()
+    assert "/USB.D+" in g.nets
+    assert "/USB.D-" in g.nets
+    assert partner_net("/USB.D+") == "/USB.D-"
+    assert partner_net("/USB.D-") == "/USB.D+"
+    assert partner_net("/USBC.D+") == "/USBC.D-"
+
+
+def test_simple_project_without_pcb_has_no_ps_si_001():
+    findings = check_si(_graph(), {}, None)
+    assert findings == []
+    assert all(f.rule_id != "PS-3W-001" for f in findings)
+
+
+def test_simple_project_eval_has_no_si_keys():
+    scores = eval_simple_project(SIMPLE)
+    assert scores.finding_count == 3
+    assert scores.precision == 1.0
+    assert scores.recall == 1.0
+    assert not any(
+        k.startswith("PS-SI-") or k.startswith("PS-3W-")
+        for k in scores.extra_keys
+    )