Formal Verification

Skill: Formal Verification (FPV + LEC)

Invocation

When this skill is loaded and a user presents a formal verification task, do not execute stages directly. Immediately spawn the digital-chip-design-agents:formal-orchestrator agent and pass the full user request and any available context to it. The orchestrator enforces the stage sequence, loop-back rules, and sign-off criteria defined below.

Use the domain rules in this file only when the orchestrator reads this skill mid-flow for stage-specific guidance, or when the user asks a targeted reference question rather than requesting a full flow execution.

Pre-run Context

Before executing or advising on any stage, read the following files if they exist:

1. memory/formal/knowledge.md — known failure patterns, successful tool flags, PDK/tool quirks. Incorporate its guidance into every stage decision. If absent, proceed without it.
2. memory/formal/run_state.md — current run identity (run_id, design_name, tool, last_stage). Use this to resume correctly after interruption. If absent, a new run is starting; the orchestrator will create this file before the first stage.

This pre-run read applies whether this skill is loaded by a user or called by the orchestrator mid-flow. It ensures the fix database is consulted before any diagnosis step.

Purpose

Exhaustively prove design properties and equivalence using formal methods. Complements simulation-based verification for correctness proofs, protocol compliance, and equivalence checking between RTL and gate-level netlists.

Supported EDA Tools

Open-Source

• SymbiYosys (sby) — formal property verification front-end for open-source solvers
• Yosys (yosys) — synthesis and equivalence checking back-end
• Boolector — SMT solver for bit-vector arithmetic
• Z3 — general-purpose SMT solver from Microsoft Research
• ABC — logic synthesis and verification framework (sequential equivalence)
• Tabby CAD Suite — commercial bundle of sby + solvers (from YosysHQ)

Proprietary

• Cadence JasperGold (jg) — industry-standard FPV, CDC, DFT formal
• Synopsys VC Formal (vcf) — property checking and equivalence verification
• Siemens Questa Formal (qformal) — FPV and coverage closure

Stage: property_planning

Property Categories

1. Safety: "something bad never happens" assert property (@(posedge clk) !(error && valid));
2. Liveness: "something good eventually happens" (always bound the interval) assert property (@(posedge clk) req |-> ##[1:MAX] ack);
3. Stability: "output is stable while condition holds" assert property (@(posedge clk) valid |-> $stable(data));
4. Reachability: "a state is reachable" (use cover, not assert) cover property (@(posedge clk) state == DONE);

Domain Rules

1. Every spec feature: at least one property or cover point
2. All properties: include descriptive name and failure message
3. Liveness properties: always bound with ##[1:BOUND]
4. Use $past(), $rose(), $fell() over manual delay logic
5. disable iff: use for reset gating

QoR Metrics to Evaluate

• All spec features mapped to property or cover
• Cover points: key states are reachable

Output Required

• Property plan (feature → property mapping)
• SVA property file (.sva)
• SVA assumption file

Stage: environment_setup

Domain Rules

1. Constrain all primary inputs to legal values only
2. Protocol assumptions: model upstream block behaviour
3. Reset assumption: force correct reset sequence at time 0
4. Over-constraining → vacuous proof (nothing can be proven wrong) — always run vacuity check
5. Under-constraining → false CEX (environment bug, not DUT) — check all CEX carefully
6. Vacuity check: disable each assume — property should NOT hold without it
7. Document every assumption with justification

Common Assumption Templates

// Reset sequence
assume property (@(posedge clk) $rose(rst_n) |-> ##[1:5] rst_n);

// AXI valid stability
assume property (@(posedge clk)
  (s_axi_awvalid && !s_axi_awready) |=> $stable(s_axi_awaddr));

QoR Metrics to Evaluate

• Vacuity check: PASS for all properties
• No over-constraining: formal tool reports reasonable state space
• Environment signed off by verification lead

Output Required

• Formal environment file (constraints/assumptions)
• Vacuity check report
• Environment review record

Stage: fpv_run

Result Classifications

Strategies for Inconclusive

1. Increase BMC bound (k-induction)
2. Apply abstractions (data abstraction, counter abstraction)
3. Decompose: prove sub-properties; compose to main property
4. Document as "assumed correct" with justification if intractable

QoR Metrics to Evaluate

• Target: 100% PROVEN or UNREACHABLE (no unanalysed CEX)
• All INCONCLUSIVE: documented with justification and bound used

Output Required

• FPV run report (per property: result, CEX trace if applicable)
• CEX waveform descriptions for failures

Stage: cex_analysis

Domain Rules

1. Every CEX: determine if it is a real DUT bug or an assumption/environment bug
2. Real DUT bug: fix RTL → re-run FPV (counts as RTL bug, not formal bug)
3. Assumption bug: tighten assumption → re-run vacuity check
4. False CEX from under-constraining: document clearly before adding assumption
5. Never waive a CEX without root cause

Output Required

• CEX analysis report (bug or false alarm, root cause, fix applied)

Stage: lec_run

LEC Flow

1. Read golden: RTL or pre-ECO netlist
2. Read revised: post-synthesis netlist or post-ECO netlist
3. Map points: match sequential/combinational key points
4. Verify all points: compare cone-of-influence
5. Report: EQUIVALENT / UNMATCHED / ABORTED

Domain Rules

1. Use same SDC for both golden and revised
2. Scan mode: flatten scan chains or use scan-unaware mode
3. Black boxes: handle consistently in both netlists
4. Unmatched points: must be root-caused — not waived without RTL team approval
5. Post-ECO: run LEC after every ECO, not just at sign-off

Common LEC Failures

QoR Metrics to Evaluate

• All compare points: EQUIVALENT
• 0 UNMATCHED points
• 0 ABORTED points

Output Required

• LEC run report
• Unmatched point analysis (if any)
• EQUIVALENT sign-off record

Stage: formal_signoff

Sign-off Checklist

• All P0 properties: PROVEN
• No unanalysed CEX
• No vacuous proofs
• LEC: 100% EQUIVALENT
• All INCONCLUSIVE: documented with justification
• Additional coverage closed vs simulation baseline

Output Required

• Formal sign-off report
• Final property status table
• LEC clean record

Constraint Validation

See plugins/meta/skills/pipeline-orchestration/SKILL.md §Constraints Schema for the authoritative schema and stage-entry validation rule.

No required keys for formal verification — all constraints in this domain are optional.

There are no numeric coverage thresholds unique to formal; this domain shares coverage targets with the functional-verification domain (coverage.*) and timing targets with the STA domain (timing.wns_ns_target). When evaluating LEC equivalence or FPV property results, tag constraint_ref in history entries with the relevant dot-path key if a constraint value was consulted (e.g. "coverage.functional_pct" when reporting coverage contribution).

Memory

Write on stage completion

After each stage completes (regardless of whether an orchestrator session is active), write or overwrite one JSON record in memory/formal/experiences.jsonl keyed by run_id. This ensures data is persisted even if the flow is interrupted or called without full orchestrator context.

Use run_id = formal_<YYYYMMDD>_<HHMMSS> (set once at flow start; reuse on each stage update). Every JSON record written must include a top-level "run_id" field whose value matches this key — stage writes must upsert/overwrite by matching this persisted run_id. Set signoff_achieved: false until the final sign-off stage completes.

Run state (write before first stage, update after each stage)

Write memory/formal/run_state.md as the first action before launching any tool:

run_id:       formal_<YYYYMMDD>_<HHMMSS>
design_name:  <design>
tool:         <primary tool>
start_time:   <ISO-8601>
last_stage:   null
current_stage: <first stage name>

Update current_stage when a stage starts, and set last_stage to the completed stage name only after successful completion (then clear current_stage). This file lets wakeup-loop prompts and resumed sessions identify the correct run and distinguish completed vs in-flight work. Create the file and parent directories if they do not exist.

Optional: claude-mem index

If mcp__plugin_ecc_memory__add_observations is available in this session, emit each applied fix as an observation to entity chip-design-formal-fixes after writing to experiences.jsonl. Skip silently if the tool is absent — JSONL is the canonical record.