cdc-audit

CDC Audit (Clock-Domain-Crossing Risk Review & Resolution)

A methodology playbook for finding, classifying, and safely resolving clock-domain crossings using the SynthPilot MCP tools that drive Vivado. CDC bugs do not show up as timing failures — a metastable crossing can pass STA and still fail silently in silicon. This skill treats CDC as a structural correctness problem first and a constraint problem second.

The governing rule for this entire skill: a CDC constraint does not fix a CDC bug. set_clock_groups, set_false_path, and set_max_delay -datapath_only only tell the timing engine to stop reporting a crossing — they do nothing to add the synchronizer hardware that actually prevents metastability propagation. Constraints are applied only after the crossing is confirmed to be structurally safe, or explicitly waived with a stated assumption.

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When to use

• User asks for a "CDC analysis", "check clock domain crossings", "metastability review", or "reset domain crossing (RDC) check".
• A design has two or more asynchronous (unrelated) clocks and you need to prove every crossing is either synchronized or safely a quasi-static/false path.
• report_cdc / check_cdc_lint is reporting Critical or Warning severity items and the user wants them triaged and resolved.
• Pre-tapeout / pre-release signoff that must include a clean CDC report.
• A clock-crossing FIFO, handshake, or gray-code counter was just added and the user wants it verified.
• Intermittent / "works on the bench sometimes" hardware bugs where metastability is a suspect.

When NOT to use

• Single-clock design (or all clocks synchronous/derived from one source with a known phase relationship). There are no async crossings to audit — confirm this with get_all_clocks / report_clock_interaction and stop.
• Setup/hold timing failures within a single domain → use the timing-closure skill. CDC is about unrelated clocks; intra-domain WNS violations are a different problem.
• Functional logic bugs that happen to be near a crossing → simulate first (sim_run); do not blame CDC without evidence of a real domain crossing.
• Pure lint/style cleanup with no async clocks → use the lint skill.
• You only have an RTL file and no project / no Vivado connection — CDC structural analysis needs an elaborated or synthesized netlist. Set the project up first.

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Prerequisites (verify first)

Run these before any analysis. Do not proceed on assumption.

1. MCP/Vivado link is live — test_connection. If it fails, the Tcl server inside Vivado is not running; stop and tell the user to start it. Optionally vivado_version to record the tool version in the report.
2. A project is open with the design loaded — get_project_info and list_source_files. If nothing is open, open_project (or create_project + add_source_file + set_top_module).
3. Clocks are actually defined — get_all_clocks. CDC analysis is meaningless without clock constraints; if clocks are missing (everything shows as one inferred clock, or report_cdc later says "no inter-clock paths"), the timing environment is incomplete. Note this and, before drawing conclusions, ensure primary clocks are constrained via create_clock_constraint / create_generated_clock. A missing clock definition silently hides real crossings — treat an incomplete clock set as a blocker, not a pass.
4. A design exists to report on. report_cdc needs an elaborated or synthesized design open. The cheapest entry point is structural lint (check_cdc_lint), which works on RTL. For the authoritative crossing-by-crossing report you need a synthesized design — if none exists, run run_synthesis_async and poll get_run_status (see Methodology step 2). Then open_synthesized_design.

State explicitly which of these are satisfied before continuing.

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Methodology (measure → classify → smallest-safe-fix → re-measure)

Work the crossings in severity order (Critical → Warning). Change one crossing-class per iteration so cause and effect stay attributable.

1. Inventory the clock topology (understand the battlefield)

• get_all_clocks — list every clock, its period, and source.
• get_clock_info — per-clock detail (waveform, generated-vs-primary) where needed.
• report_clock_networks — physical clock network / buffering view.
• report_clock_interaction — the key map: which clock pairs have paths between them and whether each pair is timed, partial-false-path, or asynchronous. This is your ground truth for "which crossings exist." Note every pair flagged asynchronous or with inter-clock paths.

From this, build the set of clock-pair crossings you must account for. A crossing not on this list does not exist; a crossing on this list must be classified before you may close the audit.

2. Get an authoritative CDC report

• RTL-level first pass (fast, no synth): check_cdc_lint and/or run_full_lint_check → get_lint_violations. This catches missing synchronizers and obvious structural issues early and cheaply.
• Netlist-level authoritative pass: ensure a synthesized design is open. If not:
  • run_synthesis_async, then poll get_run_status until complete (multi-minute — do not block; use the async + poll pattern). On completion check check_synthesis_issues / get_synthesis_warnings.
  • open_synthesized_design.
  • report_cdc — the full crossing-by-crossing CDC report with severity (Critical / Warning / Info) and endpoint detail.
• For large reports, narrow with report_cdc_filtered (e.g., by clock pair or severity) and drill into specific endpoints with report_cdc_details.

Record the raw counts (Critical / Warning / Info) as the audit baseline. Do not editorialize them yet.

3. Classify every Critical/Warning crossing (classification table below)

For each flagged crossing, identify: source clock, destination clock, signal width (single-bit vs bus), whether a synchronizer is present, whether it is a control vs data path, and whether reconvergence or reset-domain-crossing is involved. Use report_cdc_details and, where you need to see the RTL, read_file / read_file_lines on the relevant source. Map each to a structure type using the table below.

4. Apply the smallest safe fix for that crossing class

Follow the fix-precedence ladder strictly:

1. Already-safe → constrain only. If a correct synchronizer already exists, the crossing is structurally fine; the only action is to make the timing tool treat it correctly (set_clock_groups -asynchronous, or set_max_delay -datapath_only / set_bus_skew for buses — see table). This is a constraint, not a design change.
2. Missing/incorrect synchronizer → recommend RTL. Do not silence it. Propose the specific RTL fix (2-FF synchronizer for single-bit control; async FIFO or gray-code + handshake for buses; reset synchronizer for RDC) and present it to the user. Recommend the edit; do not silently apply RTL. If the user approves, use read_file → replace_in_file / update_file (or create_source_file for a new synchronizer module), then add it to the project and re-elaborate.
3. Genuinely a non-crossing (quasi-static, same effective domain, tied constant) → waive with a stated assumption. Only here may you use set_false_path / set_clock_groups / waive_lint_violation, and only after explicitly stating the assumption (e.g., "config register is written once at reset, never while the destination clock is active"). If the path could be real and you are unsure, ask the user before waiving.

Apply exactly one crossing-class of change, then save_constraints (for constraint changes).

5. Re-measure on the same evidence source

Re-run the same tool you measured with: report_cdc (and report_clock_interaction to confirm the pair is now correctly treated) for netlist changes; check_cdc_lint / get_lint_violations for RTL changes. Confirm the targeted crossing's severity dropped and that no new Critical/Warning appeared elsewhere. RTL changes require re-synthesis (step 2) before the netlist CDC report is valid again — synthesis warnings alone are necessary-not-sufficient.

6. (Signoff) Prove it on the implemented design

For a signoff claim that "CDC is clean," constraints must hold through place & route. Run run_implementation_async, poll get_run_status, then open_implemented_design and:

• report_cdc (final, post-implementation) — the authoritative clean/not-clean verdict.
• report_clock_interaction — confirm every async pair is correctly grouped.
• report_drc and report_methodology — catch CDC-adjacent methodology violations (e.g., CDC-*, TIMING-* rule hits) that the CDC report alone may not surface.
• For bus crossings constrained with set_bus_skew, confirm the skew constraint passed in report_timing_summary (a skew constraint that fails routing is not a fix).

A success claim requires fresh post-implementation tool output, never inference from RTL or synthesis.

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Classification table (symptom → structure → safe fix)

#	Symptom from report_cdc / report_cdc_details	Structural cause	Smallest safe fix (in precedence order)
C1	1-bit control crossing, no synchronizer (direct reg→reg between async clocks)	Missing 2-FF synchronizer — true metastability risk	RTL: recommend a 2-flop synchronizer on the destination clock. Do NOT false-path. After fix + re-synth, optionally set_max_delay -datapath_only src→sync-FF1.
C2	1-bit crossing WITH a recognized 2-FF synchronizer	Already safe; tool just needs to be told it's async	Constraint only: set_clock_groups -asynchronous between the two clocks (or set_max_delay -datapath_only on the input to FF1). No RTL change.
C3	Multi-bit bus crossing, all bits sampled by a synchronizer	Bus bits can skew → recombine to a wrong value (data coherency hazard)	RTL: recommend gray-code (counters), or async FIFO, or a handshake-gated MUX-capture (req/ack). A per-bit 2-FF synchronizer on a bus is a BUG, not a fix — call it out.
C4	Bus crossing via async FIFO / handshake-qualified capture	Already coherent by construction	Constraint: set_bus_skew on the bus (bound inter-bit skew) and/or set_max_delay -datapath_only; set_clock_groups -asynchronous for the clock pair. No RTL change.
C5	Reset asserted in one domain, deasserted into another (RDC)	Reset deassertion not synchronized → metastable release, partial reset	RTL: recommend an async-assert / sync-deassert reset synchronizer per destination domain. Verify with check_async_reset.
C6	Reconvergence: two synchronized signals from the same source recombine downstream	Independent 2-FF synchronizers resolve on different cycles → glitch/illegal combo	RTL: recommend re-architecting to cross a single encoded bus (gray/handshake) once, then decode in the destination domain. Constraints cannot fix this.
C7	"Quasi-static" / config-register crossing (written once, static thereafter)	Not a dynamic crossing — but only if the "written once" assumption holds	Waive with stated assumption: set_false_path (or waive_lint_violation). State the assumption explicitly; ask the user to confirm the register is truly static during destination-clock operation.
C8	Clocks reported as crossing but actually synchronous/related (common divider, known phase)	Not an async crossing at all	Constraint: define the relationship (create_generated_clock) so STA times it normally; do NOT set_clock_groups -asynchronous (that would hide a real timed path).
C9	Crossing into/out of an IP (clk-wiz, async FIFO IP, AXI clock converter)	IP handles CDC internally if configured correctly	Verify IP config (get_fifo_config / get_clocking_wizard_config / report_ip_status); apply the IP's recommended set_* constraints. Don't hand-synchronize an IP boundary that's already safe.

Rule of thumb: single-bit control → 2-FF synchronizer; multi-bit data → FIFO / gray-code / handshake; reset → async-assert+sync-deassert. A bank of independent 2-FF synchronizers on a bus is itself a defect (C3), not a remedy.

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Resolution loop & STOP conditions

for each crossing in (Critical first, then Warning), one class at a time:
    measure   → report_cdc / report_cdc_details (or check_cdc_lint for RTL)
    classify  → map to C1..C9 above
    fix       → smallest safe change in the precedence ladder
                (constraint-only > recommend-RTL > waive-with-assumption)
    re-measure→ same tool; confirm THIS crossing dropped AND nothing new appeared
    (RTL change ⇒ re-synthesize before the netlist CDC report is valid again)

STOP and hand back to the human when ANY of:

• Clean: report_cdc on the implemented design shows 0 Critical, 0 unexplained Warning, every remaining waiver has a written assumption, and report_methodology shows no open CDC-* violations. Report the fresh evidence.
• RTL change required: the safe fix is a synchronizer / FIFO / re-architecture. Present the exact recommended RTL and wait for user approval before editing. Never silently rewrite RTL to make a number go green.
• Ambiguous quasi-static waiver: a crossing might be real and you cannot prove it is static from the design alone — present it and ask the user before waiving.
• Missing clock definitions: crossings are hidden because clocks aren't constrained. Stop, report the gap, and ask for / propose the clock constraints first — do not declare "clean" on an under-constrained design.
• Constraint can't make a bus crossing meet skew even though the structure is correct (C4): present the options (tighter floorplan, deeper FIFO, slower crossing clock) with their costs and stop. Do not relax/remove the set_bus_skew just to pass.
• Two adversarial iterations with no net reduction in Critical/unexplained Warning: stop thrashing; summarize remaining items with options and trade-offs.

Do not loop forever and do not "resolve" by deletion of evidence (removing waivers' assumptions, false-pathing real crossings, disabling lint rules).

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Safety rails (do not violate)

• No fake-pass. Never apply set_false_path, set_clock_groups -asynchronous, set_max_delay, disable_lint_rule, or waive_lint_violation to a crossing that could be a real, unsynchronized metastability path just to clear the report. A constraint silences STA; it adds no hardware. Waivers/false-paths are allowed ONLY for confirmed non-crossings (C7/C8) with a written assumption, and if there's any doubt the path is real, ask the user first.
• Constraint ≠ correctness. set_clock_groups -asynchronous is correct ONLY when a real synchronizer already protects the crossing (C2/C4). Applying it to an unsynchronized crossing (C1/C3) hides a live bug. Confirm the synchronizer exists (report_cdc_details / read_file) before grouping.
• Smallest safe change first. Constraints/strategy before RTL. Only escalate to RTL when no safe constraint exists, and then recommend, do not silently apply.
• Recommend RTL, never sneak it. Synchronizer insertion, FIFO swap-in, and reset re-architecture change design behavior. Present the diff and get approval.
• One crossing-class per iteration so cause→effect is attributable; re-measure after each.
• Evidence before claims. "CDC clean," "metastability-safe," and "crossings resolved" each REQUIRE fresh tool output — and for signoff, fresh post-implementation report_cdc + report_methodology. Synthesis-level or lint-level results are necessary-not-sufficient. Never infer cleanliness from a source edit.
• A bus is not single bits. Never recommend per-bit 2-FF synchronizers on a multi-bit data bus (C3) — that is a coherency bug. Use FIFO / gray-code / handshake.
• Don't hide real timing. Don't set_clock_groups -asynchronous on clocks that are actually related (C8); define the generated-clock relationship instead so the timed path stays timed.
• Bounded effort. When safe options are exhausted, surface the remaining items with trade-offs and hand back. Do not thrash.

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Output (report format)

Produce a CDC audit report with:

1. Environment — Vivado version, project, top module, clock list (from get_all_clocks), and which design stage the verdict is based on (RTL-lint / synthesized / implemented). State it plainly if no post-implementation evidence was collected — then the verdict is "preliminary," not "signoff."
2. Crossing inventory — table of clock-pair crossings from report_clock_interaction, each marked async / synchronous / handled-by-IP.
3. Baseline counts — Critical / Warning / Info from the first report_cdc (or check_cdc_lint), quoted from tool output.
4. Per-crossing findings — for each Critical/Warning: source→dest clock, width, classification (C1–C9), structural diagnosis, and the action taken or recommended.
5. Changes applied — constraints added (with the exact set_* rationale) and any RTL edits the user approved, each tied to its crossing.
6. Waivers — every set_false_path / set_clock_groups / waive_lint_violation with its explicit stated assumption and whether the user confirmed it.
7. Final verdict with fresh evidence — post-implementation report_cdc counts + report_methodology CDC-rule status (and report_timing_summary for any set_bus_skew). Quote the numbers. If post-implementation evidence is absent, say "NOT signed off" and list exactly what remains.
8. Open items / trade-offs — anything requiring a human decision (RTL approval, ambiguous waiver, structural re-architecture), with options and costs.