Dft

Skill: Design for Test (DFT)

Invocation

• If invoked by a user presenting a DFT task: immediately spawn the digital-chip-design-agents:dft-orchestrator agent and pass the full user request and any available context. Do not execute stages directly.
• If invoked by the dft-orchestrator mid-flow: do not spawn a new agent. Treat this file as read-only — return the requested stage rules, sign-off criteria, or loop-back guidance to the calling orchestrator.

Spawning the orchestrator from within an active orchestrator run causes recursive delegation and must never happen.

Pre-run Context

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

1. memory/dft/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/dft/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

Guide the complete DFT flow from architecture planning through ATPG pattern generation, BIST insertion, JTAG setup, and sign-off. Ensures the manufactured chip meets quality targets (fault coverage and DPPM).

Supported EDA Tools

Open-Source

• Yosys DFT plugins (yosys) — basic scan insertion for open-source flows
• OpenROAD DFT utilities (openroad) — scan insertion within the OpenROAD/ORFS flow

Proprietary

• Synopsys TetraMAX ATPG (tmax, dialect synopsys) — pattern generation, fault simulation, and compression
• Cadence Modus Test (modus, dialect cadence) — ATPG, scan DRC, and diagnosis
• Siemens Tessent (tessent, dialect siemens) — full DFT suite: scan, ATPG, MBIST, IJTAG

Stage: dft_architecture

Domain Rules

1. Scan architecture: full-scan preferred for ASIC; capture all sequential elements
2. Scan chain count: √(total flip-flops) as rule of thumb; balance test time vs routing
3. Chain length balance: ±5% of target length across all chains
4. Compression: EDT/OPMISR for designs > 1M FFs to reduce ATE test time
5. MBIST: one controller per memory group (same width/depth class)
6. JTAG: IEEE 1149.1 TAP controller; boundary scan for all IO pins
7. At-speed test: launch-on-capture (LOC) or launch-on-shift (LOS) — agree with test team
8. Test modes: scan_mode, mbist_mode, jtag_mode must be mutually exclusive
9. Power domains: scan must respect UPF power domain boundaries
10. Run the scan-readiness audit below on the incoming design before planning chains. A blocker found here costs an RTL edit; the same blocker found at Scan DRC costs an insertion run first
11. Any DFT logic delivered as RTL rather than inserted by a tool (TAP controller, BIST controller, test-mode muxes, wrappers) is synthesisable RTL: it follows the rtl-design skill's coding rules and passes lint with 0 errors before it is integrated

Scan-Readiness Audit (before insertion)

Check the RTL or pre-DFT netlist for constructs that block scan. Each is owned by the RTL flow, not by scan insertion:

• RTL-generated clocks (assign gclk = clk & en;) — gated flops are unreachable in scan mode
• Clock-gating cells with no test-enable (test_en / scan_en) OR-ed into the enable
• Asynchronous set/reset driven by internal logic with no test-mode bypass — it cannot be held inactive during shift
• Latches other than deliberate lockup latches and clock-gate cell internals
• Combinational feedback loops — ATPG cannot model them
• On-chip tri-state buses — contention is untestable
• Memories with neither a bypass path nor a BIST wrapper (a FIFO's storage array is a memory)
• Black boxes and analog interfaces with no defined test-mode behaviour

Report each finding with the instance and the rule above. A finding that insertion can repair (a test point, a tool-inserted reset or clock mux) is planned into scan_insertion; one that needs an RTL change is handed back to the RTL flow — do not edit the RTL here.

DFT IO Signals Required

• scan_en (SE): primary input, must be controllable from ATE
• scan_in[] (SDI): one per chain
• scan_out[] (SDO): one per chain
• test_clk: separate from functional clock or gated version

QoR Metrics to Evaluate

• DFT spec completeness: all elements defined before insertion
• Scan-readiness audit: 0 findings that need an RTL change left open
• Estimated fault coverage: analytical pre-insertion estimate ≥ target
• Estimated test time: within ATE budget

Output Required

• DFT architecture document
• Scan-readiness audit report (finding, instance, owner: insertion or RTL)
• Scan chain plan (count, estimated length, IOs)
• Test mode definitions

Stage: scan_insertion

Domain Rules

1. Replace all standard FFs with scan-equivalent cells (SDFF, SDFFRQ, etc.)
2. Exclude from scan: memory-mapped registers, MBIST controllers, JTAG cells
3. Do not place scan in: clock gating enables, async set/reset paths (without care cells)
4. EDT compression: insert compressor/decompressor for > 100K FFs
5. Lockup latches: insert between chains crossing clock domain boundaries
6. Scan re-ordering: minimise routing wirelength (use placement-aware reorder)
7. Test points: add controllability/observability points for low-coverage nets
8. Classify every Scan DRC error by owner before retrying. An error insertion can repair (chain connection, test point, tool-inserted reset or clock mux, lockup latch) is retried here. An error caused by the design itself — a generated clock, an uncontrollable asynchronous reset, an unintended latch, a combinational loop — is not changed by running insertion again: stop and hand it back to the RTL flow with the instance and the rule
9. A fix must not trade coverage for a clean report: do not clear a Scan DRC error by excluding the offending flops from scan, or raise ATPG coverage by reclassifying faults as untestable, unless the exclusion is justified and recorded

Scan DRC Rules (all must pass before ATPG)

• No clock signals feeding into scan data path
• No combinational feedback loops through scan
• Scan enable is glitch-free during functional mode
• All scan FFs: correct SI/SE connections

QoR Metrics to Evaluate

• Scan FF count: 100% of sequential elements minus explicit exclusions
• Chain count and length: per architecture spec (±5%)
• Scan DRC: 0 errors

Output Required

• Scan-inserted netlist
• Scan chain definition file (.scandef)
• Scan DRC report

Stage: atpg

Fault Model Targets