Test-point insertion

This document describes the add_tp / reject_tp Tcl shell commands. They let faultflow users drive OpenTestability’s one-shot TPI flow from within the interactive shell, compare coverage before and after insertion, and revert if the result is unsatisfactory. These commands are shell-only — there is no batch-CLI equivalent.

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Prerequisites

1. OpenTestability installed. Set the path in config.ofs:

   [testpoint]
   opentest = /path/to/venv/bin/opentest
   metric    = scoap
   threshold = 50
   max_points = 10
   

   If opentest is on PATH you can write opentest = opentest.

2. Baseline sim campaign complete. Run sim (or run_atpg) before calling add_tp. The baseline campaign is the reference against which the TP campaign is compared.

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Workflow

read_netlist design.json -top my_top
use_lib_cells sky130
run_atpg

# Insert test points, re-sim, show comparison
add_tp

# Optionally revert
reject_tp

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add_tp — Insert and compare

add_tp [-m METRIC] [-t THRESHOLD] [-n MAX_POINTS]

Flag	Default	Description
-m METRIC	scoap	Testability metric used by OT for candidate ranking (scoap, cop)
-t THRESHOLD	50	OT’s hard-to-detect threshold (0–100)
-n MAX_POINTS	10	Maximum number of test points to insert

What happens:

1. Calls opentest --yosys analyze_and_add_tp on the current netlist, writing a new *_tp_yosys.json under output/<top>/tp/.

2. Loads the TP netlist (skips re-synthesis — it’s already a Yosys JSON), initialises a new campaign, and runs ATPG.

3. Computes coverage metrics and a “rescued” count: original faults (excluding tp_obs_* / tp_ctrl_* nets added by OT) that were undetected in the baseline but are now detected.

4. Prints a Baseline | +TP | Δ comparison table.

5. Pushes the new netlist onto the version stack and calls checkpoint(). The TP netlist becomes the active version.

Comparison table columns:

Row	Meaning
Test coverage %	Structural test coverage
Fault coverage %	Stuck-at fault coverage
Detected / Denominator / Undetected	Raw fault counts
Original faults rescued	Faults on original nets, undetected in baseline, now detected
Test vectors	ATPG vector count
ATPG / Sim / Total time (s)	Timing breakdown
ATPG rounds	Number of ATPG rounds
Test points inserted (obs/ctrl)	OT’s insertion report

The TP netlist is kept by default. Use reject_tp to revert.

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reject_tp — Revert last iteration

reject_tp

Pops the most recent TP iteration from the version stack and restores the previous netlist as the active source. The campaign database retains both campaigns’ data; only the active netlist pointer changes. Calls checkpoint().

You cannot revert past the original baseline (iter 0). Attempting to do so raises an error.

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Version stack

Each add_tp call creates a new iteration, tracked in output/<top>/tp/state.json:

{
  "versions": [
    {"iter": 0, "netlist_path": "output/my_top/my_top.json",    "campaign_id": 1, "label": "baseline"},
    {"iter": 1, "netlist_path": "output/my_top/tp/my_top_tp.json", "campaign_id": 2, "label": "tp_iter1"}
  ]
}

You can call add_tp multiple times to explore different thresholds or metrics, then reject_tp to step back through the history.

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PDK and TPI cell support

faultflow automatically passes --tech sky130 (or --tech osu035) to OT based on the active cell library. The TPI cells each PDK inserts are fully supported:

Sky130 HD (default)

• Observe: sky130_fd_sc_hd__buf_1 → BUF

• Control: sky130_fd_sc_hd__mux2_1 → MUX2_NI (non-inverting: Y = (~S&A0)|(S&A1))

OSU035

• Observe: BUFX2 → BUF

• Control: MX2X1/MX2X2/MX2X4 → MUX2 (inverting: Y = ~((S0&A)|(~S0&B)))

The active PDK is determined from [design] cell_lib in config.ofs.

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Troubleshooting

Error	Cause	Fix
opentest binary not found	opentest not on PATH or wrong path	Set [testpoint] opentest = /abs/path
opentest failed (exit N)	OT subprocess error	Check OT logs; ensure input JSON has no $_DFF_* primitives (run dfflibmap upstream so FF cells are PDK-mapped)
cannot pop: only baseline remains	reject_tp called with nothing to revert	Run add_tp first
unsupported cell: MX2X1	Old faultflow without M0 patch	Update to current faultflow