[mark-brannan-agent]

Adds reference/halmet_firmware.md: what the HALMET measures, the engine/boat point for each sensor, and which HALMET input it uses.

• Board integration points, from Hat Labs' docs and the HALMET-hardware schematic (rev 1.0.1), including the 2×10 GPIO header J201. The digital inputs and I2C are galvanically isolated; the analog current source tops out at 300 Ω.
• What firmware/halmet/ does today, with its known gaps (index-addressed probes, −127 °C publishing, alarm.* instead of notifications.*, test pulse on by default).
• Planned: thermocouple on the dry elbow, more DS18B20 points, IR flame on D4, start-battery and alternator voltage, ignition sense, fuel vacuum, derived run hours / engine state / raw-water-loss alarm.
• Cooling: a fresh coolant circuit plus raw seawater joining at the mixing elbow (owner's answer, 2026-10-26).
• Gas sensing (BME688/690, MQ-series) stays on the separate gas-sensor box, per the 2026-09-26 answer recorded in the engine plan.
• A "Considered and not done" table and an Open list.

Decide

• The "Considered and not done" rows are Claude's recommendations, except the gas-box and alternator-tach rows (owner's answers, 2026-09-26). Strike any you want kept open.
• The "read-only on the engine" principle is Claude's draft.

Pencil:

• Location reference/halmet_firmware.md (design rationale lives in reference/). Undo: git mv.
• A4 goes to the fuel vacuum or the oil-pressure transducer, whichever is fitted first; water-in-fuel and bilge-pump run take the second ADS1115. Undo: edit two table rows and one sentence.
• The 110-character table-cell limit is not applied: the rubric that sets it is not in the repo. Undo: reflow the wide rows once it is.

Head: 416bd91

🤖 Generated with Claude Code · claude-sonnet-5-5 · -

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