docs(parts): explain the PR #221 internal-trigger firmware path

[HuggeK]

What does this PR do?

Follow-up to #273, docs only, one file (docs/get-started/parts.md).

• Explains the PR feat(ops243): add internal hardware trigger #221 internal-trigger path. New "Internal Trigger Instead (PR feat(ops243): add internal hardware trigger #221)" subsection under Sound Trigger. It says what feat(ops243): add internal hardware trigger #221 changes (the OPS243-A fires the rolling-buffer capture itself, so the SEN-14262, its resistor and its wiring are not needed), that the firmware it needs is OPS243-A 1.3.2 and that every OPS243-A can be brought to it, and how to check what you have (?V over a serial terminal). Then the two cases:
  • Reports 1.3.2 or later: nothing to buy. OmniPreSense's 2026-09-10 message on feat(ops243): add internal hardware trigger #221 says 1.3.2 went onto sensors shipping from that build on (1.3.1 had gone to some earlier customers with a late bug).
  • Reports 1.3.1 or older: you flash it yourself per OmniPreSense's AN-013: SEGGER J-Link on the keyed J2 JTAG header (not the J3 UART header OpenFlight wires to), Infineon's free XMCFlasher in SWD mode with XMC4500-1024 selected (XMC4700 if ?P reports it), the 1.3.2 hex requested from customerservice@omnipresense.com, never Erase, and the Windows caveat that the J-Link driver installer must run as administrator with the legacy USB driver ticked.
• J-Link EDU Mini row. SEGGER J-Link EDU Mini (Adafruit 3571, ~$76) with the tokenised Mouser link from the shared project plus Amazon and Adafruit links, so the cost of that route sits next to the $18 detector. The box includes the 9-pin 0.05" Cortex cable that fits J2; non-commercial licence noted.
• Cost Summary. The closing sentence now says the Sound Trigger line drops out for a radar that already reports 1.3.2, and that an older unit costs the ~$76 debugger instead. The paragraph that itemised the complete-build line (X1202 cells, what the total leaves out) is removed; the table carries that.

Why was this required?

#273 said only that PR #221 "requires OPS243-A firmware 1.3.2" and that the sound trigger becomes optional "if your OPS243 can be updated". That left a builder without the two facts that decide the purchase: whether their radar already has 1.3.2 (new units do) and what it costs if it does not (a ~$76 debugger and a vendor procedure, with the hex only available from OmniPreSense support). Without this, someone could buy the detector they do not need, or plan on the internal trigger without knowing about the debugger.

Sources

• OmniPreSense AN-013-D Code Update: J-Link, XMCFlasher, J2 on the OPS243, SWD not JTAG, XMC4500-1024, hex from customer service, do not press Erase.
• OmniPreSense's 1.3.2 shipping statement as relayed on #221; the XMC4700 note on #221; the EDU Mini upgrade report and driver caveat on #221.
• Adafruit 3571 listing for the price and box contents; the Mouser token from the shared project page (Mouser 485-3571).

Automated tests

None added: one Markdown file, no code. The existing CI on the PR (docs build, lint, format, Python tests, UI build and e2e) covers it.

Manual (human) testing

• Read AN-013-D in full and cross-checked the header, MCU and Erase warning against the feat(ops243): add internal hardware trigger #221 thread.
• Confirmed OmniPreSense's September 2026 AN-010-AF only tables the OPS243-A up to 1.3.1, so the 1.3.2 shipping statement is cited to OmniPreSense's own message on feat(ops243): add internal hardware trigger #221 rather than to a datasheet.
• Took the Adafruit 3571 token from the saved shared-project page and checked the row's other links resolve to the same part.
• Grepped the edited page for the new anchor and read the rendered section back.
• Not verified by hand: Mouser stock (Mouser blocks fetches; the project page showed one unit), and the Amazon listing's current price.

Checklist

• Single feature/fix — one subsection and its cost-summary sentence
• Automated tests included — docs only
• Manual testing described — above
• Python tests pass (uv run pytest tests/ -v) — in CI on this PR
• Pylint passes (uv run pylint src/openflight/ --fail-under=9) — in CI on this PR
• Ruff passes (uv run ruff check src/openflight/) — in CI on this PR
• UI builds (cd ui && npm run build) — in CI on this PR
• UI lint passes (cd ui && npm run lint) — in CI on this PR
• Updated docs or CHANGELOG if needed
• No unrelated changes mixed in

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