• Four identical beltpack units are worn on a belt or behind the trousers during church services.
• Operators use common Kenwood K1-style single-ear surveillance headsets with inline external PTT.
• Users on the same logical channel hear all currently transmitting users on that channel.
• The system should operate without permanently required base-station hardware; if the radio protocol requires a coordinator, any identical beltpack should be able to take that role.

• Receive audio continuously from active peers.
• Transmit microphone audio only while external headset PTT is asserted.
• Support bidirectional operation and more than one simultaneous talker.
• Mix received voice streams locally into one mono earpiece output.
• Automatically reconnect only to previously paired beltpacks.
• Recover when a beltpack leaves, powers off, or loses coverage.
• Investigate multi-hop or mesh-style forwarding, while not claiming support unless validated.
• Prioritize predictable speech latency, reliability, intelligibility, and coverage over music-grade audio quality.

• DECT NR+ radio direction using Nordic nRF9151 as the initial candidate, while comparing alternate DECT/DECT NR+ SoCs before final radio commitment.
• Replaceable 1880-1900 MHz external antenna path with 50 ohm feed, pi matching footprint, RF conducted-test provision, and antenna keepout.
• Kenwood K1 two-jack headset interface with electret mic bias, PTT detect, earpiece output, ESD, RF filtering, protection, and pinout-variation options.
• 1.3 inch monochrome OLED.
• Incremental rotary encoder with push switch for volume and mute.
• Physical 4-position or 6-position rotary switch for logical channel selection.
• Menu/info pushbutton.
• Fully disconnecting battery power switch where practical.
• USB-C charging and service/debug access where useful.
• Removable protected 18650 Li-ion cell.
• Development headers, test points, current-measurement jumpers, and debug access.

Diagram

• Radio: nRF9151 DECT NR+ candidate with EU band operation, conducted-test provision, pi matching network, and replaceable external antenna interface; do not assume nRF9151 is final until alternate DECT/DECT NR+ SoCs are compared.
• Application/audio processing: do not assume a second MCU is required; first evaluate whether the selected radio SoC can run codec, encryption, OLED UI, routing, packet scheduler, and mixing.
• Audio: prefer digital audio after the microphone preamp where practical; include low-power codec or discrete audio chain with electret microphone input, programmable gain, mic bias, mono earpiece driver, sidetone support, protection, RF filtering, and test points.
• Power: USB-C input protection, USB-C advertised-current detection, charger, power-path management, removable protected 18650 cell, fuel gauge, NTC monitoring, system rail, 3.3 V digital rail, filtered analog/audio rail, and possibly dedicated radio rail.
• UI: OLED, volume/mute encoder, logical-channel rotary switch, menu button, power switch.
• Debug/test: SWD, UART log, reset/recovery/boot access, RF test point, audio test points, rail test points, and subsystem current jumpers.

• External antenna: mechanically robust locking SMA-class replaceable antenna interface for the European DECT NR+ band; reserve at least 25 mm clearance between antenna and battery.
• Headset: Kenwood K1 two-pin accessory interface using 3.5 mm and 2.5 mm jacks at approximately 11 mm spacing; exact accessory wiring must be measured before production.
• USB-C: 5 V sink for charging and optional service/debug data; do not assume USB-PD voltages unless a PD sink controller is deliberately added.
• Battery: removable protected 18650 cell with temperature sensing, undervoltage protection, reverse-insertion mitigation, and hot-swap validation.
• Programming/debug: SWD and UART logging headers.

• Source: one removable protected 18650 Li-ion cell.
• Runtime target: at least 6 h, preferably 10-12 h under realistic intercom use.
• Charging: device remains operational from USB-C power while safely charging the cell.
• USB-C input: support advertised 5 V Type-C current levels; do not draw more than the detected source capability.
• Revision A default charge current: 1.0 A until the approved cell, enclosure, NTC placement, and charger layout are validated.
• Revision A normal charge-current option: 1.5 A is allowed only for an approved cell and validated thermals.
• Revision A engineering-only charge-current option: 2.0 A may be tested only after confirming cell datasheet support, USB-C 3 A advertisement or equivalent input margin, enclosure thermals, and simultaneous system load behavior.
• BQ25895 safe-default requirement: charger shall not be allowed to charge at its 2.048 A reset/default fast-charge current before firmware has configured safe settings.
• Conservative Phase 1 recommendation remains: use switch-mode charger/power path for charge currents above about 1 A; linear chargers at 1.5-2 A are thermally risky from 5 V.
• Phase 2 validated battery-management direction: BQ25895-class 1S switch-mode charger/power path, MAX17055-class fuel gauge with current sensing, and TUSB320LAI-class USB-C current detection.

• Preferred charger / power-path IC: BQ25895RTWT.
• Preferred fuel gauge: MAX17055ETB+T.
• Preferred USB-C current detector: TUSB320LAIRWBR configured as UFP/sink.
• Preferred MAX17055 sense resistor: 10 mΩ, 1% or better, 0.25 W minimum, 0.5 W preferred, 4-terminal/Kelvin preferred.
• BQ25895 CE or equivalent control shall default to charge-disabled until firmware configures safe charger settings.
• BQ25895 ILIM shall provide a conservative hardware input-current ceiling; default population around a 1.5 A ceiling is preferred, with a documented test option if higher-current validation is needed.
• Firmware shall set BQ25895 charge current, charge voltage, input current limit, watchdog behavior, and high-voltage adapter behavior before enabling charging.
• Firmware shall use TUSB320LAI OUT1/OUT2 or I2C current-detect result to cap BQ25895 input current.
• MaxCharge/HVDCP high-voltage adapter behavior shall remain disabled unless deliberately adopted later.
• BQ25895 TS shall connect to a 10 kΩ NTC thermally coupled to the removable cell or cell compartment; charging shall suspend outside the approved cell temperature range.
• For a 103AT NTC with a 0°C to 45°C Li-ion charge window, the BQ25895 datasheet example uses RT1 = 5.21 kΩ and RT2 = 29.87 kΩ; final values must be checked against the selected NTC and mechanical placement.
• MAX17055 temperature shall use an external thermistor if practical; otherwise internal-temperature use must be documented as a Rev A SOC-accuracy limitation.
• Do not directly share one NTC node between BQ25895 and MAX17055 unless both bias schemes are verified.
• The design shall handle USB plugged with no battery, battery insertion while USB is plugged, battery removal while USB is plugged, deeply discharged/protector-open cells, protected-cell trip/recovery, I2C failure, MCU reset, TUSB320LAI fault, and NTC open/short.
• Reverse-insertion mitigation is required through mechanical holder choice, electrical protection, or both.

• Revision A: professional PCB assembly, conservative spacing, accessible probing, not optimized for size.
• PCB manufacturer: JLCPCB.
• Use JLCPCB standard capabilities: Shengyi S1000-2M, 6 layers, 1.6 mm nominal standard stackup unless mechanical design changes it, ENIG, epoxy-filled/capped through vias where required and available as a standard option, horizontal electroless copper plating, and controlled impedance.
• No exotic PCB technologies: no blind vias, no buried vias, no HDI microvias, and no unusual material/copper processes unless explicitly justified and approved.
• Standard JLCPCB 6-layer stackup options from the user's reference include JLC06161H-3313, JLC06161H-7628, JLC06161H-1080, JLC06161H-2116A/B, JLC06161H-3313C/D/E, JLC06161H-1080A/B, and JLC06161H-7628A/B; final selection happens during layout setup using the JLCPCB impedance calculator.
• Use available, non-obsolete components with documented datasheets and reference circuits.
• Provide mounting holes and reinforcement around user-force points: USB-C, antenna, headset jacks, rotary controls, and belt-clip area.

• Unique device identity and persistent trusted-peer credentials.
• Network identity, authenticated pairing, encrypted voice/control data, and replay protection.
• Physical pairing and lost-device removal flows.
• Codec support, jitter buffering, packet loss concealment, local audio mixing, sidetone, AGC/noise suppression/high-pass filtering, output limiting.
• Coordinator election and recovery logic if using FT/PT star networking.
• Diagnostic display pages for peer list, link quality, battery/runtime, firmware version, device/network IDs, radio diagnostics, and errors.
• Battery firmware shall log MAX17055 voltage, current, SOC, capacity, temperature, and alerts.
• Charger firmware shall log BQ25895 charge state, fault state, input DPM, thermal regulation, TS state, input current, charge current, VBUS, SYS, and BAT readings where available.
• USB-C firmware shall log TUSB320LAI attach state, orientation if used, and advertised current mode.
• Firmware shall never enable 2.0 A charging as a normal user mode until validation passes.

• Belt-worn plastic enclosure assumed until RF/mechanical analysis says otherwise.
• Edge-accessible antenna, Kenwood K1 jacks, USB-C, power switch, volume encoder, channel selector, menu button, OLED, and battery access.
• Keep antenna isolated from battery, OLED, USB-C, audio, and the user's body as much as practical.
• Reserve at least 25 mm clearance between antenna and battery.
• Separate RF, digital, power, and analog audio placement zones.
• Battery holder/enclosure shall support protected button-top 18650 length class, nominally around 69-70 mm depending final cell.
• NTC placement shall thermally track the cell, not only the charger PCB.

• Phase 1 recommendation: proceed only as a four-node one-hop prototype with elected coordinator fallback, not as a guaranteed mesh product.
• Nordic nRF9151 remains a valid DECT NR+ candidate, but current public Nordic implementation does not directly provide mesh/multi-hop routing.
• Do not assume nRF9151 is final or that a second MCU is required; compare available DECT/DECT NR+ SoCs and evaluate whether the selected radio SoC alone can run codec, encryption, OLED UI, routing, packet scheduler, and mixing.
• Preferred codec ranking is Opus, then LC3, then ADPCM, then PCM.
• Mechanically preferred antenna direction: locking enclosure-mounted bulkhead SMA with internal coax/U.FL-to-board connection and strain relief; board-edge SMA may be acceptable for a reinforced development prototype.
• Phase 2 battery-management decision: use MAX17055ETB+T as the preferred fuel gauge, BQ25895RTWT as the preferred charger/power-path IC, and TUSB320LAIRWBR as the preferred USB-C advertised-current detector for Rev A.
• Keep MAX17048 as the simpler fallback gauge; avoid BQ25792/BQ25672/MP2731 for Rev A unless requirements change.
• Treat USB-C 5 V / 3 A as an available advertised source capability, not as a default draw permission.

• European operation in the 1880-1900 MHz DECT allocation.
• Four beltpacks in the first system.
• One mono earpiece and external headset PTT; no internal speaker, buzzer, or vibration motor.
• Protected removable 18650 cell around 3000-3500 mAh nominal capacity for initial runtime estimates.
• Production accessory pinout cannot be assumed; K1 headset variants must be measured.
• RF range, antenna tuning, thermal behavior, codec CPU load, and DECT NR+ latency must be measured before production claims.
• Battery-management validation assumes a 4.2 V-class 1S Li-ion protected 18650; final charge current depends on the approved cell datasheet.

• Initial specification created from the user's Phase 1 design prompt.
• Project renamed from the generated draft name to DECT NR+ Intercom Beltpack.
• Phase 2 battery-management constraints added: JLCPCB standard 6-layer stackup assumptions, no exotic PCB technologies, MAX17055 fuel gauge direction, BQ25895 charger/power-path direction, and conservative charge-current policy.
• Battery-management validation completed before schematic capture: added TUSB320LAI USB-C current detection, BQ25895 safe-default requirements, MAX17055 10 mΩ Kelvin-sense requirements, NTC/cell-temperature requirements, protected-cell hot-swap/reverse-insertion validation, and no-2.0-A-charge-without-validation rule.