The schematic build made partial progress: 19 primary/interface components were added, including the nRF52840 module, MAX-F10S GNSS, BMP581 barometer, 32 MB flash, charger, regulator, USB protection, button, RGB LED, SWD/battery connectors, U.FL test path, and magnetic USB contacts.
Still incomplete
Components are not wired; the project still has zero nets.
Support passives and ceramic antenna integration are missing.
Datasheet checks, power-budget update, ERC review, and fixes remain.
The top-mounted-button enclosure revision was still in progress and was not verified.
This has not reached the schematic-completion checkpoint, so PCB placement, routing, and manufacturing files have not started. Execution stopped because your ACUs ran out.
The enclosure revision completed successfully: the button is now on the sky-facing top surface, outside the ceramic GNSS antenna keep-out. The mechanical brief and enclosure concept were updated accordingly.
The project remains before the schematic checkpoint: the 19 primary components are present but unwired, with support passives, antenna integration, power verification, and ERC still outstanding. PCB placement/routing cannot begin safely yet.
The product is feasible as a four-layer removable logger, but three original assumptions need correction before schematic capture:
600 mAh is not a robust 10-hour cold-weather battery specification. Use 800 mAh minimum; 1000 mAh is preferred.
16 MB flash is marginal at 10 Hz. Use at least 32 MB and log compact binary records rather than continuous NMEA text.
A sealed 10 mm enclosure is mechanically aggressive. A 12–14 mm first prototype is more credible with a dual-band patch, PCB, battery, walls, sealing, and assembly tolerances.
Selected Architecture Direction
Table
Block
Direction
Reason
MCU/radio
nRF52840 module
Low-power BLE Central, post-session BLE Peripheral, USB FS, QSPI, mature wearable ecosystem
GNSS
LC29H-AA-class dual-band 10 Hz module
L1/L5 robustness under forest cover and multipath; lower power than several older dual-band options
Antenna
25 x 25 x 4 mm passive dual-band ceramic patch
Better first-prototype sensitivity and repeatability than an ultra-small patch
Barometer
BMP581
Very low current and suitable temperature range
Storage
32 MB QSPI NOR minimum
Fits 10-hour compact records with filesystem and update margin
Charger
Single-cell charger with power path and NTC
Safe charging/data operation while connected; temperature-aware charge inhibit
3.3 V regulator
TPS62840-class buck
High efficiency across LiPo range and very low standby current
Recording-Mode Power Budget
These are conservative architecture estimates for sizing, not final measured values. GNSS and MCU figures must be revalidated against the exact purchased variants during schematic capture.
Average includes UART parsing, buffering, and BLE connection events
QSPI NOR flash
0.8 mA
25 mA
Writes buffered into page-sized bursts
BMP581
0.08 mA
0.08 mA
75 µA typical in high-resolution 30 Hz operation
RGB LED averaged
0.05 mA
5 mA
Short low-duty flashes only during recording
Regulators, dividers, protection, leakage
0.5 mA
1 mA
Budget allowance
Total 3.3 V rail
29.43 mA
about 80 mA
Peak events are not all simultaneous
Battery Current
Assume a 3.7 V nominal battery and 90% conversion efficiency during recording:
I_battery = (3.3 V x 29.43 mA) / (3.7 V x 0.90) = 29.2 mA
Add 10% design uncertainty:
I_battery_design = 32.1 mA
Energy required for a 10-hour session:
32.1 mA x 10 h = 321 mAh nominal at room temperature
Cold and Aging Derating
For a conservative -10 °C target:
Cold usable-capacity factor: 0.65
End-of-life capacity factor: 0.80
Shutdown/reserve factor: 0.90
Required nominal capacity = 321 / (0.65 x 0.80 x 0.90) = 686 mAh
Battery Requirement
Absolute minimum: 800 mAh protected LiPo with documented low-temperature discharge behavior.
Preferred: 1000 mAh to absorb GNSS variation, battery vendor spread, enclosure cooling, and firmware overhead.
Standard LiPo charging must be inhibited below 0 °C unless the chosen cell's datasheet explicitly allows it.
Runtime below -10 °C is a validation item, not guaranteed from nominal capacity alone.
Regulator Sizing
Approximate active rail peak budget: 80 mA.
Use a regulator rated for at least 250 mA to preserve transient and future margin.
TPS62840 provides up to 750 mA and has a 1.8–6.5 V input range, comfortably covering a single LiPo.
The same device's very low quiescent current makes off-session standby practical.
Storage Budget
Assume one compact record per 10 Hz sample:
Table
Field group
Example bytes
Timestamp and flags
6
Latitude/longitude
8
Altitude and velocity
8
Pressure/temperature
6
Heart rate and status
2
CRC/alignment
2
Total
32 bytes/sample
32 bytes x 10 samples/s x 36,000 s = 11.52 MB per 10-hour session
Allowing for metadata, flash erase-block handling, firmware-update staging, and incomplete-session recovery makes 16 MB too tight. 32 MB is the minimum; 64 MB is preferred if several sessions must remain onboard.
Continuous textual NMEA logging is intentionally excluded because it can multiply the storage requirement. Firmware should parse GNSS output and store a compact binary session format.
Therefore, stacking all major items cannot credibly fit inside a 10 mm sealed enclosure. The preferred layout is a patch above a dedicated PCB ground region, with the battery beside it or beneath only the non-antenna electronics region. The first enclosure target should be 70 x 40 x 12–14 mm.
RF risk
The upper-back location is favorable for sky visibility.
The body and battery can attenuate the 2.4 GHz link to a chest strap; place the BLE antenna at an enclosure edge with no battery or ground between it and the strap-facing direction.
The GNSS patch must face outward and must not be covered by battery, copper, or metallic mounting hardware.
Final antenna performance requires a tuned enclosure/PCB assembly test; ceramic patch center frequency shifts with its ground plane and nearby dielectric materials.
Prototype Validation Gates
Confirm 10 Hz GNSS output and compact message bandwidth.
Measure recording-mode current with a real HR strap connected.
Run 10-hour chamber/freezer runtime tests at 0 °C and -10 °C.
Test HR strap reconnection with the pod mounted on the upper back.
Compare 25 mm and smaller dual-band patches under tree cover.
Validate pressure vent response after splash and condensation exposure.
Verify USB signal integrity through the magnetic cable/contact assembly.
Perform sealing tests separately from electrical functionality; IP67/IP68 cannot be claimed from design intent alone.
Open Risks Before Schematic Freeze
Exact low-temperature battery model and dimensions.
Exact magnetic connector/contact geometry and cable pinout.
Library availability and verified footprint for the selected LC29H variant.
Dual-band patch supplier, tuning data, and required ground-plane dimensions.
Whether the product may grow from 10 mm to 12–14 mm thickness.