The hardware · spec digest

Drafted in
the open.

The single source of truth is the markdown specification living beside this site. This page is the working digest: what is decided, and what is still deliberately undecided.

SPEC DRAFT V6 · UPDATED 2026-09-06 · SUPERSEDES V5

01Chassis & mechanical

size / mass
42 × 36 × 9 mm target, 30 g ceiling, provisional until real CAD mass properties existReference: F-91W is 37.5 × 34.5 × 8.5 mm at 21 g, in resin with no radio, sensors, or haptics.
material
CNC-machined aluminum housing, tempered glass front, removable back cover on 4× Torx screws with gasket seal
water / temp
10 ATM (100 m) target, -20 °C to 50 °C operating range
finishes
Natural grey anodize and jet black hardcoat anodize or PVD, sample swatches pending
strap
Quick-release 20 mm proposed, metal link bracelet default, width confirmed against final case CAD
charging
Magnetic spring-loaded pogo pins integrated into one bracelet link, no in-case port, puck ends in a female USB-C receptacle

02Display

panel
~1" rectangular monochrome OLED, single emitter color, dark-background UIChanged in v6 from e-paper. Panel selection, interface, and driver IC are open item 12.
mounting
Rigid internal carrier, soft gasket mount inside the case for impact isolation, drop-test validation pending
update policy
Frame-addressable, no ghosting or refresh cycles. Static faces redraw on state change only, so the policy is a power decision now, not a panel limit
daylight
Matte AR stack on the glass, maximum-contrast 1-bit UI, peak-brightness burst on wrist-raise
burn-in
Dim always-on state, brighter raise state, occasional pixel-shift for long-lived static elements

03Compute

mcu
Nordic nRF52840
storage
1 MB onboard flash, roughly 400 to 470 KB real single-image budget after dual-bank OTA overhead

04Sensors

imu
6-axis accelerometer + gyroscope, candidate for tilt-to-wake with threshold tuning against false triggers
heart rate / spo2
Optical sensor, tap-to-measure or logging mode, exposed over standard BLE GATT services for third-party app compatibility
excluded
Barometer, altimeter, magnetometer. Out until they pay rent.

05Input

layout
Two pill buttons with one rotary encoder crown between them, all contactless: Hall-effect sensing for buttons, magnetic rotary sensing for the crownNo shaft or actuator penetrates the waterproof cavity.
gestures
Single, double, and hold presses per button; rotate, click, and hold on the crown. Each paired with an LRA tap or a chime.
mode cycle
Date & Time → Stopwatch → Countdown / Interval → Alarms → HR / SpO2 → back. Set Time lives outside the loop, on a long press from the home mode.

06Haptics & audio

haptics
LRA with closed-loop driver, feel benchmarked against the Apple Watch Taptic Engine
audio
Piezo buzzer with tone-sequencer chimes suited to its narrowband resonance, placement under review away from the back panel
silent mode
Global haptic-only mode under consideration

07Timing

resolution
True 1/100 s via hardware timer, edge-triggered interrupts latching a free-running counter, no polling, no debounce needed
display
Live subsecond digits are possible on OLED. Live versus reveal-on-lap is now a taste and power choice, open item 11.

08Firmware

watchfaces
Custom watchface UI compiled into a single firmware image, with SDK templates and a web-based build tool planned
updates
Distribution over BLE Secure DFU via Web Bluetooth, OS-agnostic, with internal SWD recovery pads as the physical failsafe

09Power

target
30 days including active notification serving, room-temperature baselinev6 caveat: budget to be re-derived for the emissive panel. Runtime now hinges on the always-on dim level and raise-to-bright frequency.
cells
CR2032 for bench bring-up only, non-rechargeable, never on the charge circuit. Production: rechargeable thin LiPo pouch shaped to the case cavity.
Deliberately undecided

Open questions.

The spec keeps a numbered list of pending decisions. These are the ones currently shaping the next prototyping loop. The full list of twenty lives in the markdown.

  1. OLED panel selection: a one-inch-class rectangular monochrome module, interface, driver IC, long-term availability
  2. Brightness strategy: always-on dim level, raise-to-bright peak, measured current draw at each
  3. Burn-in mitigation details: pixel-shift cadence and which static elements are exempt
  4. Piezo relocation off the back panel, with a real listening test
  5. Magnetic pogo-pin connector: contact count, spacing, pull strength on the bench
  6. Hall-effect interference: confirming the charging puck magnet never false-triggers a button
  7. Interaction state diagram: the full gesture-to-action map across all modes
  8. Pill buttons: simple single-actuation versus two-way rocker
  9. Stopwatch rendering: live subsecond digits versus reveal-on-lap
  10. Games: the deliberate include or exclude call against the calm-tech thesis

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