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Multi-zone ToF hardware selection R0

Date: 2026-08-03

Terminal:

VL53L8CX_DEFAULT_CANARY_VL53L5CX_AVAILABILITY_FALLBACK

Decision

Use the ST VL53L8CX as the default sparse metric-anchor sensor.

  • First bench configuration: X-NUCLEO-53L8A1 + STM32 Nucleo USB bridge.
  • Compact A568 prototype: SATEL-VL53L8 + direct host I2C/SPI, if the board exposes a verified bus and voltage-compatible carrier.
  • Compact phone/USB prototype: SATEL-VL53L8 + small MCU USB bridge.
  • Availability fallback only: VL53L5CX-SATEL; keep the same JSONL and spatial registration contract.

No hardware was purchased by this work. Exact regional stock, total delivered price, carrier voltage levels, and connector availability must be checked at procurement time.

Why VL53L8CX is the balance choice

The official product information gives the properties needed by this branch:

Property VL53L8CX VL53L5CX Relevance
native zones 4x4 or 8x8 4x4 or 8x8 enough spatial support for left/center/right scale coverage
maximum advertised range 4 m 4 m matches the current clearance field's 0.2-4.0 m working range
maximum frame rate 60 Hz 60 Hz well above a sparse-anchor refresh requirement
diagonal FoV 65 degrees 65 degrees materially better than a single-point range source
host interface I2C up to 1 MHz or SPI up to 3 MHz I2C up to 1 MHz L8 offers a lower bus-risk deployment option
continuous-mode example about 215 mW 216-313 mW depending supplies sensor cost is bounded but still requires full-rig measurement
generation emphasis enhanced ambient performance and reduced power earlier wide-FoV generation L8 is the safer outdoor/bright-scene canary

The VL53L8CX does not replace RGB geometry. Its 64 measurements are too coarse to define a complete body clearance field, but they can supply absolute scale, per-zone sigma/status, multi-target information, and freshness to the faster RGB observer. This is exactly the missing variable demonstrated by the consumed replay.

Why the development kit and final carrier differ

X-NUCLEO-53L8A1 is the default first purchase because ST describes it as a complete evaluation kit compatible with STM32 Nucleo Arduino R3 boards and supplies examples and a GUI. It reduces initial electrical and firmware risk, but it is too large for the final wearable form.

SATEL-VL53L8 contains compact breakout boards and is the correct second-stage mechanical candidate. It still needs a verified power/interface carrier. On A568, direct host I2C is simplest for an initial low-rate canary; SPI is preferred when supported and measured because the sensor supports 3 MHz SPI. For a phone, an MCU bridge must translate the sensor bus to USB and timestamp frames into the same host monotonic clock used by RGB.

The official STSW-IMG040 VL53L8CX ULD is C source with an isolated platform layer for low-level bus access. It is the preferred driver basis; the repository adapter remains independent of the vendor API by consuming normalized JSONL.

Initial canary configuration

Start with:

8x8 zones
15 Hz continuous ranging
all reported targets retained by the raw capture
per-zone range + sigma/status
host-monotonic capture timestamp
rigid shared mount with the exact external RGB camera

15 Hz is an engineering starting point, not a selected optimum or deployment policy. The bench must retain raw frames so 5/10/15 Hz scheduling can later be evaluated without changing the spatial registration. Anchor expiry, sigma, scale-MAD, and skew gates remain prospectively chosen hardware parameters.

Minimal acquisition list

  1. One X-NUCLEO-53L8A1 and one compatible STM32 Nucleo board for USB bench capture, or one SATEL-VL53L8 if a known-good host carrier already exists.
  2. The final candidate RGB camera and a rigid two-sensor mount.
  3. A flat high-contrast correspondence target usable from approximately 0.5-4 m and across left/center/right overlap.
  4. A stable power source and, for deployment measurement, an external power meter.

Do not buy a single-zone ToF or ultrasonic module for this branch: neither can establish multi-band correspondence in a multi-obstacle scene. Do not treat the phone proximity sensor as a substitute; the bounded device probe exposed no public multi-zone range stream.

Admission sequence after hardware arrives

  1. Export the exact sensor's zone rays and raw quality fields.
  2. Calibrate the final RGB camera.
  3. Collect multi-distance, multi-zone RGB/ToF correspondences.
  4. Run calibrate_multizone_tof_rgb.py; reject a non-admitted registration.
  5. Capture both streams in one host clock and run the existing sidecar.
  6. Report anchor availability/skew/staleness, clearance gates, end-to-end P50/ P95, memory, sensor-plus-bridge power, and sustained thermals.

The route remains a candidate until these physical measurements exist.

Primary sources