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04 - Firmware

Strategy

The flight-critical code should track Open32Drone rather than being reimplemented from scratch in this repo. The current upstream project documents:

  • XIAO ESP32-S3 Sense
  • ESP32 Arduino Core 3.3.6
  • 300 Hz flight-critical main loop
  • 10 kHz / 10-bit motor PWM
  • optional QVGA MJPEG stream in a separate low-priority task
  • MAVLink v2 over UDP 14550
  • Wi-Fi AP fallback
  • current motor GPIOs 4/3/6/5
  • IMU I2C on GPIO2/43
flowchart LR
    subgraph XIAO["XIAO ESP32-S3 Sense"]
        FC["Flight-critical loop<br/>300 Hz"] --> PWM["4x motor PWM<br/>10 kHz / 10-bit"]
        CAM["Camera task<br/>low priority"] --> MJPEG["QVGA MJPEG stream"]
    end
    FC <-->|"UDP 14550<br/>MAVLink v2"| GCS["GCS / phone client"]
    MJPEG -->|"HTTP, 1 viewer max"| Viewer["Video viewer"]

Fetch Open32Drone from its upstream repository (see docs/SOURCES.md) and record the exact revision used in your build log, e.g. git -C vendor/open32drone rev-parse HEAD.

This repo vendors it as the vendor/open32drone git submodule and keeps the V1 config deltas below as a patch rather than edits to the submodule itself — see firmware/open32drone/README.md for the exact build/flash commands and the 2026-09-06 USB-only smoke-test log (camera, Wi-Fi AP, MAVLink transport confirmed working without IMU/motors/battery wired).

Arduino build settings

For the current upstream documentation:

  • Board: XIAO_ESP32S3
  • ESP32 Arduino Core: 3.3.6
  • OPI PSRAM: enabled
  • flash mode: DIO as documented by upstream
  • partition: upstream's documented 8 MB A/B/default partition
  • serial monitor: 115200

Do not casually update the ESP32 core on a flight-tested build. Re-run all prop-off and tethered tests after framework upgrades.

Local V1 choices

The intended V1 policy, to be applied as Open32Drone configuration settings:

  • camera enabled
  • QVGA
  • one viewer maximum
  • MJPEG quality starting point 18
  • MAVLink port 14550
  • optical flow disabled for V1
  • SBUS disabled for V1
  • battery divider factor 2.0
  • low-voltage warning/landing thresholds as initial values only

These are a project manifest, not a guaranteed drop-in upstream header, because upstream changes. Apply the corresponding settings to the current Open32Drone configuration and document the resulting diff in your fork.

Video

Use the upstream bounded background stream. Keep these design rules:

  1. control loop wins over video;
  2. one video client maximum;
  3. QVGA first;
  4. drop frames instead of growing queues;
  5. if loop timing degrades, reduce frame rate/quality before touching flight-control rates;
  6. never use the SD card in V1; D7-D10 are more useful as expansion/RC pins and the card adds I/O load.

The stream URL in current Open32Drone documentation is http://192.168.4.1/stream when using its default AP address.

Default UDP port is 14550. For first testing, use a single GCS/phone client. Do not fly solely by looking at MJPEG video until measured end-to-end latency is acceptable; ESP32 MJPEG is suitable for inspection and slow FPV, not low-latency racing.

Bench-test firmware

firmware/bench_test/main.cpp is intentionally not a flight controller. It performs three safe integration checks:

  • initializes the XIAO Sense camera and captures a JPEG;
  • prints IMU values if a supported test IMU is connected;
  • pulses one selected motor at low duty for 250 ms.

Build with PlatformIO:

cd firmware/bench_test
pio run -t upload
pio device monitor

Remove all propellers before running it.

Note: the bench sketch uses Adafruit MPU6050 for a simple library-based smoke test. If the final board uses GY-91/MPU9250, validate it with Open32Drone's actual IMU backend as part of the upstream bring-up.

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