# Binary Transport Validation — 2026-08-17

This milestone replaced high-volume device-side CSV with the versioned `TRK1`
binary stream shared by future USB, BLE, and storage paths. Mapped raw sensor
counts remain authoritative. Calibration metadata travels as float32 values, and
the host reconstructs the prior 23-column diagnostic CSV without discarding raw
data.

## Verification layers

- Host compilation of the production C encoder with `-Wall -Wextra -Werror`
- Fragmented C-encoder-to-Python-parser contract test
- Deliberately corrupted CRC test with stream resynchronization
- ESP-IDF firmware build and flash on the assembled ESP32-C3 prototype
- Live USB capture followed by independent offline re-decoding

## USB text-conversion finding

The first live capture exposed that the USB console's default CRLF mode inserted
a carriage return whenever a binary byte equaled LF (`0x0A`). CRC rejected every
affected frame and the parser resynchronized at the next `TRK1` magic. No invalid
sample entered decoded CSV.

Before binary output begins, firmware now changes the USB Serial/JTAG VFS transmit
mode to `ESP_LINE_ENDINGS_LF`, which means no byte modification. Startup logs and
readable metadata are flushed first.

## Final hardware capture

`captures/binary_v1_smoke2.trk` and its decoded CSV contain:

- 7,184 samples over 71.830 seconds
- 912 total frames, including 14 repeated metadata frames
- Packet gaps and resets: 0
- Sample gaps and resets: 0
- Timestamp anomalies: 0
- CRC and header failures: 0
- Queue/read/output drops: 0
- Acquisition-loop overruns: 0
- ADXL345 overruns: 0
- L3G4200D data-ready clear: 0
- L3G4200D overruns: 192

Offline decoding of the saved `.trk` file produced CSV byte-for-byte identical to
the CSV rendered during live capture.

The validated stream occupied 177,184 bytes, or 2,466.7 bytes/s including frame
headers and repeated metadata. That is 8.47 MiB/hour and about 19.7 kbit/s before
BLE link overhead, far below the previous CSV stream.

## Task separation and buffer

The 100 Hz I2C acquisition runs at FreeRTOS priority 10. USB encoding/output runs
at priority 5 and receives samples through a 128-entry queue (about 1.28 seconds
at 100 Hz). The hardware capture's zero timing anomalies and zero loop overruns
confirm that packet encoding, CRC, float metadata, and USB output did not disturb
the acquisition cadence.