Binary Protocol

Single source of truth: host/protocol.py (Python) and firmware/shared/packet_protocol.h (C). Both must stay in sync.

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KB2040 → STM32F405 (70 bytes, UART 921600 baud)

Offset	Size	Field
0	2	Magic 0xBB 0x55
2	4	Sequence number (uint32 LE)
6	4	Timestamp µs (uint32 LE)
10	28	IMU #4 chest: quat w,x,y,z + lin accel x,y,z (float32 LE × 7)
38	28	IMU #5 thigh: same layout
66	4	CRC32 (Ethernet polynomial 0xEDB88320) over bytes 0–65

IMU payload layout (28 bytes, both IMUs)

Offset within IMU	Field
+0	quaternion w
+4	quaternion x
+8	quaternion y
+12	quaternion z
+16	linear acceleration x
+20	linear acceleration y
+24	linear acceleration z

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STM32F405 → PC (214 bytes, USB CDC)

Offset	Size	Field
0	2	Magic 0xAA 0x55
2	4	Sequence number (uint32 LE)
6	4	Timestamp µs (uint32 LE)
10	168	6 × BNO055 payload (28 bytes each, same layout as above)
178	8	MyoWare forearm + back (float32 LE, 0.0–1.0)
186	16	FSR: L heel, L toe, R heel, R toe (float32 LE, 0.0–1.0)
202	4	BMP390 pressure hPa (float32 LE, 0.0 until sensor arrives)
206	4	BMP390 altitude m (float32 LE, 0.0 until sensor arrives)
210	4	CRC32 over bytes 0–209

BNO055 order in the 168-byte block

#	IMU location	Offset in packet
0	Left wrist	10
1	Right wrist	38
2	Left upper arm	66
3	Right upper arm	94
4	Chest	122
5	Thigh	150

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CRC32

Ethernet polynomial: 0xEDB88320 (reflected form). Computed over all bytes before the CRC field. Verify with the standard test vector:

from host.protocol import crc32
assert crc32(b'123456789') == 0xCBF43926

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Magic sync

The host reader scans for magic bytes and resyncs on mismatch:

# host/serial_reader.py
if byte0 == 0xAA and byte1 == 0x55:
    read 212 more bytes
    validate CRC32
    parse frame
else:
    shift window by 1 byte, repeat

False-magic probability: ~1/65536 per random byte pair. CRC32 catches any false-positive sync within the next frame.

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Example parsed frame

seq=1234 ts=56789012
  IMU[0] quat=(0.999,-0.001,0.002,-0.003) acc=(0.01,0.02,-9.80)
  IMU[1] quat=(0.998,0.001,-0.003,0.004) acc=(0.00,-0.01,-9.81)
  IMU[2] quat=(0.997,-0.002,0.001,-0.005) acc=(0.02,0.01,-9.79)
  IMU[3] quat=(0.996,0.003,-0.002,0.006) acc=(-0.01,0.00,-9.80)
  IMU[4] quat=(0.995,-0.001,0.004,-0.002) acc=(0.00,0.03,-9.78)
  IMU[5] quat=(0.994,0.002,-0.001,0.007) acc=(0.01,-0.02,-9.80)
  EMG  fg=0.12  bk=0.34
  FSR  lh=0.45  lt=0.12  rh=0.55  rt=0.18
  BMP  p=0.00  alt=0.00  (placeholder)
  CRC  ok