Accelerometer Data Format

Accelerometer data arrives on the PMD Data characteristic as notification payloads.

Frame Layout

[measurement_type] [timestamp_bytes (8)] [frame_type] [sample_data...]

• measurement_type: 0x02 (Accelerometer)
• timestamp_bytes: 8-byte little-endian nanosecond timestamp
• frame_type: 0x01 for uncompressed, 0x02 for compressed delta encoding

Uncompressed Samples (Frame Type 1)

Each sample consists of three 16-bit signed little-endian integers representing acceleration in milliG along the X, Y, and Z axes:

[x_lo] [x_hi] [y_lo] [y_hi] [z_lo] [z_hi]   // 6 bytes per sample

Total samples per frame = (payload_length - 10) / 6 (subtracting the measurement type byte, 8 timestamp bytes, and frame type byte).

Compressed Delta Encoding (Frame Type 2)

This format packs more samples into a single notification by encoding deltas relative to a reference sample.

Structure

1. Reference sample: 3 × 16-bit signed LE values (X, Y, Z) = 6 bytes
2. Delta block header:
   • Number of samples (variable)
   • Bit width for each axis
3. Bit-packed deltas: each delta is a signed integer of the specified bit width, packed sequentially

Decoding

1. Read the reference sample (first X, Y, Z values).
2. For each subsequent sample, read signed deltas of the declared bit width and add them to the previous sample's values.
3. Clamp each axis value to the Int16 range (−32768 to 32767).

Units and Conversion

Notes

• The accelerometer range affects the scaling. At ±2 G the full 16-bit range maps to ±2000 mG.
• Compressed frames are more common at higher sample rates (50 Hz, 100 Hz) where the device needs to fit more samples per notification.