ECG Data Format

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

Frame Layout

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

• measurement_type: 0x00 (ECG)
• timestamp_bytes: 8-byte little-endian nanosecond timestamp
• frame_type: frame encoding type (typically 0x00 for uncompressed)
• sample_data: sequence of 3-byte ECG samples

Sample Encoding

Each ECG sample is a 24-bit signed integer in little-endian byte order, representing microvolts (µV).

byte[0] = LSB
byte[1] = middle byte
byte[2] = MSB (bit 7 = sign)

To decode:

1. Read 3 bytes as an unsigned 24-bit value: raw = b[0] | (b[1] << 8) | (b[2] << 16)
2. Sign-extend from 24 bits to 32 bits: if bit 23 is set, raw |= 0xFF000000
3. The resulting int is the sample value in microvolts

Typical Parameters

At 130 Hz with 3 bytes per sample, each notification frame carries multiple samples. The exact count depends on the negotiated ATT MTU.

Notes

• The 14-bit resolution setting at the PMD level does not change the wire encoding; samples are still transmitted as 24-bit values. The resolution setting controls the internal ADC configuration on the device.
• A negotiated MTU of at least 232 bytes is recommended to avoid truncation.