Protocol v1 (Legacy)

This document describes the communication protocol used for device configuration and data transmission.

Packet Structure

Bit Range	Name	Bytes	Remarks
[65]	PADDING	1	Padding bytes, always 0
[64:62]	CHECKSUM	3
[61:42]	CHANNEL_N	20
[41:22]	CHANNEL_E	20
[21:2]	CHANNEL_Z	20
[1:0]	HEADER	2

The packet is fixed length of 66 bytes, sending interval is determined by sampling rate, using the following formula:

$$\text{SEND INTERVAL} = \frac{5 \cdot 1000}{\text{SAMPLE RATE}}$$

To get the current sampling rate in receiver end, calculate the time difference between two consecutive packets and using the following formula:

$$\text{SAMPLE RATE} = \frac{5 \cdot 1000}{\text{TIME DIFFERENCE}}$$

HEADER

The packet header is a 2-byte field that marks the beginning of a packet.

It is set to FC 1B in this version of the protocol.

CHANNEL_Z

The CHANNEL_Z field is a 20-byte field that holds 5 samples of Z-axis acceleration data in int32 format.

CHANNEL_E

The CHANNEL_E field is a 20-byte field that holds 5 samples of E-axis acceleration data in int32 format.

CHANNEL_N

The CHANNEL_N field is a 20-byte field that holds 5 samples of N-axis acceleration data in int32 format.

XOR_CHECKSUM

The field is divided into 3 bytes, corresponding to the checksum of CHANNEL_Z, CHANNEL_E and CHANNEL_N respectively.

C example given below:

uint8_t get_checksum(int32_t* array, uint32_t size) {
    uint8_t checksum = 0;
    uint8_t* bytes;

    for (uint8_t i = 0; i < size; i++) {
        bytes = (uint8_t*)&array[i];
        for (uint8_t j = 0; j < sizeof(int32_t); j++) {
            checksum ^= bytes[j];
        }
    }

    return checksum;
}

int main() {
    int32_t channel_z_samples[] = { 10, 20, 30, 40, 50 };
    int32_t channel_e_samples[] = { 60, 70, 80, 90, 100 };
    int32_t channel_n_samples[] = { 110, 120, 130, 140, 150 };

    uint8_t calc_checksum[3];
    calc_checksum[0] = get_checksum(channel_z_samples, sizeof(channel_z_samples) / sizeof(int32_t));
    calc_checksum[1] = get_checksum(channel_e_samples, sizeof(channel_e_samples) / sizeof(int32_t));
    calc_checksum[2] = get_checksum(channel_n_samples, sizeof(channel_n_samples) / sizeof(int32_t));

    // ... other parts ...
}

PADDING

The PADDING field is a 1-byte field that is always set to 0 in this version of the protocol.