CRC Calculation

github link: https://github.com/atomsenses/atomsenses-decoder/blob/main/CKC_Calculation.md

Command Example:

1. The header command follows a fixed format: 0xFF 0x00.

2. The command byte is a special function byte. 0xA0 indicates that the current command is used to modify the data upload period.

3. The data bytes represent the adjusted period size in seconds. For example, 0x012C indicates that the period is to be adjusted to 300 seconds.

4. The last two bytes are the checksum following the Modbus protocol, with the low byte first, indicating that the protocol is in big-endian mode.

CRC Verification Explanation:

1. Initialize a CRC register in hexadecimal format with the value 0xFFFF.

2. XOR the first 8-bit binary data (here 0xFF) with the low byte of the register, and store the result (0xFF) in the register.

3. Shift the contents of the register right by one bit, padding the high bit with 0, and check the shifted-out bit.

4. If the shifted-out bit is 0, repeat step 3. If it is 1, XOR the register with the polynomial 0xA001 (1010 0000 0000 0001).

5. Repeat steps 3 and 4 until 8 shifts are completed, ensuring all 8 bits of the data are processed.

6. Repeat steps 2–5 for the next byte in the data frame.

7. After processing all bytes, swap the high and low bytes of the resulting 16-bit CRC register.

C Sample Code:

#include <stdio.h>

int main(void) {

    // Tx:000000-01 03 00 00 00 0A C5 CD

    unsigned short tmp = 0xffff;

    unsigned short val = 0;

    unsigned char buff[6] = {0};

    buff[0] = 0x01;

    buff[1] = 0x03;

    buff[2] = 0x00;

    buff[3] = 0x00;

    buff[4] = 0x00;

    buff[5] = 0x0A;

    for (int n = 0; n < 6; n++) {

        tmp = buff[n] ^ tmp;

        printf("XOR result: %x\n", tmp);

        for (int i = 0; i < 8; i++) {  /* 8 refers to the 8 bits in each char type */

            printf("Iteration: %d\t\tLSB: %d", i * n, tmp & 0x01);

            printf("\tCurrent tmp: %x\n", tmp);

            if (tmp & 0x01) {

                tmp = tmp >> 1;

                tmp = tmp ^ 0xa001;

            } else {

                tmp = tmp >> 1;

            }

        }

        printf("CRC after processing data %d: %x\n", n, tmp);

    }

    /* Swap the high and low bytes of the CRC checksum */

    val = tmp >> 8;

    val = val | (tmp << 8);

    printf("After swapping: %X\n", val);

    return 0;

}