CRC check for received data through UART

[sireevenkat1]

Hi,

I am sending/receiving data using UART between STM32G4 board and other board. While I am sending the data I am adding the CRC and transmitting it through UART to other board. While receiving how do I check the CRC correct or not.

For Eg: From other board to my STM32G4 board I recivedata frame is as below

header	Length	Command	Data	CRC	EOF
0x32	0x05	0x17	0x00/0x01/0x02		0x89

how do I check the CRC is correct or not in the coding?

Do I need to calculate crc for all 3 possible data values and compare the possible crc with received crc?

is this correct way. Can anyone suggest how to check the CRC.

Thanks

[LCE]

Transmitter and Receiver should use the same algorithm for calculating the CRC, only that the receiver compares the result to the CRC sent by the transmitter.

[Tesla DeLorean · Accepted solution]

You'd need the details of the polynomial used, align your frames as received and compute the correct value for the data received, and compare that.

Write a CRC computation that can take the data a byte at a time, the packet format looks to have flexibility, so all packets are not the same size.

Link to documentation for the protocol. Find a number of valid examples for packets so you can valid the computation based on polynomial, shift direction, initial values, etc.

[Karl Yamashita]

Here is code that I use for a simple CRC. You'll have to pass the Command, Data and CRC when validating and just Command and Data when calculating the CRC. 

/*
 * Description: Validate checksum using modulo 256
 * Input: the data array and the length of array including the checksum
 * Output: true if valid checksum else false
 *
 */
bool ValidateChkSum(uint8_t *data, uint8_t len){
    uint32_t chksum = 0;
    uint32_t i = 0;
    for(i = 0; i < len - 1; i++){
        chksum += *data++;
    }
    if((chksum % 256) != *data){
        return false;
    }
    return true;
}

/*
 * Description: Calculate checksum using modulo 256 and adds checksum to array at last index + 1
 * Input: The data array and the length of array minus the checksum index
 * Output:
 *
 */
void CalculateChkSum(uint8_t *data, uint8_t len){
    uint32_t chksum = 0;
    uint32_t i = 0;

    for (i = 0; i < len - 1; i++)
    {
        chksum += *data++;
    }
    *data = (chksum % 256); // copy checksum to last index
}

[Tesla DeLorean]

>>Do I need to calculate crc for all 3 possible data values and compare the possible crc with received crc?

No you'd compute on-the-fly, otherwise the table of possible values for 3 bytes would run to 16 million..

Typically it would include the length and command fields, so you can fully validate the whole content of the messages/packet.

Provide a cite to the specific protocol you are using here.

>>Can anyone suggest how to check the CRC.

Have some documentation showing known working examples, or collect some valid / good data from the serial port to inspect and test against.