[MAVLINK] Issue with Checksum / mavlink_message_crcs

[Lauriane C]

Hello everyone,

I'm trying to implement a parser in C++/Qt and my issue lies in calculating the checksum. I found this code that I slightly adapted to my parser :

#define X25_INIT_CRC 0xffff
#define X25_VALIDATE_CRC 0xf0b8

static inline void crc_accumulate(uint8_t data, uint16_t *crcAccum)
{
        /*Accumulate one byte of data into the CRC*/
        uint8_t tmp;

        tmp = data ^ (uint8_t)(*crcAccum &0xff);
        tmp ^= (tmp<<4);
        *crcAccum = (*crcAccum>>8) ^ (tmp<<8) ^ (tmp <<3) ^ (tmp>>4);
}

//Initialize the buffer for the X25 CRC
static inline void crc_init(uint16_t* crcAccum)
{
        *crcAccum = X25_INIT_CRC;
}

//Calculates the X.25 checksum on a byte buffer

//Here index and length help me to go through my QByteArray representing the message, from the second byte (so length_payload) to the last byte of the payload.

static inline uint16_t crc_calculate(QByteArray pBuffer, uint16_t index, uint8_t length, uint8_t message_id) {

        static const uint8_t mavlink_message_crcs[256] = {50, 124, 137, 0, 237, 217, 104, 119, 0, 0, 0, 89, 0, 0, 0, 0, 0, 0, 0, 0, 214, 159, 220, 168, 24, 23, 170, 144, 67, 115, 39, 246, 185, 104, 237, 244, 222, 212, 9, 254, 230, 28, 28, 132, 221, 232, 11, 153, 41, 39, 78, 196, 0, 0, 15, 3, 0, 0, 0, 0, 0, 153, 183, 51, 59, 118, 148, 21, 0, 243, 124, 0, 0, 38, 20, 158, 152, 143, 0, 0, 0, 106, 49, 22, 143, 140, 5, 150, 0, 231, 183, 63, 54, 0, 0, 0, 0, 0, 0, 0, 175, 102, 158, 208, 56, 93, 138, 108, 32, 185, 84, 34, 174, 124, 237, 4, 76, 128, 56, 116, 134, 237, 203, 250, 87, 203, 220, 25, 226, 46, 29, 223, 85, 6, 229, 203, 1, 195, 109, 168, 181, 47, 72, 131, 127, 0, 103, 154, 178, 200, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 34, 71, 15, 0, 0, 0, 0, 0, 0, 0, 163, 105, 0, 35, 0, 0, 0, 0, 0, 0, 0, 90, 104, 85, 95, 130, 184, 0, 8, 204, 49, 170, 44, 83, 46, 0};

        uint16_t crcTmp;
            crc_init(&crcTmp);
        while (length--) {
                crc_accumulate(pBuffer.at(index), &crcTmp);
                index++ ;
        }
        crc_accumulate(mavlink_message_crcs[message_id], &crcTmp);
        return crcTmp;
}


It doesn't match the checksum I have in my packets in tlog files but I don't get why. My guess would be that the mavlink_message_crcs is not right (I'm using mavlink 1.0) but I can't find other values.

Thanks a lot for your help (in advance) !

[Lauriane C]

Just to be more precise about what I'm trying to do.

I have got some .tlog files (so binary code of thousands of mavlink packets) that I want to "decode", meaning store in a Vector of "Packets". The struct Packets that I defined is close to mavlink_message_t : I can store length_payload, message_id etc and the payload but I dont read the payload itselft.

Thus when I want to calculate the checksum I have an issue : this mavlink_message_crcs that I don't have (it is generated from xml files apparently but I only have the one .tlog binary file I told you about).



I wanted to let my parser/lexer aside for a while and try using the one from mavlink C library availiable on github but they define macros for the CRCS as well so it is not useful for me at all.

Hoping someone could help me out,

Lauriane.

[Milan E]

Hello Lauriane,

This is a function that I use to check the CRC, maybe it can be useful to you too. It relies on the crc_accumulate from mavlink's checksum.h.

Hope it helps,
Milan


u_int8_t mavlink_message_crcs[256] = MAVLINK_MESSAGE_CRCS;

// check the received message CRC, return true if OK

bool check_mavlink_crc(u_int8_t *buff_in, ssize_t recsize, u_int8_t msgid) {

    u_int16_t *crc_accum = new u_int16_t(X25_INIT_CRC);

    u_int8_t temp;

    u_int16_t recv_crc;

    for (int i = 1; i < (recsize-2); ++i)

    {

        temp = buff_in[i];

        crc_accumulate(temp,crc_accum);

    }

    // adding the "seed"

    crc_accumulate(mavlink_message_crcs[msgid],crc_accum);

    recv_crc = buff_in[recsize-1]<<8 ^ buff_in[recsize-2];

    //cout << "CRC(recv): " << hex << setw(4) << recv_crc << endl;

    //cout << "CRC(calc): " << hex << setw(4) << *crc_accum << endl;

    // if the CRCs are the same, the subtraction result should be 0:

    recv_crc -= *crc_accum;

    delete crc_accum;

    if(!recv_crc) return true;

    return false;

}

[Lauriane CHILAUD]

Thank you Milan, I solved my problem in the meantime.

Indeed, there was no issue with my code but the mavlink_message_crcs array that I posted only works for messages that are in the Mavlink common message set, which I did not realise. Thus when the id corresponds to a zero value in the array, the checksum would fail because the array does not include the extra seed byte needed. This is fine for what I'm trying to do because I only need the packets with message_id == 33 to get the telemetry data.

Furthermore I did not get that the checksum was written from right to left in the mavlink protocol (big endian) thus my conversion from byte array to int failed as well...

[time.m...@gmail.com]

Hi Lauriane,

I need very similar as you describe, so please can you give me your help.

I want to calculate CRC of the message, example i have need to calculate CRC code for follow message (taken from my xml file):

<message id="3" name="CMN_HEARTBEAT_RSP">                                                                                                                                                                                                                                                                                                                                        
            <descripton>Message for reporting the heartbeat</descripton>                                                                                                                                                                                                                                                                                                                  
            <field type="uint32_t" name="timestamp">Unix time stamp</field>                                                                                                                                                                                                                                                                                                          
            <field type="uint32_t" name="elapsedtime">Elapsed time since component power on</field>                                                                                                                                                                                                                                                                                        
            <field type="uint8_t" name="runningstate"></field>                                                                                                                                                                                                                                                                                                                            
            <field type="uint8_t" name="errorstate"></field>                                                                                                                                                                                                                                                                                                                              
            <field type="uint8_t" name="Uint16_t"></field>                                                                                                                                                                                                                                                                                                                                
        </message>

When i create class files over python scripts it show follow CRC code in correspond class:

#define MAVLINK_MSG_ID_CMN_HEARTBEAT_CRC 172

#define MAVLINK_MSG_ID_3_CRC 172

I am also using C++/Qt source for my application.

My general need is to load each message from xml file and calculate CRC for each of message with my C++/Qt application, so i need application that will received message as string (or other way?) and to return/caluclate CRC number (example 172 for previous example).

Please can you help me or share your ideas/source about how to achieve this?

Thanks in advance,

Time