I&#39;m working on a library to provide simple reliable communication over an RS232 or RS485 connection.  Part of this code involves using a CRC16 checksum on the data to detect corruption from line noise.  I&#39;ve created a function to calculate a CRC16 checksum, but it doesn&#39;t seem to be outputting correct values.

The relevant code I&#39;ve written is below (it can also be found [here][1]).

    #include &lt;stdint.h&gt;
    
    #define CRC16 0x8005
    
    uint16_t gen_crc16(const uint8_t *data, uint16_t size)
    {
        uint16_t out = 0;
        int bits_read = 0, bit_flag;
    
        /* Sanity check: */
        if(data == NULL)
            return 0;
    
        while(size &gt; 0)
        {
            bit_flag = out &gt;&gt; 15;
    
            /* Get next bit: */
            out &lt;&lt;= 1;
            out |= (*data &gt;&gt; (7 - bits_read)) &amp; 1;
    
            /* Increment bit counter: */
            bits_read++;
            if(bits_read &gt; 7)
            {
                bits_read = 0;
                data++;
                size--;
            }
    
            /* Cycle check: */
            if(bit_flag)
                out ^= CRC16;
        }

        return out;
    }

I&#39;m checking my output against [this online CRC calculator][2].

I&#39;ve come to the conclusion that either my understanding of how to calculate a CRC16 is wrong, or the online calculator is wrong (the former seems more likely).  Can someone tell me where I might be going wrong?

  [1]: https://gist.github.com/2666368
  [2]: http://www.lammertbies.nl/comm/info/crc-calculation.html
  [3]: http://www.ross.net/crc/download/crc_v3.txt


All Crc16 Solutions on Gang of Coders

Total of 1 Crc16 Solutions

Function to Calculate a CRC16 Checksum