Converting 8 bit CRC function written in C to PHP

2
votes

I am trying to convert a C function to PHP that does 8 bit CRC calculation.

The original C code:

uint8_t CRCCalc (uint8_t* pointer, uint16_t len) {

    uint8_t CRC = 0x00;

    uint16_t tmp;

    while(len > 0) {

        tmp = CRC << 1;

        tmp += *pointer;

        CRC = (tmp & 0xFF) + (tmp >> 8);

        pointer++;

        --len;

    }

    return CRC;

}

The PHP code that I have come up with is:

function crc8_calc($hex_string)
{
    $bin_data = pack('H*',$hex_string);
    $bin_length = strlen($bin_data);

    $CRC    =   0x00;
    $pos    =   0;

    while($bin_length>0)
    {
        //$pos = $CRC << 1;

        $CRC = ($bin_data[$pos] & 0xFF) + ($bin_data[$pos] >> 8);
        $bin_length --;
        $pos++ ;
    }

    return $CRC;
}

There is something that is missing as the results from the PHP functions are not correct. I am not very familiar with C, so not sure if my conversion is correct. The C function gives out the correct CRC

For example, if the hex representation of the string is: 280500000805151001240000000010017475260004041001372068828503000000000000

The CRC should be D4.

I have already seen the following links for CRC8 calculation, but I seem to missing something

how to generate 8bit crc in php CRC8-Check in PHP

I have taken some bits of my conversion code from this answer too Convert C to PHP for CRC16 Function

3
phpccrccode-conversion
Why don't you try and translate the C code exactly? Note that the C code is working on a 16-bit variable tmp. Why did you omit the << shift operation? Note also that tmp += *pointer; means tmp = tmp + *pointer and not tmp = *pointer. – JimmyB
the C - code in your example does not compile. – specializt
@specializt Because of the typo in Pointer++? – JeremyP
The shift operator got commented out by mistake. JeremyP has already corrected the typo in C code. Thanks – Amitabh Kant

3 Answers

2
votes

Try it like this:

function crc8_calc($hex_string)
{
    $bin_data = pack('H*',$hex_string);
    $bin_length = strlen($bin_data);

    $CRC = 0;
    $tmp = 0;
    $pos = 0;

    while($bin_length>0)
    {
        $tmp = $CRC << 1;
        $tmp = $tmp + ord($bin_data[$pos]); //Added ord

        $CRC = ($tmp + ($tmp >> 8)) & 0xFF;
        $bin_length --;
        $pos++ ;
    }
    return $CRC;
}
3
votes

It's obvious that the two code fragments don't do the same thing. In the C function, the first thing that happens is you shift the CRC (that you've calculated so far) left by 1 bit (same as multiplying it by 2), then you add the next byte from the array, then you recalculate the CRC by adding the two bytes of tmp together.

Your PHP example doesn't do the initial shift, or mix in the previous version of the CRC into the calculation for the next byte..

0
votes

($tmp & 0xFF) just lower byte

($tmp >> 8) shift right 8

I have not tested, but this is a quick attempt.

function CRCCalc ($value, $len) {
    $CRC = 0;
    while($len > 0) {
        $tmp = $CRC << 1;  // shift left 1
        $tmp += $value;
        $CRC = ($tmp & 0xFF) + ($tmp >> 8); //  ($tmp & 0xFF) just lower byte    ($tmp >> 8: shift right 8
        $value++;
        $len--;
    }
    return $CRC;
}