CCITT CRC 16 Bit Start Value 0xffff

Question

I need to calculate a CCITT 16 bit checksum value for data passed as a parameter together with the length. If I fill my array TempStr with the test data "123456789", use the polynomial 0x8408 with the length excluding the null termination character, I get the result string 6E90(Hex). Together with the null termination char I get 907A. When I swap out the polynomial to 0x1201 then I get results 29E2(Hex) and EFE8(Hex) with and without termination character.

My questions are: Do I need to calculate the CRC with or without the null termination character to obtain the correct value? Do I use the polynomial 0x1201 or the reverse polynomial 0x8408 in the algorithm? Is the correct CRC of the given data 0x29B1? I need the correct value to determine if the function works correctly.. Is the algorithm to calculate this specific CRC type correct? wData=(unsigned int)0xff & *pData++?? If someone can explain to me what is wrong and how to fix my problem I would much appreciate it. Thank you

This is the code that uses and displays the calculate_CRC16 function:

CHAR_t TestStr[] = {"123456789"};
unsigned short CrcTest = calculate_CRC16(TestStr,sizeof(TestStr)-1);
QString CrcDisplay = QString("CrcTest : %1").arg(CrcTest);
ui->txtDebug->setText(CrcDisplay);

This is the calculate_CRC16 function:

UINT16_t MainWindow::calculate_CRC16(CHAR_t* pData, UINT16_t wLength)
{

  UCHAR_t i;
  UINT16_t wData;
  UINT16_t wCrc = 0xffff;

  if (wLength == 0)
    return (~wCrc);

  do
  {
    for (i=0, wData=(unsigned int)0xff & *pData++; i < 8; i++, wData >>= 1)
    {
        if ((wCrc & 0x0001) ^ (wData & 0x0001))
            wCrc = (wCrc >> 1) ^ CRC_POLY;
        else  wCrc >>= 1;
    }
  } while (--wLength);

  wCrc = ~wCrc;
  wData = wCrc;
  wCrc = (wCrc << 8) | (wData >> 8 & 0xff);

  return (wCrc);
}

Answer 1

The result of 0x29b1 is for the "false" CCITT CRC-16 (link to CRC catalog). Which is apparently the one you need. From the catalog:

width=16 poly=0x1021 init=0xffff refin=false refout=false xorout=0x0000 check=0x29b1 name="CRC-16/CCITT-FALSE"

So there is no bit reversal (refin, refout false). The CRC is initialized with 0xffff and is not post-processed.

To fix your code with the least changes:

if (wLength == 0)
    return wCrc;

do
{
    for (i=0, wData=((unsigned int)0xff & *pData++) << 8; i < 8; i++, wData <<= 1)
    {

        if ((wCrc & 0x8000) ^ (wData & 0x8000))
            wCrc = (wCrc << 1) ^ 0x1021;
        else  wCrc <<= 1;
    }
} while (--wLength);

return wCrc & 0xffff;

or to do it more reasonably:

while (wLength--) {
    wCrc ^= *(unsigned char *)pData++ << 8;
    for (i=0; i < 8; i++)
        wCrc = wCrc & 0x8000 ? (wCrc << 1) ^ 0x1021 : wCrc << 1;
}
return wCrc & 0xffff;