CODEWITHSUNDEEP
c
E_Blue
E_Blue

Reputation: 1151

Can't get the right CRC CCiTT in C

I'm trying to get the 16 bit CRC CCITT FALSE. I'm using this page to check it.

http://www.sunshine2k.de/coding/javascript/crc/crc_js.html

And this is my code

unsigned int16 crc16_CCITT(unsigned int8 *data, unsigned int16 len)//CRC16 CCITT False
{
   unsigned int16 crc = 0xFFFF;

   for(unsigned int16 j = len; j > 0; j--)
   {
      crc ^= *data++;

      for(unsigned int8 i = 0; i < 8; i++)
      {
         if(crc & 1)
         {
            //crc = (crc >> 1) ^ 0x8401; // 0x8401 is the reflection of 0x1021
            crc = (crc >> 1) ^ 0x1021;
         }   
         else
         {
            crc >>= 1;
         }   
      }
   }
   return (crc);
}

As you can see I already tried by reflecting the polynomial and didn't work either.

I don't understand what I'm doing wrong, I already used this routine with the 16bit ARC CRC.(0x8005) and works ok.

Upvotes: 0

Views: 253

Answers (2)

Doug Currie
Doug Currie

Reputation: 41170

Try shifting the bits the other way:

uint16_t crc16_CCITT (unsigned char *ptr, int count)
{
   uint16_t crc = 0xffff;
   int i = 0;

   while (--count >= 0)
   {
      crc = crc ^ (uint16_t )*ptr++ << 8;
      for (i = 0; i < 8; ++i)
      {
         if (crc & 0x8000)
         {
            crc = (crc << 1) ^ 0x1021;
         }
         else
         {
            crc = crc << 1;
         }
      }
   }
   return crc;
}

Upvotes: 1

JaeMann Yeh
JaeMann Yeh

Reputation: 373

unsigned int16 and unsigned int8 are ambiguous. It's better to change them to uint16,uint8 or unsigned short,unsigned char. In many header files, int16 is defined as signed short and int8 is defined as signed char.

Upvotes: 0

Related Questions