Function to Calculate a CRC16 Checksum

Question

I 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 ve created a function to calculate a CRC16 checksum  but it doesn t seem to be outputting correct values   The relevant code I ve written is below  it can also be found here     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 m checking my output against this online CRC calculator   I 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

User · Answer

This function works for CRC-16 Modbus version  Not for CRC-16

User · Answer

Here follows a working code to calculate crc16 CCITT  I tested it and the results matched with those provided by http   www lammertbies nl comm info crc-calculation html    unsigned short crc16 const unsigned char  data p  unsigned char length       unsigned char x      unsigned short crc   0xFFFF       while  length--           x   crc  gt  gt  8    data p            x    x gt  gt 4          crc    crc  lt  lt  8      unsigned short  x  lt  lt  12       unsigned short  x  lt  lt 5       unsigned short x             return crc

User · Answer

There are several different varieties of CRC-16  See wiki page   Every of those will return different results from the same input   So you must carefully select correct one for your program

User · Answer

crcany will generate efficient C code for any CRC  and includes a library of over one hundred known CRC definitions   Efficient CRC code uses tables instead of bit-wise calculations  crcany generates both byte-wise routines and word-wise routines  the latter tuned to the architecture they are generated on  Word-wise is the fastest  Byte-wise is still much faster than bit-wise  but the implementation is more easily portable over architectures   You do not seem to have a protocol definition with a specific CRC definition that you need to match  In this case  you can pick any 16-bit CRC in the catalog  and you will get good performance   If you have a relatively low bit error rate  e g  single digit number of errors per packet  and you want to maximize your error detection performance  you would need to look at the packet size you are applying the CRC to  assuming that that is constant or bounded  and look at the performance of the best polynomials in Philip Koopman s extensive research  The classic CRCs  such as the CCITT Kermit 16-bit CRC or the X 25 16-bit CRC are not the best performers   One of the good 16-bit performers in Koopman s tables that is also in the catalog of CRCs used in practice is CRC-16 DNP  It has very good performance detecting up to 6-bit errors in a packet  Following is the code generated by crcany for that CRC definition  This code assumes a little-endian architecture for the word-wise calculation  e g  Intel x86 and x86-64  and it assumes that uintmax t is 64 bits  crcany can be used to generate alternative code for big-endian and other word sizes   crc16dnp h      The  bit   byte  and  word routines return the CRC of the len bytes at mem     applied to the previous CRC value  crc  If mem is NULL  then the other    arguments are ignored  and the initial CRC  i e  the CRC of zero bytes  is    returned  Those routines will all return the same result  differing only in    speed and code complexity  The  rem routine returns the CRC of the remaining    bits in the last byte  for when the number of bits in the message is not a    multiple of eight  The low bits bits of the low byte of val are applied to    crc  bits must be in 0  8    include  lt stddef h gt      Compute the CRC a bit at a time  unsigned crc16dnp bit unsigned crc  void const  mem  size t len       Compute the CRC of the low bits bits in val  unsigned crc16dnp rem unsigned crc  unsigned val  unsigned bits       Compute the CRC a byte at a time  unsigned crc16dnp byte unsigned crc  void const  mem  size t len       Compute the CRC a word at a time  unsigned crc16dnp word unsigned crc  void const  mem  size t len     crc16dnp c    include  lt stdint h gt   include  crc16dnp h      This code assumes that unsigned is 4 bytes   unsigned crc16dnp bit unsigned crc  void const  mem  size t len        unsigned char const  data   mem      if  data    NULL          return 0xffff      crc    crc      crc  amp   0xffff      while  len--            crc     data            for  unsigned k   0  k  lt  8  k                crc   crc  amp  1    crc  gt  gt  1    0xa6bc   crc  gt  gt  1            crc    0xffff      return crc     unsigned crc16dnp rem unsigned crc  unsigned val  unsigned bits        crc    crc      crc  amp   0xffff      val  amp    1U  lt  lt  bits  - 1      crc    val      while  bits--          crc   crc  amp  1    crc  gt  gt  1    0xa6bc   crc  gt  gt  1      crc    0xffff      return crc      define table byte table word 0   static unsigned short const table word   256          0xed35  0xdb6b  0x8189  0xb7d7  0x344d  0x0213  0x58f1  0x6eaf  0x12bc  0x24e2      0x7e00  0x485e  0xcbc4  0xfd9a  0xa778  0x9126  0x5f5e  0x6900  0x33e2  0x05bc      0x8626  0xb078  0xea9a  0xdcc4  0xa0d7  0x9689  0xcc6b  0xfa35  0x79af  0x4ff1      0x1513  0x234d  0xc49a  0xf2c4  0xa826  0x9e78  0x1de2  0x2bbc  0x715e  0x4700      0x3b13  0x0d4d  0x57af  0x61f1  0xe26b  0xd435  0x8ed7  0xb889  0x76f1  0x40af      0x1a4d  0x2c13  0xaf89  0x99d7  0xc335  0xf56b  0x8978  0xbf26  0xe5c4  0xd39a      0x5000  0x665e  0x3cbc  0x0ae2  0xbe6b  0x8835  0xd2d7  0xe489  0x6713  0x514d      0x0baf  0x3df1  0x41e2  0x77bc  0x2d5e  0x1b00  0x989a  0xaec4  0xf426  0xc278      0x0c00  0x3a5e  0x60bc  0x56e2  0xd578  0xe326  0xb9c4  0x8f9a  0xf389  0xc5d7      0x9f35  0xa96b  0x2af1  0x1caf  0x464d  0x7013  0x97c4  0xa19a  0xfb78  0xcd26      0x4ebc  0x78e2  0x2200  0x145e  0x684d  0x5e13  0x04f1  0x32af  0xb135  0x876b      0xdd89  0xebd7  0x25af  0x13f1  0x4913  0x7f4d  0xfcd7  0xca89  0x906b  0xa635      0xda26  0xec78  0xb69a  0x80c4  0x035e  0x3500  0x6fe2  0x59bc  0x4b89  0x7dd7      0x2735  0x116b  0x92f1  0xa4af  0xfe4d  0xc813  0xb400  0x825e  0xd8bc  0xeee2      0x6d78  0x5b26  0x01c4  0x379a  0xf9e2  0xcfbc  0x955e  0xa300  0x209a  0x16c4      0x4c26  0x7a78  0x066b  0x3035  0x6ad7  0x5c89  0xdf13  0xe94d  0xb3af  0x85f1      0x6226  0x5478  0x0e9a  0x38c4  0xbb5e  0x8d00  0xd7e2  0xe1bc  0x9daf  0xabf1      0xf113  0xc74d  0x44d7  0x7289  0x286b  0x1e35  0xd04d  0xe613  0xbcf1  0x8aaf      0x0935  0x3f6b  0x6589  0x53d7  0x2fc4  0x199a  0x4378  0x7526  0xf6bc  0xc0e2      0x9a00  0xac5e  0x18d7  0x2e89  0x746b  0x4235  0xc1af  0xf7f1  0xad13  0x9b4d      0xe75e  0xd100  0x8be2  0xbdbc  0x3e26  0x0878  0x529a  0x64c4  0xaabc  0x9ce2      0xc600  0xf05e  0x73c4  0x459a  0x1f78  0x2926  0x5535  0x636b  0x3989  0x0fd7      0x8c4d  0xba13  0xe0f1  0xd6af  0x3178  0x0726  0x5dc4  0x6b9a  0xe800  0xde5e      0x84bc  0xb2e2  0xcef1  0xf8af  0xa24d  0x9413  0x1789  0x21d7  0x7b35  0x4d6b      0x8313  0xb54d  0xefaf  0xd9f1  0x5a6b  0x6c35  0x36d7  0x0089  0x7c9a  0x4ac4      0x1026  0x2678  0xa5e2  0x93bc  0xc95e  0xff00       0x740f  0xdf41  0x6fea  0xc4a4  0x43c5  0xe88b  0x5820  0xf36e  0x1b9b  0xb0d5      0x007e  0xab30  0x2c51  0x871f  0x37b4  0x9cfa  0xab27  0x0069  0xb0c2  0x1b8c      0x9ced  0x37a3  0x8708  0x2c46  0xc4b3  0x6ffd  0xdf56  0x7418  0xf379  0x5837      0xe89c  0x43d2  0x8726  0x2c68  0x9cc3  0x378d  0xb0ec  0x1ba2  0xab09  0x0047      0xe8b2  0x43fc  0xf357  0x5819  0xdf78  0x7436  0xc49d  0x6fd3  0x580e  0xf340      0x43eb  0xe8a5  0x6fc4  0xc48a  0x7421  0xdf6f  0x379a  0x9cd4  0x2c7f  0x8731      0x0050  0xab1e  0x1bb5  0xb0fb  0xdf24  0x746a  0xc4c1  0x6f8f  0xe8ee  0x43a0      0xf30b  0x5845  0xb0b0  0x1bfe  0xab55  0x001b  0x877a  0x2c34  0x9c9f  0x37d1      0x000c  0xab42  0x1be9  0xb0a7  0x37c6  0x9c88  0x2c23  0x876d  0x6f98  0xc4d6      0x747d  0xdf33  0x5852  0xf31c  0x43b7  0xe8f9  0x2c0d  0x8743  0x37e8  0x9ca6      0x1bc7  0xb089  0x0022  0xab6c  0x4399  0xe8d7  0x587c  0xf332  0x7453  0xdf1d      0x6fb6  0xc4f8  0xf325  0x586b  0xe8c0  0x438e  0xc4ef  0x6fa1  0xdf0a  0x7444      0x9cb1  0x37ff  0x8754  0x2c1a  0xab7b  0x0035  0xb09e  0x1bd0  0x6f20  0xc46e      0x74c5  0xdf8b  0x58ea  0xf3a4  0x430f  0xe841  0x00b4  0xabfa  0x1b51  0xb01f      0x377e  0x9c30  0x2c9b  0x87d5  0xb008  0x1b46  0xabed  0x00a3  0x87c2  0x2c8c      0x9c27  0x3769  0xdf9c  0x74d2  0xc479  0x6f37  0xe856  0x4318  0xf3b3  0x58fd      0x9c09  0x3747  0x87ec  0x2ca2  0xabc3  0x008d  0xb026  0x1b68  0xf39d  0x58d3      0xe878  0x4336  0xc457  0x6f19  0xdfb2  0x74fc  0x4321  0xe86f  0x58c4  0xf38a      0x74eb  0xdfa5  0x6f0e  0xc440  0x2cb5  0x87fb  0x3750  0x9c1e  0x1b7f  0xb031      0x009a  0xabd4  0xc40b  0x6f45  0xdfee  0x74a0  0xf3c1  0x588f  0xe824  0x436a      0xab9f  0x00d1  0xb07a  0x1b34  0x9c55  0x371b  0x87b0  0x2cfe  0x1b23  0xb06d      0x00c6  0xab88  0x2ce9  0x87a7  0x370c  0x9c42  0x74b7  0xdff9  0x6f52  0xc41c      0x437d  0xe833  0x5898  0xf3d6  0x3722  0x9c6c  0x2cc7  0x8789  0x00e8  0xaba6      0x1b0d  0xb043  0x58b6  0xf3f8  0x4353  0xe81d  0x6f7c  0xc432  0x7499  0xdfd7      0xe80a  0x4344  0xf3ef  0x58a1  0xdfc0  0x748e  0xc425  0x6f6b  0x879e  0x2cd0      0x9c7b  0x3735  0xb054  0x1b1a  0xabb1  0x00ff       0x7c67  0x65df  0x4f17  0x56af  0x1a87  0x033f  0x29f7  0x304f  0xb1a7  0xa81f      0x82d7  0x9b6f  0xd747  0xceff  0xe437  0xfd8f  0xaa9e  0xb326  0x99ee  0x8056      0xcc7e  0xd5c6  0xff0e  0xe6b6  0x675e  0x7ee6  0x542e  0x4d96  0x01be  0x1806      0x32ce  0x2b76  0x9cec  0x8554  0xaf9c  0xb624  0xfa0c  0xe3b4  0xc97c  0xd0c4      0x512c  0x4894  0x625c  0x7be4  0x37cc  0x2e74  0x04bc  0x1d04  0x4a15  0x53ad      0x7965  0x60dd  0x2cf5  0x354d  0x1f85  0x063d  0x87d5  0x9e6d  0xb4a5  0xad1d      0xe135  0xf88d  0xd245  0xcbfd  0xf008  0xe9b0  0xc378  0xdac0  0x96e8  0x8f50      0xa598  0xbc20  0x3dc8  0x2470  0x0eb8  0x1700  0x5b28  0x4290  0x6858  0x71e0      0x26f1  0x3f49  0x1581  0x0c39  0x4011  0x59a9  0x7361  0x6ad9  0xeb31  0xf289      0xd841  0xc1f9  0x8dd1  0x9469  0xbea1  0xa719  0x1083  0x093b  0x23f3  0x3a4b      0x7663  0x6fdb  0x4513  0x5cab  0xdd43  0xc4fb  0xee33  0xf78b  0xbba3  0xa21b      0x88d3  0x916b  0xc67a  0xdfc2  0xf50a  0xecb2  0xa09a  0xb922  0x93ea  0x8a52      0x0bba  0x1202  0x38ca  0x2172  0x6d5a  0x74e2  0x5e2a  0x4792  0x29c0  0x3078      0x1ab0  0x0308  0x4f20  0x5698  0x7c50  0x65e8  0xe400  0xfdb8  0xd770  0xcec8      0x82e0  0x9b58  0xb190  0xa828  0xff39  0xe681  0xcc49  0xd5f1  0x99d9  0x8061      0xaaa9  0xb311  0x32f9  0x2b41  0x0189  0x1831  0x5419  0x4da1  0x6769  0x7ed1      0xc94b  0xd0f3  0xfa3b  0xe383  0xafab  0xb613  0x9cdb  0x8563  0x048b  0x1d33      0x37fb  0x2e43  0x626b  0x7bd3  0x511b  0x48a3  0x1fb2  0x060a  0x2cc2  0x357a      0x7952  0x60ea  0x4a22  0x539a  0xd272  0xcbca  0xe102  0xf8ba  0xb492  0xad2a      0x87e2  0x9e5a  0xa5af  0xbc17  0x96df  0x8f67  0xc34f  0xdaf7  0xf03f  0xe987      0x686f  0x71d7  0x5b1f  0x42a7  0x0e8f  0x1737  0x3dff  0x2447  0x7356  0x6aee      0x4026  0x599e  0x15b6  0x0c0e  0x26c6  0x3f7e  0xbe96  0xa72e  0x8de6  0x945e      0xd876  0xc1ce  0xeb06  0xf2be  0x4524  0x5c9c  0x7654  0x6fec  0x23c4  0x3a7c      0x10b4  0x090c  0x88e4  0x915c  0xbb94  0xa22c  0xee04  0xf7bc  0xdd74  0xc4cc      0x93dd  0x8a65  0xa0ad  0xb915  0xf53d  0xec85  0xc64d  0xdff5  0x5e1d  0x47a5      0x6d6d  0x74d5  0x38fd  0x2145  0x0b8d  0x1235       0xf917  0x3bff  0x31be  0xf356  0x253c  0xe7d4  0xed95  0x2f7d  0x0c38  0xced0      0xc491  0x0679  0xd013  0x12fb  0x18ba  0xda52  0x5e30  0x9cd8  0x9699  0x5471      0x821b  0x40f3  0x4ab2  0x885a  0xab1f  0x69f7  0x63b6  0xa15e  0x7734  0xb5dc      0xbf9d  0x7d75  0xfa20  0x38c8  0x3289  0xf061  0x260b  0xe4e3  0xeea2  0x2c4a      0x0f0f  0xcde7  0xc7a6  0x054e  0xd324  0x11cc  0x1b8d  0xd965  0x5d07  0x9fef      0x95ae  0x5746  0x812c  0x43c4  0x4985  0x8b6d  0xa828  0x6ac0  0x6081  0xa269      0x7403  0xb6eb  0xbcaa  0x7e42  0xff79  0x3d91  0x37d0  0xf538  0x2352  0xe1ba      0xebfb  0x2913  0x0a56  0xc8be  0xc2ff  0x0017  0xd67d  0x1495  0x1ed4  0xdc3c      0x585e  0x9ab6  0x90f7  0x521f  0x8475  0x469d  0x4cdc  0x8e34  0xad71  0x6f99      0x65d8  0xa730  0x715a  0xb3b2  0xb9f3  0x7b1b  0xfc4e  0x3ea6  0x34e7  0xf60f      0x2065  0xe28d  0xe8cc  0x2a24  0x0961  0xcb89  0xc1c8  0x0320  0xd54a  0x17a2      0x1de3  0xdf0b  0x5b69  0x9981  0x93c0  0x5128  0x8742  0x45aa  0x4feb  0x8d03      0xae46  0x6cae  0x66ef  0xa407  0x726d  0xb085  0xbac4  0x782c  0xf5cb  0x3723      0x3d62  0xff8a  0x29e0  0xeb08  0xe149  0x23a1  0x00e4  0xc20c  0xc84d  0x0aa5      0xdccf  0x1e27  0x1466  0xd68e  0x52ec  0x9004  0x9a45  0x58ad  0x8ec7  0x4c2f      0x466e  0x8486  0xa7c3  0x652b  0x6f6a  0xad82  0x7be8  0xb900  0xb341  0x71a9      0xf6fc  0x3414  0x3e55  0xfcbd  0x2ad7  0xe83f  0xe27e  0x2096  0x03d3  0xc13b      0xcb7a  0x0992  0xdff8  0x1d10  0x1751  0xd5b9  0x51db  0x9333  0x9972  0x5b9a      0x8df0  0x4f18  0x4559  0x87b1  0xa4f4  0x661c  0x6c5d  0xaeb5  0x78df  0xba37      0xb076  0x729e  0xf3a5  0x314d  0x3b0c  0xf9e4  0x2f8e  0xed66  0xe727  0x25cf      0x068a  0xc462  0xce23  0x0ccb  0xdaa1  0x1849  0x1208  0xd0e0  0x5482  0x966a      0x9c2b  0x5ec3  0x88a9  0x4a41  0x4000  0x82e8  0xa1ad  0x6345  0x6904  0xabec      0x7d86  0xbf6e  0xb52f  0x77c7  0xf092  0x327a  0x383b  0xfad3  0x2cb9  0xee51      0xe410  0x26f8  0x05bd  0xc755  0xcd14  0x0ffc  0xd996  0x1b7e  0x113f  0xd3d7      0x57b5  0x955d  0x9f1c  0x5df4  0x8b9e  0x4976  0x4337  0x81df  0xa29a  0x6072      0x6a33  0xa8db  0x7eb1  0xbc59  0xb618  0x74f0       0x3108  0x120e  0x7704  0x5402  0xbd10  0x9e16  0xfb1c  0xd81a  0x6441  0x4747      0x224d  0x014b  0xe859  0xcb5f  0xae55  0x8d53  0x9b9a  0xb89c  0xdd96  0xfe90      0x1782  0x3484  0x518e  0x7288  0xced3  0xedd5  0x88df  0xabd9  0x42cb  0x61cd      0x04c7  0x27c1  0x2955  0x0a53  0x6f59  0x4c5f  0xa54d  0x864b  0xe341  0xc047      0x7c1c  0x5f1a  0x3a10  0x1916  0xf004  0xd302  0xb608  0x950e  0x83c7  0xa0c1      0xc5cb  0xe6cd  0x0fdf  0x2cd9  0x49d3  0x6ad5  0xd68e  0xf588  0x9082  0xb384      0x5a96  0x7990  0x1c9a  0x3f9c  0x01b2  0x22b4  0x47be  0x64b8  0x8daa  0xaeac      0xcba6  0xe8a0  0x54fb  0x77fd  0x12f7  0x31f1  0xd8e3  0xfbe5  0x9eef  0xbde9      0xab20  0x8826  0xed2c  0xce2a  0x2738  0x043e  0x6134  0x4232  0xfe69  0xdd6f      0xb865  0x9b63  0x7271  0x5177  0x347d  0x177b  0x19ef  0x3ae9  0x5fe3  0x7ce5      0x95f7  0xb6f1  0xd3fb  0xf0fd  0x4ca6  0x6fa0  0x0aaa  0x29ac  0xc0be  0xe3b8      0x86b2  0xa5b4  0xb37d  0x907b  0xf571  0xd677  0x3f65  0x1c63  0x7969  0x5a6f      0xe634  0xc532  0xa038  0x833e  0x6a2c  0x492a  0x2c20  0x0f26  0x507c  0x737a      0x1670  0x3576  0xdc64  0xff62  0x9a68  0xb96e  0x0535  0x2633  0x4339  0x603f      0x892d  0xaa2b  0xcf21  0xec27  0xfaee  0xd9e8  0xbce2  0x9fe4  0x76f6  0x55f0      0x30fa  0x13fc  0xafa7  0x8ca1  0xe9ab  0xcaad  0x23bf  0x00b9  0x65b3  0x46b5      0x4821  0x6b27  0x0e2d  0x2d2b  0xc439  0xe73f  0x8235  0xa133  0x1d68  0x3e6e      0x5b64  0x7862  0x9170  0xb276  0xd77c  0xf47a  0xe2b3  0xc1b5  0xa4bf  0x87b9      0x6eab  0x4dad  0x28a7  0x0ba1  0xb7fa  0x94fc  0xf1f6  0xd2f0  0x3be2  0x18e4      0x7dee  0x5ee8  0x60c6  0x43c0  0x26ca  0x05cc  0xecde  0xcfd8  0xaad2  0x89d4      0x358f  0x1689  0x7383  0x5085  0xb997  0x9a91  0xff9b  0xdc9d  0xca54  0xe952      0x8c58  0xaf5e  0x464c  0x654a  0x0040  0x2346  0x9f1d  0xbc1b  0xd911  0xfa17      0x1305  0x3003  0x5509  0x760f  0x789b  0x5b9d  0x3e97  0x1d91  0xf483  0xd785      0xb28f  0x9189  0x2dd2  0x0ed4  0x6bde  0x48d8  0xa1ca  0x82cc  0xe7c6  0xc4c0      0xd209  0xf10f  0x9405  0xb703  0x5e11  0x7d17  0x181d  0x3b1b  0x8740  0xa446      0xc14c  0xe24a  0x0b58  0x285e  0x4d54  0x6e52       0xffb8  0x4a5f  0xd90f  0x6ce8  0xb2d6  0x0731  0x9461  0x2186  0x6564  0xd083      0x43d3  0xf634  0x280a  0x9ded  0x0ebd  0xbb5a  0x8779  0x329e  0xa1ce  0x1429      0xca17  0x7ff0  0xeca0  0x5947  0x1da5  0xa842  0x3b12  0x8ef5  0x50cb  0xe52c      0x767c  0xc39b  0x0e3a  0xbbdd  0x288d  0x9d6a  0x4354  0xf6b3  0x65e3  0xd004      0x94e6  0x2101  0xb251  0x07b6  0xd988  0x6c6f  0xff3f  0x4ad8  0x76fb  0xc31c      0x504c  0xe5ab  0x3b95  0x8e72  0x1d22  0xa8c5  0xec27  0x59c0  0xca90  0x7f77      0xa149  0x14ae  0x87fe  0x3219  0x51c5  0xe422  0x7772  0xc295  0x1cab  0xa94c      0x3a1c  0x8ffb  0xcb19  0x7efe  0xedae  0x5849  0x8677  0x3390  0xa0c0  0x1527      0x2904  0x9ce3  0x0fb3  0xba54  0x646a  0xd18d  0x42dd  0xf73a  0xb3d8  0x063f      0x956f  0x2088  0xfeb6  0x4b51  0xd801  0x6de6  0xa047  0x15a0  0x86f0  0x3317      0xed29  0x58ce  0xcb9e  0x7e79  0x3a9b  0x8f7c  0x1c2c  0xa9cb  0x77f5  0xc212      0x5142  0xe4a5  0xd886  0x6d61  0xfe31  0x4bd6  0x95e8  0x200f  0xb35f  0x06b8      0x425a  0xf7bd  0x64ed  0xd10a  0x0f34  0xbad3  0x2983  0x9c64  0xee3b  0x5bdc      0xc88c  0x7d6b  0xa355  0x16b2  0x85e2  0x3005  0x74e7  0xc100  0x5250  0xe7b7      0x3989  0x8c6e  0x1f3e  0xaad9  0x96fa  0x231d  0xb04d  0x05aa  0xdb94  0x6e73      0xfd23  0x48c4  0x0c26  0xb9c1  0x2a91  0x9f76  0x4148  0xf4af  0x67ff  0xd218      0x1fb9  0xaa5e  0x390e  0x8ce9  0x52d7  0xe730  0x7460  0xc187  0x8565  0x3082      0xa3d2  0x1635  0xc80b  0x7dec  0xeebc  0x5b5b  0x6778  0xd29f  0x41cf  0xf428      0x2a16  0x9ff1  0x0ca1  0xb946  0xfda4  0x4843  0xdb13  0x6ef4  0xb0ca  0x052d      0x967d  0x239a  0x4046  0xf5a1  0x66f1  0xd316  0x0d28  0xb8cf  0x2b9f  0x9e78      0xda9a  0x6f7d  0xfc2d  0x49ca  0x97f4  0x2213  0xb143  0x04a4  0x3887  0x8d60      0x1e30  0xabd7  0x75e9  0xc00e  0x535e  0xe6b9  0xa25b  0x17bc  0x84ec  0x310b      0xef35  0x5ad2  0xc982  0x7c65  0xb1c4  0x0423  0x9773  0x2294  0xfcaa  0x494d      0xda1d  0x6ffa  0x2b18  0x9eff  0x0daf  0xb848  0x6676  0xd391  0x40c1  0xf526      0xc905  0x7ce2  0xefb2  0x5a55  0x846b  0x318c  0xa2dc  0x173b  0x53d9  0xe63e      0x756e  0xc089  0x1eb7  0xab50  0x3800  0x8de7       0xc20e  0x9d6c  0x7cca  0x23a8  0xf2ff  0xad9d  0x4c3b  0x1359  0xa3ec  0xfc8e      0x1d28  0x424a  0x931d  0xcc7f  0x2dd9  0x72bb  0x01ca  0x5ea8  0xbf0e  0xe06c      0x313b  0x6e59  0x8fff  0xd09d  0x6028  0x3f4a  0xdeec  0x818e  0x50d9  0x0fbb      0xee1d  0xb17f  0x08ff  0x579d  0xb63b  0xe959  0x380e  0x676c  0x86ca  0xd9a8      0x691d  0x367f  0xd7d9  0x88bb  0x59ec  0x068e  0xe728  0xb84a  0xcb3b  0x9459      0x75ff  0x2a9d  0xfbca  0xa4a8  0x450e  0x1a6c  0xaad9  0xf5bb  0x141d  0x4b7f      0x9a28  0xc54a  0x24ec  0x7b8e  0x1a95  0x45f7  0xa451  0xfb33  0x2a64  0x7506      0x94a0  0xcbc2  0x7b77  0x2415  0xc5b3  0x9ad1  0x4b86  0x14e4  0xf542  0xaa20      0xd951  0x8633  0x6795  0x38f7  0xe9a0  0xb6c2  0x5764  0x0806  0xb8b3  0xe7d1      0x0677  0x5915  0x8842  0xd720  0x3686  0x69e4  0xd064  0x8f06  0x6ea0  0x31c2      0xe095  0xbff7  0x5e51  0x0133  0xb186  0xeee4  0x0f42  0x5020  0x8177  0xde15      0x3fb3  0x60d1  0x13a0  0x4cc2  0xad64  0xf206  0x2351  0x7c33  0x9d95  0xc2f7      0x7242  0x2d20  0xcc86  0x93e4  0x42b3  0x1dd1  0xfc77  0xa315  0x3e41  0x6123      0x8085  0xdfe7  0x0eb0  0x51d2  0xb074  0xef16  0x5fa3  0x00c1  0xe167  0xbe05      0x6f52  0x3030  0xd196  0x8ef4  0xfd85  0xa2e7  0x4341  0x1c23  0xcd74  0x9216      0x73b0  0x2cd2  0x9c67  0xc305  0x22a3  0x7dc1  0xac96  0xf3f4  0x1252  0x4d30      0xf4b0  0xabd2  0x4a74  0x1516  0xc441  0x9b23  0x7a85  0x25e7  0x9552  0xca30      0x2b96  0x74f4  0xa5a3  0xfac1  0x1b67  0x4405  0x3774  0x6816  0x89b0  0xd6d2      0x0785  0x58e7  0xb941  0xe623  0x5696  0x09f4  0xe852  0xb730  0x6667  0x3905      0xd8a3  0x87c1  0xe6da  0xb9b8  0x581e  0x077c  0xd62b  0x8949  0x68ef  0x378d      0x8738  0xd85a  0x39fc  0x669e  0xb7c9  0xe8ab  0x090d  0x566f  0x251e  0x7a7c      0x9bda  0xc4b8  0x15ef  0x4a8d  0xab2b  0xf449  0x44fc  0x1b9e  0xfa38  0xa55a      0x740d  0x2b6f  0xcac9  0x95ab  0x2c2b  0x7349  0x92ef  0xcd8d  0x1cda  0x43b8      0xa21e  0xfd7c  0x4dc9  0x12ab  0xf30d  0xac6f  0x7d38  0x225a  0xc3fc  0x9c9e      0xefef  0xb08d  0x512b  0x0e49  0xdf1e  0x807c  0x61da  0x3eb8  0x8e0d  0xd16f      0x30c9  0x6fab  0xbefc  0xe19e  0x0038  0x5f5a       0x4a8f  0x5c9d  0x66ab  0x70b9  0x12c7  0x04d5  0x3ee3  0x28f1  0xfa1f  0xec0d      0xd63b  0xc029  0xa257  0xb445  0x8e73  0x9861  0x66d6  0x70c4  0x4af2  0x5ce0      0x3e9e  0x288c  0x12ba  0x04a8  0xd646  0xc054  0xfa62  0xec70  0x8e0e  0x981c      0xa22a  0xb438  0x123d  0x042f  0x3e19  0x280b  0x4a75  0x5c67  0x6651  0x7043      0xa2ad  0xb4bf  0x8e89  0x989b  0xfae5  0xecf7  0xd6c1  0xc0d3  0x3e64  0x2876      0x1240  0x0452  0x662c  0x703e  0x4a08  0x5c1a  0x8ef4  0x98e6  0xa2d0  0xb4c2      0xd6bc  0xc0ae  0xfa98  0xec8a  0xfbeb  0xedf9  0xd7cf  0xc1dd  0xa3a3  0xb5b1      0x8f87  0x9995  0x4b7b  0x5d69  0x675f  0x714d  0x1333  0x0521  0x3f17  0x2905      0xd7b2  0xc1a0  0xfb96  0xed84  0x8ffa  0x99e8  0xa3de  0xb5cc  0x6722  0x7130      0x4b06  0x5d14  0x3f6a  0x2978  0x134e  0x055c  0xa359  0xb54b  0x8f7d  0x996f      0xfb11  0xed03  0xd735  0xc127  0x13c9  0x05db  0x3fed  0x29ff  0x4b81  0x5d93      0x67a5  0x71b7  0x8f00  0x9912  0xa324  0xb536  0xd748  0xc15a  0xfb6c  0xed7e      0x3f90  0x2982  0x13b4  0x05a6  0x67d8  0x71ca  0x4bfc  0x5dee  0x653e  0x732c      0x491a  0x5f08  0x3d76  0x2b64  0x1152  0x0740  0xd5ae  0xc3bc  0xf98a  0xef98      0x8de6  0x9bf4  0xa1c2  0xb7d0  0x4967  0x5f75  0x6543  0x7351  0x112f  0x073d      0x3d0b  0x2b19  0xf9f7  0xefe5  0xd5d3  0xc3c1  0xa1bf  0xb7ad  0x8d9b  0x9b89      0x3d8c  0x2b9e  0x11a8  0x07ba  0x65c4  0x73d6  0x49e0  0x5ff2  0x8d1c  0x9b0e      0xa138  0xb72a  0xd554  0xc346  0xf970  0xef62  0x11d5  0x07c7  0x3df1  0x2be3      0x499d  0x5f8f  0x65b9  0x73ab  0xa145  0xb757  0x8d61  0x9b73  0xf90d  0xef1f      0xd529  0xc33b  0xd45a  0xc248  0xf87e  0xee6c  0x8c12  0x9a00  0xa036  0xb624      0x64ca  0x72d8  0x48ee  0x5efc  0x3c82  0x2a90  0x10a6  0x06b4  0xf803  0xee11      0xd427  0xc235  0xa04b  0xb659  0x8c6f  0x9a7d  0x4893  0x5e81  0x64b7  0x72a5      0x10db  0x06c9  0x3cff  0x2aed  0x8ce8  0x9afa  0xa0cc  0xb6de  0xd4a0  0xc2b2      0xf884  0xee96  0x3c78  0x2a6a  0x105c  0x064e  0x6430  0x7222  0x4814  0x5e06      0xa0b1  0xb6a3  0x8c95  0x9a87  0xf8f9  0xeeeb  0xd4dd  0xc2cf  0x1021  0x0633      0x3c05  0x2a17  0x4869  0x5e7b  0x644d  0x725f      unsigned crc16dnp byte unsigned crc  void const  mem  size t len        unsigned char const  data   mem      if  data    NULL          return 0xffff      crc  amp   0xffff      while  len--          crc    crc  gt  gt  8                  table byte  crc    data     amp  0xff       return crc        This code assumes that integers are stored little-endian   unsigned crc16dnp word unsigned crc  void const  mem  size t len        unsigned char const  data   mem      if  data    NULL          return 0xffff      crc  amp   0xffff      while  len  amp  amp    ptrdiff t data  amp  0x7             crc    crc  gt  gt  8                  table byte  crc    data     amp  0xff           len--            if  len  gt   8            do               uintmax t word   crc     uintmax t const   data              crc   table word 7  word  amp  0xff                      table word 6   word  gt  gt  8   amp  0xff                      table word 5   word  gt  gt  16   amp  0xff                      table word 4   word  gt  gt  24   amp  0xff                      table word 3   word  gt  gt  32   amp  0xff                      table word 2   word  gt  gt  40   amp  0xff                      table word 1   word  gt  gt  48   amp  0xff                      table word 0  word  gt  gt  56               data    8              len -  8            while  len  gt   8             while  len--          crc    crc  gt  gt  8                  table byte  crc    data     amp  0xff       return crc

User · Answer

There are several details you need to  match up  with for a particular CRC implementation - even using the same polynomial there can be different results because of minor differences in how data bits are handled  using a particular initial value for the CRC  sometimes it s zero  sometimes 0xffff   and or inverting the bits of the CRC  For example  sometimes one implementation will work from the low order bits of the data bytes up  while sometimes they ll work from the high order bits down  as yours currently does    Also  you need to  push out  the last bits of the CRC after you ve run all the data bits through   Keep in mind that CRC algorithms were designed to be implemented in hardware  so some of how bit ordering is handled may not make so much sense from a software point of view   If you want to match the CRC16 with polynomial 0x8005 as shown on the lammertbies nl CRC calculator page  you need to make the following changes to your CRC function    a  run the data bits through the CRC loop starting from the least significant bit instead of from the most significant bit b  push the last 16 bits of the CRC out of the CRC register after you ve finished with the input data c  reverse the CRC bits  I m guessing this bit is a carry over from hardware implementations    So  your function might look like    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  bits read   amp  1     item a  work from the least significant bits             Increment bit counter             bits read            if bits read  gt  7                        bits read   0              data                size--                        Cycle check             if bit flag              out    CRC16                 item b   push out  the last 16 bits     int i      for  i   0  i  lt  16    i            bit flag   out  gt  gt  15          out  lt  lt   1          if bit flag              out    CRC16                item c  reverse the bits     uint16 t crc   0      i   0x8000      int j   0x0001      for    i    0  i  gt  gt  1  j  lt  lt   1            if  i  amp  out  crc    j             return crc      That function returns 0xbb3d for me when I pass in  123456789

User · Answer

for  pos   0  pos  lt  len  pos          crc     uint16 t buf pos          XOR byte into least sig  byte of crc      for  i   8  i    0  i--           Loop over each bit         if   crc  amp  0x0001     0       If the LSB is set             crc  gt  gt   1                 Shift right and XOR 0xA001             crc    CRC16            else                        Else LSB is not set             crc  gt  gt   1                 Just shift right                     return crc

User · Answer

I used the code example from  http   www sunshine2k de articles coding crc understanding crc html ch5  And also this utility to verify  http   www sunshine2k de coding javascript crc crc js html

[c] Function to Calculate a CRC16 Checksum

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