crc16.cpp

00001 #include "crc16.h"
00002 
00003 /*
00004 CRC16 generation algorithm taken from J.G. Harston at
00005 http://mdfs.net/Info/Comp/Comms/CRC16.htm
00006 */
00007 
00008 // Refer to crc16.h for overall function details
00009 
00010 #define poly 0x11021 // Define polynomial generating CRC tag
00011 
00012 int     crc16 (char* dataIn, int numberOfBytes) {
00013     if (numberOfBytes > 16)     // Keep numberOfBytes under 16
00014         return 0;
00015     int i, j;
00016     int crc = 0;                // Initialize CRC to zero
00017     
00018     for (j = numberOfBytes ; j > 0; j--) {  // Execute numberOfBytes times
00019         crc = crc ^ (*dataIn++ << 8);   // Get byte and XOR with crc
00020         for (i = 0; i < 8; i++) {
00021             crc = crc << 1;             // Rotate
00022             if (crc & 0x10000)          // If bit 15 is set
00023                 crc ^= poly;            // XOR with polynomial            
00024         }
00025     }
00026     return crc;             // Return CRC tag       
00027 }
00028 
00029 void    crc16_attach (char* dataIn, int numberOfBytes) {
00030     int crc = crc16(dataIn, numberOfBytes);             // Generate CRC tag from first numberOfBytes elements of DataIn
00031     dataIn[numberOfBytes] = ((crc >> 8) & 0xFF);        // Store 8 MSbits as the next element in dataIn
00032     dataIn[numberOfBytes + 1] = (crc & 0xFF);           // Store the 8 LSbits is the last element in dataIn
00033 }
00034 
00035 bool    crc16_match (char* dataIn, int numberOfBytes) {
00036     int crcData = crc16(dataIn, numberOfBytes);         // Generate CRC tag from the first numberOfBytes elements of dataIn
00037     int a,b;
00038     a = dataIn[numberOfBytes];                          // Extract the last two 8 bit elements of dataIn as integers
00039     b = dataIn[numberOfBytes + 1];
00040     int crcExpect = (a << 8) + b;                       // Recombine into a single 16 bit element
00041     if (crcData == crcExpect)                           // Compare the generated with the expected
00042         return true;                                    // If they match, data is intact
00043     else
00044         return false;                                   // Otherwise, data is not intact
00045 }