Jonathan Lamothe
Reputation: 733
Function to Calculate a CRC16 Checksum
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 <stdint.h>
#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 > 0)
{
bit_flag = out >> 15;
/* Get next bit: */
out <<= 1;
out |= (*data >> (7 - bits_read)) & 1;
/* Increment bit counter: */
bits_read++;
if(bits_read > 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?
Upvotes: 61
Views: 297432
Answers (8)
edc352229
Reputation: 1
CRC implementation doesn't have to be expensive in terms of speed and code size. Here is an efficient C code for a fast 16-bit CRC, which has low complexity and requires no lookup table:
uint16_t fast_crc16(uint8_t *data, size_t length)
{
uint16_t A, crc=0;
for(size_t i=0; i<length; i++)
{
A = (crc>>8)^data[i];
crc = (A<<2)^(A<<1)^A^(crc<<8);
}
return crc;
}
This CRC, generated by x^16 + x^2 + x + 1, has HD4 and length up to 2^15 - 1 = 32767 bits, which are the same as those of CRC-16-ANSI/IBM (x^16 + x^15 + x^2 + 1) and CRC-16-CCITT (x^16 + x^12 + x^5 + 1). For more details about this CRC and other fast CRCs, see [1].
[1] G.D. Nguyen, “Fast CRCs,” IEEE Transactions on Computers, vol. 58, no. 10, pp. 1321-1331, Oct. 2009.
Upvotes: 0
sanju suthar
Reputation: 21
To generate checksum for packet for example this packet "78781101086655103641220800002122000fdcfd0d0a" "dcfd" this is error checksum this is generate by the "1101086655103641220800002122000f" these people are so misunderstand that the generate the response checksum of "dcfd" but the checksum is generated by using its own packet data..
7878 and odoa is fixed so..
package org.traccar.helper;
import java.nio.ByteBuffer;
import java.nio.charset.StandardCharsets;
import java.util.zip.CRC32;
public final class Checksum {
private Checksum() {
}
public static class Algorithm {
private final int poly;
private final int init;
private final boolean refIn;
private final boolean refOut;
private final int xorOut;
private final int[] table;
public Algorithm(int bits, int poly, int init, boolean refIn, boolean refOut, int xorOut) {
this.poly = poly;
this.init = init;
this.refIn = refIn;
this.refOut = refOut;
this.xorOut = xorOut;
this.table = bits == 8 ? initTable8() : initTable16();
}
private int[] initTable8() {
int[] table = new int[256];
int crc;
for (int i = 0; i < 256; i++) {
crc = i;
for (int j = 0; j < 8; j++) {
boolean bit = (crc & 0x80) != 0;
crc <<= 1;
if (bit) {
crc ^= poly;
}
}
table[i] = crc & 0xFF;
}
return table;
}
private int[] initTable16() {
int[] table = new int[256];
int crc;
for (int i = 0; i < 256; i++) {
crc = i << 8;
for (int j = 0; j < 8; j++) {
boolean bit = (crc & 0x8000) != 0;
crc <<= 1;
if (bit) {
crc ^= poly;
}
}
table[i] = crc & 0xFFFF;
}
return table;
}
}
private static int reverse(int value, int bits) {
int result = 0;
for (int i = 0; i < bits; i++) {
result = (result << 1) | (value & 1);
value >>= 1;
}
return result;
}
public static int crc8(Algorithm algorithm, ByteBuffer buf) {
int crc = algorithm.init;
while (buf.hasRemaining()) {
int b = buf.get() & 0xFF;
if (algorithm.refIn) {
b = reverse(b, 8);
}
crc = algorithm.table[(crc & 0xFF) ^ b];
}
if (algorithm.refOut) {
crc = reverse(crc, 8);
}
return (crc ^ algorithm.xorOut) & 0xFF;
}
public static int crc16(Algorithm algorithm, ByteBuffer buf) {
int crc = algorithm.init;
while (buf.hasRemaining()) {
int b = buf.get() & 0xFF;
if (algorithm.refIn) {
b = reverse(b, 8);
}
crc = (crc << 8) ^ algorithm.table[((crc >> 8) & 0xFF) ^ b];
}
if (algorithm.refOut) {
crc = reverse(crc, 16);
}
return (crc ^ algorithm.xorOut) & 0xFFFF;
}
public static final Algorithm CRC8_EGTS = new Algorithm(8, 0x31, 0xFF, false, false, 0x00);
public static final Algorithm CRC8_ROHC = new Algorithm(8, 0x07, 0xFF, true, true, 0x00);
public static final Algorithm CRC16_IBM = new Algorithm(16, 0x8005, 0x0000, true, true, 0x0000);
public static final Algorithm CRC16_X25 = new Algorithm(16, 0x1021, 0xFFFF, true, true, 0xFFFF);
public static final Algorithm CRC16_MODBUS = new Algorithm(16, 0x8005, 0xFFFF, true, true, 0x0000);
public static final Algorithm CRC16_CCITT_FALSE = new Algorithm(16, 0x1021, 0xFFFF, false, false, 0x0000);
public static final Algorithm CRC16_KERMIT = new Algorithm(16, 0x1021, 0x0000, true, true, 0x0000);
public static final Algorithm CRC16_XMODEM = new Algorithm(16, 0x1021, 0x0000, false, false, 0x0000);
public static int crc32(ByteBuffer buf) {
CRC32 checksum = new CRC32();
while (buf.hasRemaining()) {
checksum.update(buf.get());
}
return (int) checksum.getValue();
}
public static int xor(ByteBuffer buf) {
int checksum = 0;
while (buf.hasRemaining()) {
checksum ^= buf.get();
}
return checksum;
}
public static int xor(String string) {
byte checksum = 0;
for (byte b : string.getBytes(StandardCharsets.US_ASCII)) {
checksum ^= b;
}
return checksum;
}
public static String nmea(String string) {
return String.format("*%02X", xor(string));
}
public static int sum(ByteBuffer buf) {
byte checksum = 0;
while (buf.hasRemaining()) {
checksum += buf.get();
}
return checksum;
}
public static int modulo256(ByteBuffer buf) {
int checksum = 0;
while (buf.hasRemaining()) {
checksum = (checksum + buf.get()) & 0xFF;
}
return checksum;
}
public static String sum(String msg) {
byte checksum = 0;
for (byte b : msg.getBytes(StandardCharsets.US_ASCII)) {
checksum += b;
}
return String.format("%02X", checksum).toUpperCase();
}
public static long luhn(long imei) {
long checksum = 0;
long remain = imei;
for (int i = 0; remain != 0; i++) {
long digit = remain % 10;
if (i % 2 == 0) {
digit *= 2;
if (digit >= 10) {
digit = 1 + (digit % 10);
}
}
checksum += digit;
remain /= 10;
}
return (10 - (checksum % 10)) % 10;
}
public static int ip(ByteBuffer data) {
int sum = 0;
while (data.remaining() > 0) {
sum += data.get() & 0xff;
if ((sum & 0x80000000) > 0) {
sum = (sum & 0xffff) + (sum >> 16);
}
}
while ((sum >> 16) > 0) {
sum = (sum & 0xffff) + sum >> 16;
}
sum = (sum == 0xffff) ? sum & 0xffff : (~sum) & 0xffff;
return sum;
}
}
for check your code use this as a example "78781101086655103641220800002122000fdcfd0d0a" pass it "1101086655103641220800002122000f" and it answer is "dcfd" meansyour code work fine
ignore 8a51 this is for just checking Don't worry for help :[email protected]
import this checksum in checksumExample and boom
my Checksum is in org.traccar.helper folder ..
`enter code here`package org.traccar.helper;
import java.nio.ByteBuffer;
public class ChecksumExample {
public static void main(String[] args) {
// Convert the hexadecimal string to a byte array
String hexData = "1101086655103641220800002122000f";
byte[] dataBytes = hexStringToByteArray(hexData);
// Create a ByteBuffer from the byte array
ByteBuffer buf = ByteBuffer.wrap(dataBytes);
// Use the CRC-16 algorithm
int crcValue = Checksum.crc16(Checksum.CRC16_X25, buf);
// Convert the expected checksum "8a51" to an integer
int expectedChecksum = Integer.parseInt("8a51", 16);
// Verify the checksum
boolean checksumMatch = crcValue == expectedChecksum;
System.out.println("Generated CRC-16: " + Integer.toHexString(crcValue));
System.out.println("Expected CRC-16: " + Integer.toHexString(expectedChecksum));
System.out.println("Checksum Match: " + checksumMatch);
}
// Helper method to convert a hexadecimal string to a byte array
private static byte[] hexStringToByteArray(String hex) {
int len = hex.length();
byte[] data = new byte[len / 2];
for (int i = 0; i < len; i += 2) {
data[i / 2] = (byte) ((Character.digit(hex.charAt(i), 16) << 4)
+ Character.digit(hex.charAt(i + 1), 16));
}
return data;
}
}
``
Upvotes: 1
Mark Adler
Reputation: 112189
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 <stddef.h>
// 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 <stdint.h>
#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 &= 0xffff;
while (len--) {
crc ^= *data++;
for (unsigned k = 0; k < 8; k++)
crc = crc & 1 ? (crc >> 1) ^ 0xa6bc : crc >> 1;
}
crc ^= 0xffff;
return crc;
}
unsigned crc16dnp_rem(unsigned crc, unsigned val, unsigned bits) {
crc = ~crc;
crc &= 0xffff;
val &= (1U << bits) - 1;
crc ^= val;
while (bits--)
crc = crc & 1 ? (crc >> 1) ^ 0xa6bc : crc >> 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 &= 0xffff;
while (len--)
crc = (crc >> 8) ^
table_byte[(crc ^ *data++) & 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 &= 0xffff;
while (len && ((ptrdiff_t)data & 0x7)) {
crc = (crc >> 8) ^
table_byte[(crc ^ *data++) & 0xff];
len--;
}
if (len >= 8) {
do {
uintmax_t word = crc ^ *(uintmax_t const *)data;
crc = table_word[7][word & 0xff] ^
table_word[6][(word >> 8) & 0xff] ^
table_word[5][(word >> 16) & 0xff] ^
table_word[4][(word >> 24) & 0xff] ^
table_word[3][(word >> 32) & 0xff] ^
table_word[2][(word >> 40) & 0xff] ^
table_word[1][(word >> 48) & 0xff] ^
table_word[0][word >> 56];
data += 8;
len -= 8;
} while (len >= 8);
}
while (len--)
crc = (crc >> 8) ^
table_byte[(crc ^ *data++) & 0xff];
return crc;
}
Upvotes: 9
ygs
Reputation: 21
for (pos = 0; pos < 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 & 0x0001) != 0) { // If the LSB is set
crc >>= 1; // Shift right and XOR 0xA001
crc ^= CRC16;
} else { // Else LSB is not set
crc >>= 1; // Just shift right
}
}
}
return crc;
Upvotes: 2
wrapperapps
Reputation: 988
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
Upvotes: 1
Michael Burr
Reputation: 340168
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 > 0)
{
bit_flag = out >> 15;
/* Get next bit: */
out <<= 1;
out |= (*data >> bits_read) & 1; // item a) work from the least significant bits
/* Increment bit counter: */
bits_read++;
if(bits_read > 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 < 16; ++i) {
bit_flag = out >> 15;
out <<= 1;
if(bit_flag)
out ^= CRC16;
}
// item c) reverse the bits
uint16_t crc = 0;
i = 0x8000;
int j = 0x0001;
for (; i != 0; i >>=1, j <<= 1) {
if (i & out) crc |= j;
}
return crc;
}
That function returns 0xbb3d for me when I pass in "123456789".
Upvotes: 73
Antonio Pires
Reputation: 547
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 >> 8 ^ *data_p++;
x ^= x>>4;
crc = (crc << 8) ^ ((unsigned short)(x << 12)) ^ ((unsigned short)(x <<5)) ^ ((unsigned short)x);
}
return crc;
}
Upvotes: 53
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