I am curious about how to convert CRC.

Question

1.00/5 (1 vote)

See more:

# encoding algorithm
CRC=0xFF
CRC_New=0x00
For i=23 to 0 {
CRC_New[7]=CRC[6]
CRC_New[6]=CRC[5]
CRC_New[5]=CRC[4]
CRC_New[4]=CRC[3] ^CRC[7]
CRC_New[3]=CRC[2] ^CRC[7]
CRC_New[2]=CRC[1] ^CRC[7]
CRC_New[1]=CRC[0]
CRC_New[0]=Input_data[i]^CRC[7]
CRC=CRC_New
}
# final invertion before appending
For i=0 to 7 {
CRC[i] = CRC[i]^1
}
The symbol ^ indicates the XOR operator.
For example, the CRC of 0xA0CA85 is 2F and the CRC of 0x3B007C is C2.

What I have tried:

CRC results do not match.
I don't think it's the general CRC table method I know.
Can anyone convert and explain to CPP code?

Posted 19-Apr-22 22:00pm

Min woo Park 2022

Updated 19-Apr-22 23:40pm

v2

Add a Solution

Comments

CPallini 20-Apr-22 4:21am

Convert... From?

Min woo Park 2022 21-Apr-22 2:08am

OK, Thanks!

OriginalGriff 20-Apr-22 5:26am

"CRC results do not match."
Do not match what? What are you testing / comparing with what, and how?

2 solutions

Solution 1

See crc algorithm in c - Google Search[^]

Posted 19-Apr-22 23:38pm

Richard MacCutchan

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

CPallini · Accepted Answer · 2022-04-19T23:40:00

Assuming the code you posted works on bits, here you are a C++ program that closely resembles it:

C++

#include <iostream>
using namespace std;

union byte
{
  struct
  {
    unsigned b0 : 1;
    unsigned b1 : 1;
    unsigned b2 : 1;
    unsigned b3 : 1;
    unsigned b4 : 1;
    unsigned b5 : 1;
    unsigned b6 : 1;
    unsigned b7 : 1;
  } bit;
  unsigned char u8;
};

byte compute_crc(unsigned data)
{
  byte crc;
  byte crc_new;
  crc.u8 = 0xFF;
  crc_new.u8 = 0x00;

  for (size_t i=0; i<24; ++i)
  {
    crc_new.bit.b7 = crc.bit.b6;
    crc_new.bit.b6 = crc.bit.b5;
    crc_new.bit.b5 = crc.bit.b4;
    crc_new.bit.b4 = crc.bit.b3 ^ crc.bit.b7;
    crc_new.bit.b3 = crc.bit.b2 ^ crc.bit.b7;
    crc_new.bit.b2 = crc.bit.b1 ^ crc.bit.b7;
    crc_new.bit.b1 = crc.bit.b0;
    unsigned data_i = (data >> (23-i) ) & 1;
    crc_new.bit.b0 = data_i ^ crc.bit.b7;
    crc.u8 = crc_new.u8;
  }
  crc.u8 ^= 0xFF;
  return crc;
}

int main()
{
  unsigned a[] = { 0xA0CA85, 0x3B007C};

  for (size_t n = 0; n < sizeof(a)/sizeof(a[0]); ++n)
  {
    byte crc = compute_crc(a[n]);
    cout << "data = " << hex << uppercase << a[n] << ", CRC = " << (unsigned) crc.u8 << endl;
  }
}

[update]
You may also use bitsets

C++

#include <iostream>
#include <bitset>
using namespace std;

using byte = bitset<8>;

byte compute_crc(bitset<24> data)
{
  byte crc = 0xFF;
  byte crc_new = 0x00;
  for (int i = 23; i>=0; --i)
  {
    crc_new[7] = crc[6];
    crc_new[6] = crc[5];
    crc_new[5] = crc[4];
    crc_new[4] = crc[3] ^ crc[7];
    crc_new[3] = crc[2] ^ crc[7];
    crc_new[2] = crc[1] ^ crc[7];
    crc_new[1] = crc[0];
    crc_new[0] = data[i] ^ crc[7];
    crc = crc_new;
  }
  crc ^= 0xFF;
  return crc;
}

int main()
{
  bitset<24> a[] = { 0xA0CA85, 0x3B007C };

  for (size_t n = 0; n < sizeof(a)/sizeof(a[0]); ++n)
  {
    byte crc = compute_crc(a[n]);
    cout << "data = " << hex << uppercase << a[n].to_ulong() << ", CRC = " << crc.to_ulong() << endl;
  }
}