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demedeiros
Joined: 27 Dec 2013
Posts: 71
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| Help with CRC |
 Posted: Tue Jan 30, 2024 7:33 pm |
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Hi All,
I am trying to use the CCS supplied crc.c file for calculation of a 32bit CRC. I am not sure if I am doing something wrong as the CRC result does not match anything that is calculated when I plug the data into https://crccalc.com/
| Code: |
void crcTest()
{
char testData[200] = "#,A,23,09,03,4000.400000,N,7000.000000,W,0.0,-95.52,0.000,0.000,01,02,00,00001,";
uint32_t myCRC = generate_32bit_crc(testData,strlen(testData),CRC_32);
fprintf(RF, "CRC: 0x%lx\r\n\n", myCRC);
}
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I have tried the length with strlen() and with sizeof() to (from my undertanding) include and not include the terminating character.
Its fairly straightforward, so I am at a loss for what I am doing wrong. This example shown here returns 0x42e9a8ef. If I plug the same data into crccalc, I do not get this value with any of the algorithms.
Link to CRCCalc https://crccalc.com/?crc=#,A,23,09,03,4000.400000,N,7000.000000,W,0.0,-95.52,0.000,0.000,01,02,00,00001,&method=crc32&datatype=ascii&outtype=0
Any thoughts? |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19538
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| Posted: Wed Jan 31, 2024 3:47 am |
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OK.
I think I've met this before. The CCS supplied CRC32 algorithm only works
correctly for sources that are a multiple of four bytes long.
I went online, and found a simpler algorithm, that actually saves quite a
lot of data movement, by reversing the bit order the code works through
the words. You therefore have to use a bit reversed 'key' for this (so
0xEDB88320, instead of 0x04C11DB7), but it results in faster calculation,
and as I wrote it, works directly on a 'string' (terminating on the null
terminator), so does not need a 'length'.
Attaching your code and this:
| Code: |
#define REVCRC_32 0xEDB88320 //bit reversed CRC32
unsigned int32 crc32str(unsigned int8 *message) {
int16 i=0; //support up to 65535 chars
int8 j;
unsigned int32 crc=0xFFFFFFFF, bmask; //initial value set
unsigned int8 b; //individual bytes
while (message[i] != 0)
{ //for an ASCII string - stop at terminator
b = message[i]; // Read next byte.
crc = crc ^ b; //XOR in the byte
for (j = 0; j < 8; j++) { // Repeat eight times.
bmask = -(crc & 1);
crc = (crc >> 1) ^ (REVCRC_32 & bmask);
}
i = i + 1;
}
return ~crc;
}
void crcTest()
{
char testData[] = "#,A,23,09,03,4000.400000,N,7000.000000,W,0.0,-95.52,0.000,0.000,01,02,00,00001,";
unsigned int32 myCRC = crc32str(testData);
printf( "CRC: 0x%lx\r\n\n", myCRC);
}
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Hopefully this will work for you.  |
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demedeiros
Joined: 27 Dec 2013
Posts: 71
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| Posted: Wed Jan 31, 2024 5:42 am |
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Ttelmah,
Thank you, once again you save the day!
Things like this make me doubt myself... "It should just work!!! What kind of idiotic mistake am I making! |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19538
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