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APN-030 Rev 1 October 14, 2004
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32-Bit CRC and XOR Checksum Computation
1 Purpose
The purpose of this document is to introduce the algorithm of the 32-bit CRC and show
how to calculate the checksum of NovAtel OEM4 ASCII and BINARY logs together
with comprehensive examples. The XOR checksum for NMEA logs is also explained in
this document.
2 32-Bit CRC Checksum
The ASCII and Binary OEM4 family message formats all contain a 32-bit CRC for data
verification. This allows the user to ensure that the data received (or transmitted) is valid
with a high level of certainty. This CRC can be generated using the following C/C++
algorithm.
• Calculate a CRC value to be used by CRC calculation functions
#define CRC32_POLYNOMIAL 0xEDB88320
unsigned long CRC32Value(int i)
{
int j;
unsigned long ulCRC;
ulCRC = i;
for (j=8;j>0;j--)
{
if (ulCRC & 1)
ulCRC = (ulCRC >> 1)^CRC32_POLYNOMIAL;
else ulCRC >>= 1;
}
return ulCRC;
}
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• Calculate the 32-Bit CRC of a block of data all at once
unsigned long CalculateBlockCRC32(
unsigned long ulCount,
unsigned char *ucBuffer)
{
unsigned long ulTemp1;
unsigned long ulTemp2;
unsigned long ulCRC = 0;
while (ulCount-- != 0)
{
ulTemp1 = (ulCRC >> 8) & 0x00FFFFFFL;
ulTemp2 = CRC32Value(((int)ulCRC^*ucBuffer++)&0xff);
ulCRC = ulTemp1^ulTemp2;
}
return(ulCRC);
}
2.1 Calculate 32-Bit CRC for NovAtel ASCII and BINARY
Example:
“BESTPOSA” and “BESTPOSB” from a NovAtel OEM4 receiver.
ASCII:
#BESTPOSA,COM1,0,65.5,FINESTEERING,1254,412250.000,00000000,4ca6,34084;
SOL_COMPUTED,SINGLE,51.11638281392,-114.03823667672,1057.5312,-16.2713,
WGS84,1.7067,1.3294,2.6578,\"\",0.000,0.000,8,8,0,0,0,0,0,0*f8a1c3e1
BINARY:
0xAA, 0x44, 0x12, 0x1C, 0x01, 0x00, 0x00, 0x20, 0x20, 0x00, 0x00, 0x00,
0x00, 0x14, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x20, 0x00, 0x00, 0x00, 0x2A, 0x00, 0x00, 0x00,
0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xF0, 0x3F,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x7a, 0x9e, 0x13, 0xfd
* Checksums are shown in bold italics above.
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• ASCII
#include <iostream.h>
#include <string.h>
void main()
{
char_*Buff_=_”BESTPOSA,COM1,0,65.5,FINESTEERING,1254,412250.000,
00000000,4ca6,34084;SOL_COMPUTED,SINGLE,51.11638281392,-
114.03823667672,1057.5312,-
16.2713,WGS84,1.7067,1.3294,2.6578,\"\",0.000,0.000,8,8,0,0,0,0,
0,0";
unsigned long iLen = strlen(Buff);
unsigned long CRC = CalculateBlockCRC32(iLen, (unsigned char*)
Buff);
cout << hex << CRC <<endl;
}
• BINARY
#include <iostream.h>
#include <string.h>
unsigned long ByteSwap (unsigned long n)
{
return (((n &0x000000FF)<<24)+(( n &0x0000FF00)<<8)+
((n &0x00FF0000)>>8)+(( n &0xFF000000)>>24));
}
void main()
{
unsigned char Buff[] = {0xAA, 0x44, 0x12, 0x1C, 0x01, 0x00,
0x00, 0x20, 0x20, 0x00, 0x00, 0x00, 0x00, 0x14, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x20, 0x00, 0x00, 0x00, 0x2A, 0x00, 0x00, 0x00,
0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0xF0, 0x3F, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00};
unsigned int iLen = sizeof Buff;
unsigned long CRC = CalculateBlockCRC32(iLen, Buff);
cout << hex << ByteSwap(CRC) <<endl;
}
** In the binary example, an optional function has been included. The purpose of this
function (“ByteSwap” ) is to reverse the byte order of the checksum. For more detail
information on the byte order, please refer the section 4, “What are Little Endian and
Big Endian?”.
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3 NMEA XOR Checksum
The NMEA checksum is an XOR of all the bytes (including delimiters such as ‘,’ but
excluding the * and $) in the message output. It is therefore an 8-bit and not a 32-bit
checksum for NMEA logs.
NMEA GPGGA example:
$GPGGA,204502.00,5106.9813,N,11402.2921,W,1,09,0.9,1065.02,
M,-16.27,M,,*6F
#include <iostream.h>
#include <string.h>
void main()
{
int i;
unsigned char XOR;
char *Buff =
"GPGGA,204502.00,5106.9813,N,11402.2921,W,1,09,0.9,1065.02,M,-
16.27,M,,";
unsigned long iLen = strlen(Buff);
for (XOR = 0, i = 0; i < iLen; i++)
XOR ^= (unsigned char)Buff[i];
cout << hex << (int) XOR <<endl;
}
4 What are Little Endian and Big Endian?
"Little Endian" means that the least significant byte of the number is stored in memory at
the lowest address, and the most significant byte at the highest address.
For example, we have 4 bytes of long integer number.
Table 4-1: 4 bytes of long integer number
Byte 3 Byte 2 Byte 1 Byte 0
Inside of the computer memory, the Little Endian long integer number will be stored as
follows.
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Table 4-2: Storage order in the computer memory for the Little Endian
Address 3 Address 2 Address 1 Address 0
Byte 3 Byte 2 Byte 1 Byte 0
IBM or Intel PC computers store bytes in the order of the Little Endian.
"Big Endian" means that the most significant byte of the number is stored in memory at
the lowest address, and the least significant byte at the highest address.
Table 4-3: Storage order in the computer memory for the Big Endian
Address 3 Address 2 Address 1 Address 0
Byte 0 Byte 1 Byte 2 Byte 3
Final Points
If you require any further information regarding the topics covered within this
application, please contact:
NovAtel Customer Service
1120 – 68 Ave. N.E.
Calgary, Alberta, Canada, T2E 8S5
Phone: 1-800-NOVATEL (in Canada or the U.S.) or +1-403-295-4500
Fax: 403-295-4501
E-mail: support@novatel.ca
Website: www.novatel.com
