How to calculate the checksum of a NMEA sentence?

[UroBoros]

nmea crc

Hai

Inorder to input a command to my GPS module I want to calculate the checksum of that NMEA sentence. How to do that?

Any software available to calculate the checksum of a sentence?

Or how to that manually?

With regards

picstudent

 

[IanP]

how to calculate nmea input checksum

Calculating and Validating NMEA Checksums
Error correction and detection in NMEA data is handled through the use of checksums. A checksum is a two-character hexadecimal number, located at the end of each NMEA sentence, representing the "two's complement" of the sentence:


$GPGSA,A,2,29,19,28,,,,,,,,,,23.4,12.1,20.0*0F

In other words, each byte value between the dollar sign ($) and asterisk (*) is XOR'ed.

Attached is an example in C-code ..

Regards,
IanP

 

[ulong]

to calculate 8-bit checksum for a hexadecimal

Look at this :

{
char buffer[82],buff2[6];
uint8 i,crc,s;

snprintf(buffer,sizeof(buffer),
"$GPGGA," //MESSAGE ID
"%09.2f," //UTCTIME
"%09.5f," //LATITUDE
"%c," //N/S Indicator
"%011.5f," //LONTITUDE
"%c," //E/W Indicator
"%d," //Type of fix
"%d," //Number of SV
"%.2f," //HDOP
"%.1f,M," //MSL Altitude
"%.1f,M," //GEOID Separation
",0" //DIFF Age, DIFF Station

,92725.00 //UTCTIME
,4717.11399 //LATITUDE
,'N' //N/S Indicator
,833.91590 //LONTITUDE
,'E' //E/W Indicator
,1 //Type of fix
,8 //Number of SV
,1.01 //HDOP
,499.6 //MSL Altitude
,48. //GEOID Separation
);

//Calculate crc...
crc=0; s=strlen(buffer);
for(i=1;i<s;i++)
crc^=buffer;

//Add it!
snprintf(buff2,sizeof(buff2),"*%02X\r\n",crc);
strcat(buffer,buff2);
}

Add the CRC definition for NMEA :

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.

Chris.