原创 calculate nmea string checksum

The sample code is to check the correct gprmc format with the last chars, and try to calculate the checksum of gprmc.

// parameters:
// softuart_revbuf: the whole nmea string with header_char '$' and endchar_char 0x0a
// softuart_revbuf_len: the length of this nmea string
// return:
// TRUE: right format; FALSE: bad string format with error end chars or checksum
bool nmea_checksum(U8* softuart_revbuf, U32 softuart_revbuf_len)
{
            U8 checksum = 0;
            U8 sum[2], nmea_sum[2];

            // "$GPRMC"
            if( (softuart_revbuf[3]=='R') && (softuart_revbuf[4]=='M') && (softuart_revbuf[5]=='C') ) {

                // check nmea data with length, and '*'<0x0d><0x0a> as endchar
                if( (softuart_revbuf_len<10) || (softuart_revbuf[softuart_revbuf_len-2]!=0x0d) || (softuart_revbuf[softuart_revbuf_len-5]!='*') ) {
                    return FALSE;               
                }
               
                // checksum
                for(U32 i=1; i<(softuart_revbuf_len-5); i++)
                    checksum ^= softuart_revbuf;
                nmea_sum[0] = softuart_revbuf[softuart_revbuf_len-4];
                nmea_sum[1] = softuart_revbuf[softuart_revbuf_len-3];           
                copy_2xchar_1xchar(sum, nmea_sum, 1);
                if( checksum != sum[0] ) {
                    return FALSE;
                }
           }

           return TRUE;
}


Allen Zhan
GV-TECH
2010.05.24