NMEA Checksum gone wrong

Projects Programming
1

Hi All,

I am trying to run a checksum algorithm found in this forum (probably from the ArduPilot project).

Using the code enclosed I keep getting the wrong checksum 0, the correct checksum is 5B.

However, using the commented out string I get the correct checksum of 68.

What am I doing wrong?

Thanks

//$GPRMC,225446,A,4916.45,N,12311.12,W,000.5,054.7,191194,020.3,E68
//$GPRMC,050000,A,331.57,N,00356.61,E,000.0,269.0,010116,004.0,W5B

int index;
char inchar;
//String tempPos = "$GPRMC,225446,A,4916.45,N,12311.12,W,000.5,054.7,191194,020.3,E*";
String tempPos ="$GPRMC,050000,A,331.57,N,00356.61,E,000.0,269.0,010116,004.0,W*";
char pos[80];

byte start_with = 0;
byte end_with = 0;
byte CRC = 0;

void setup() {

Serial.begin(9600);

}

void loop() {
tempPos.toCharArray(pos, 80);
// Serial.print(pos);
for ( index = 0; index < 80; index++) {
inchar = pos[index];

if ( inchar == '$') {
start_with = index;
}

if (inchar == '*') {
end_with = index;
}

for (byte x = start_with + 1; x < end_with; x++) { // XOR every character in between '$' and '*'
CRC = CRC ^ pos[x];

}
}
Serial.println(CRC, HEX);}

2

As best as I can tell, it might have something to do with this part of the program:

josefvs:
CRC = CRC ^ pos[x];

Try posting with code tags.

3 
//$GPRMC,225446,A,4916.45,N,12311.12,W,000.5,054.7,191194,020.3,E*68
//$GPRMC,050000,A,331.57,N,00356.61,E,000.0,269.0,010116,004.0,W*5B

int index;
char inchar;
//String temp = "$GPRMC,225446,A,4916.45,N,12311.12,W,000.5,054.7,191194,020.3,E*";
String temp="$GPRMC,050000,A,331.57,N,00356.61,E,000.0,269.0,010116,004.0,W*";
char pos[80];

byte start_with = 0;
byte end_with = 0;
byte CRC = 0;

void setup() {

  Serial.begin(9600);

}

void loop() {
  temp.toCharArray(pos, 80);

//  Serial.print(pos);
  for ( index = 0; index < 80; index++) {
    inchar = pos[index];

    if ( inchar == '

) {
      start_with = index;
    }

if (inchar == '*') {
      end_with = index;
    }

for (byte x = start_with + 1; x < end_with; x++) { // XOR every character in between '


 and '*'
      CRC = CRC ^ pos[x];
    }
  }
  Serial.println(CRC, HEX);}
4

The loop function has improperly nested loops, so the checksum (not CRC) calculation is repeated for every step of the "index" loop.

5

Thanks jremington, that will teach me to check better copied code in the future.

The working version:

//$GPRMC,225446,A,4916.45,N,12311.12,W,000.5,054.7,191194,020.3,E*68
//$GPRMC,050000,A,331.57,N,00356.61,E,000.0,269.0,010116,004.0,W*5B

int index;
char inchar;
//String temp = "$GPRMC,225446,A,4916.45,N,12311.12,W,000.5,054.7,191194,020.3,E*";
String temp = "$GPRMC,050000,A,331.57,N,678453.61,E,000.0,269.0,010116,004.0,W*";
char pos[80];

byte start_with = 0;
byte end_with = 0;
byte CRC = 0;

void setup() {

  Serial.begin(9600);

}

void loop() {

  temp.toCharArray(pos, 80);

  Serial.print(pos);

  for ( index = 0; index < 80; index++) {
    inchar = pos[index];

    if ( inchar == '

) {
      start_with = index;
    }

if (inchar == '*') {
      end_with = index;
    }

}

for (byte x = start_with + 1; x < end_with; x++) { // XOR every character in between '


 and '*'
    CRC = CRC ^ pos[x];
  }
  Serial.println(CRC, HEX);
  CRC = 0;
}