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How to do a Double Byte XOR Checksum?

Sean_Miller

I have a data packet that states its data validation (check code) is verified by a double byte XOR.  I've Googled for a tutorial and came up empty.  I've tried to solve the puzzle in Excel to never get a match to serial data I captured with Putty.  I could use a tip on how the algorithm to apply.

 

The packet description is below.  Starting at position 8, the two byte CS item contains the check code and states it uses a double-byte XOR to verify the current data packet.  I've tried adding up all bytes starting at Si in series which equates to a two byte value.  I XOR it against the CS value I've assed to be xC4AD, but that didn't resolve to 0.

 

Should I be XORing blocks of 2 bytes and comparing to the CS value? 

 

 

image

 

image

 

Example packet:

        

AA 55 00 28 85 AE 9D 0C AD C4 00 00 00 00 00 00 32 00 00 2A 12 59 12 00 00 00 00 A6 1F E5 1F 8D 1F 6D 1F 0A 17 7D 17 62 55 09 1D B1 10 31 10 4D 10 7A 0F 61 0F 5D 0F 21 0F 19 0F 31 0F 49 0F 8D 0F 4E 6D 6D 1D 82 0D 84 0D 00 00 AE 1A 9D 1A 89 1A
Parents

  • Leave the lidar scanning for a while and letme know if you find a checksum error. I suspect thst you wont

  • in reply to neuromodulator

    Yep...had a bug in that I was not XORing the full bounds of the sample data array in my ported Arduino code.  It's now 100% as you said!  The "Good Data" in my post a few minutes ago must have occurred when the Last Angle's (LSA) most significant byte was zero so it didn't affect the checksum.

     

    All that is happening here within .001 seconds absolutely boggles my mind and inspires me that the world should become an amazing place for our kids.

     

    Arduino code is below.  One could readily expand this to decipher object distance at given angles around the YDLIDAR or log a point map for importing into software to establish a geometry from a point cloud.  This approach is much, much leaner than their SDK and good for the likes of Arduino and the Azure Sphere Starter Kit.

     

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 212, 74  New Checksum: 212, 74

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 59, 83  New Checksum: 59, 83

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 3, 15  New Checksum: 3, 15

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 84, 101  New Checksum: 84, 101

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 218, 107  New Checksum: 218, 107

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 97, 95  New Checksum: 97, 95

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 133, 244  New Checksum: 133, 244

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 129, 92  New Checksum: 129, 92

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 171, 26  New Checksum: 171, 26

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 5, 155  New Checksum: 5, 155

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 90, 72  New Checksum: 90, 72

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 158, 81  New Checksum: 158, 81

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 169, 15  New Checksum: 169, 15

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 37, 96  New Checksum: 37, 96

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 72, 107  New Checksum: 72, 107

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 139, 98  New Checksum: 139, 98

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 66, 185  New Checksum: 66, 185

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 205, 70  New Checksum: 205, 70

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 157, 30  New Checksum: 157, 30

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 92, 236  New Checksum: 92, 236

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 250, 106  New Checksum: 250, 106

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 93, 85  New Checksum: 93, 85

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 51, 15  New Checksum: 51, 15

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 214, 104  New Checksum: 214, 104

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 129, 106  New Checksum: 129, 106

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 20, 95  New Checksum: 20, 95

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 237, 251  New Checksum: 237, 251

    20:06:07.435 -> Good:  Samples Sent:  40 Checksum: 108, 93  New Checksum: 108, 93

    20:06:07.469 -> Good:  Samples Sent:  40 Checksum: 53, 74  New Checksum: 53, 74

    20:06:07.469 -> Good:  Samples Sent:  40 Checksum: 140, 149  New Checksum: 140, 149

    20:06:07.469 -> Good:  Samples Sent:  40 Checksum: 178, 117  New Checksum: 178, 117

     

     

    bool first_byte_received;
bool second_byte_received;
int incomingByte;


void setup() {
  Serial.begin(115200);
  first_byte_received=false;
  second_byte_received=false;
}


void loop() {
  // Poll for our expected 3 bytes that reflect the packet header.
  if (Serial.available()) {      // If anything comes in Serial (USB),
    incomingByte=Serial.read();
    //Serial.println(incomingByte);
    if (!first_byte_received) {
      if (incomingByte==170) first_byte_received=true;
    }
    else if(!second_byte_received) {
      if (incomingByte==85) {
        second_byte_received=true;
      }
      else
      {
        first_byte_received=false;
      }
    }
    else
    {
      if (incomingByte==0) {
        first_byte_received=false;
        second_byte_received=false;
        ReceivePacket();
      }
      else
      {
        first_byte_received=false;
        second_byte_received=false;
      }
    }
  }
}


void ReceivePacket() {
  int samples_sent, starting_angle_LSB, starting_angle_MSB, end_angle_LSB, end_angle_MSB;
  int check_code_LSB, check_code_MSB;
  int left_column[50], right_column[50];
  int sample_bytes[100];
  
  while (!Serial.available());
  samples_sent=Serial.read();


  // Expect no more than 40 really, but just in case, just punt if more than 50.
  if (samples_sent>50) {    
    return;
  }
  
  while (!Serial.available());
  starting_angle_LSB=Serial.read();
  while (!Serial.available());
  starting_angle_MSB=Serial.read();
  while (!Serial.available());
  end_angle_LSB=Serial.read();
  while (!Serial.available());
  end_angle_MSB=Serial.read();
  while (!Serial.available());
  check_code_LSB=Serial.read();
  while (!Serial.available());
  check_code_MSB=Serial.read();
  
  for (int ii=0;ii<((2*samples_sent)); ii++) 
  {
    while (!Serial.available());  
    sample_bytes[ii]=Serial.read();
  }
  
  // Perform checksum
  int left_checksum=0;
  int right_checksum=0;
  
  left_checksum=left_checksum ^ 170;
  right_checksum=right_checksum ^ 85;
  
  left_checksum=left_checksum ^ starting_angle_LSB;
  right_checksum=right_checksum ^ starting_angle_MSB;


  for (int ii=0; ii<((samples_sent*2)-1); ii+=2)
  {
    left_checksum=left_checksum ^ sample_bytes[ii];
    right_checksum=right_checksum ^ sample_bytes[ii+1];
  }
  
  left_checksum=left_checksum ^ 0;
  right_checksum=right_checksum ^ samples_sent; 


  left_checksum=left_checksum ^ end_angle_LSB;
  right_checksum=right_checksum ^ end_angle_MSB;
  
  String assessment="Bad";
  if (left_checksum==check_code_LSB&&right_checksum==check_code_MSB) assessment="Good";
  Serial.println(assessment + ":  Samples Sent:  " + String(samples_sent) + " Checksum: " + String(check_code_LSB) + ", " + String(check_code_MSB) + "  New Checksum: " + String(left_checksum) + ", " + String(right_checksum) );
  while
(Serial.available()){Serial.read();}

}

Reply
  • in reply to neuromodulator

    Yep...had a bug in that I was not XORing the full bounds of the sample data array in my ported Arduino code.  It's now 100% as you said!  The "Good Data" in my post a few minutes ago must have occurred when the Last Angle's (LSA) most significant byte was zero so it didn't affect the checksum.

     

    All that is happening here within .001 seconds absolutely boggles my mind and inspires me that the world should become an amazing place for our kids.

     

    Arduino code is below.  One could readily expand this to decipher object distance at given angles around the YDLIDAR or log a point map for importing into software to establish a geometry from a point cloud.  This approach is much, much leaner than their SDK and good for the likes of Arduino and the Azure Sphere Starter Kit.

     

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 212, 74  New Checksum: 212, 74

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 59, 83  New Checksum: 59, 83

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 3, 15  New Checksum: 3, 15

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 84, 101  New Checksum: 84, 101

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 218, 107  New Checksum: 218, 107

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 97, 95  New Checksum: 97, 95

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 133, 244  New Checksum: 133, 244

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 129, 92  New Checksum: 129, 92

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 171, 26  New Checksum: 171, 26

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 5, 155  New Checksum: 5, 155

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 90, 72  New Checksum: 90, 72

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 158, 81  New Checksum: 158, 81

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 169, 15  New Checksum: 169, 15

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 37, 96  New Checksum: 37, 96

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 72, 107  New Checksum: 72, 107

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 139, 98  New Checksum: 139, 98

    20:06:07.391 -> Good:  Samples Sent:  40 Checksum: 66, 185  New Checksum: 66, 185

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 205, 70  New Checksum: 205, 70

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 157, 30  New Checksum: 157, 30

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 92, 236  New Checksum: 92, 236

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 250, 106  New Checksum: 250, 106

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 93, 85  New Checksum: 93, 85

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 51, 15  New Checksum: 51, 15

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 214, 104  New Checksum: 214, 104

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 129, 106  New Checksum: 129, 106

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 20, 95  New Checksum: 20, 95

    20:06:07.423 -> Good:  Samples Sent:  40 Checksum: 237, 251  New Checksum: 237, 251

    20:06:07.435 -> Good:  Samples Sent:  40 Checksum: 108, 93  New Checksum: 108, 93

    20:06:07.469 -> Good:  Samples Sent:  40 Checksum: 53, 74  New Checksum: 53, 74

    20:06:07.469 -> Good:  Samples Sent:  40 Checksum: 140, 149  New Checksum: 140, 149

    20:06:07.469 -> Good:  Samples Sent:  40 Checksum: 178, 117  New Checksum: 178, 117

     

     

    bool first_byte_received;
bool second_byte_received;
int incomingByte;


void setup() {
  Serial.begin(115200);
  first_byte_received=false;
  second_byte_received=false;
}


void loop() {
  // Poll for our expected 3 bytes that reflect the packet header.
  if (Serial.available()) {      // If anything comes in Serial (USB),
    incomingByte=Serial.read();
    //Serial.println(incomingByte);
    if (!first_byte_received) {
      if (incomingByte==170) first_byte_received=true;
    }
    else if(!second_byte_received) {
      if (incomingByte==85) {
        second_byte_received=true;
      }
      else
      {
        first_byte_received=false;
      }
    }
    else
    {
      if (incomingByte==0) {
        first_byte_received=false;
        second_byte_received=false;
        ReceivePacket();
      }
      else
      {
        first_byte_received=false;
        second_byte_received=false;
      }
    }
  }
}


void ReceivePacket() {
  int samples_sent, starting_angle_LSB, starting_angle_MSB, end_angle_LSB, end_angle_MSB;
  int check_code_LSB, check_code_MSB;
  int left_column[50], right_column[50];
  int sample_bytes[100];
  
  while (!Serial.available());
  samples_sent=Serial.read();


  // Expect no more than 40 really, but just in case, just punt if more than 50.
  if (samples_sent>50) {    
    return;
  }
  
  while (!Serial.available());
  starting_angle_LSB=Serial.read();
  while (!Serial.available());
  starting_angle_MSB=Serial.read();
  while (!Serial.available());
  end_angle_LSB=Serial.read();
  while (!Serial.available());
  end_angle_MSB=Serial.read();
  while (!Serial.available());
  check_code_LSB=Serial.read();
  while (!Serial.available());
  check_code_MSB=Serial.read();
  
  for (int ii=0;ii<((2*samples_sent)); ii++) 
  {
    while (!Serial.available());  
    sample_bytes[ii]=Serial.read();
  }
  
  // Perform checksum
  int left_checksum=0;
  int right_checksum=0;
  
  left_checksum=left_checksum ^ 170;
  right_checksum=right_checksum ^ 85;
  
  left_checksum=left_checksum ^ starting_angle_LSB;
  right_checksum=right_checksum ^ starting_angle_MSB;


  for (int ii=0; ii<((samples_sent*2)-1); ii+=2)
  {
    left_checksum=left_checksum ^ sample_bytes[ii];
    right_checksum=right_checksum ^ sample_bytes[ii+1];
  }
  
  left_checksum=left_checksum ^ 0;
  right_checksum=right_checksum ^ samples_sent; 


  left_checksum=left_checksum ^ end_angle_LSB;
  right_checksum=right_checksum ^ end_angle_MSB;
  
  String assessment="Bad";
  if (left_checksum==check_code_LSB&&right_checksum==check_code_MSB) assessment="Good";
  Serial.println(assessment + ":  Samples Sent:  " + String(samples_sent) + " Checksum: " + String(check_code_LSB) + ", " + String(check_code_MSB) + "  New Checksum: " + String(left_checksum) + ", " + String(right_checksum) );
  while
(Serial.available()){Serial.read();}

}

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