arttuladhar
1/28/2012 - 5:22 PM

Performing XOR-Checksum for RFID [12 - Bit Checksum]

Performing XOR-Checksum for RFID [12 - Bit Checksum]

void setup() {
  Serial.begin(9600);                                 // connect to the serial port
  pinMode(13, OUTPUT);       // initialize the digital pin as an output  
}

void loop () {
  byte i = 0;
  byte val = 0;
  byte code[6];
  byte checksum = 0;
  byte bytesread = 0;
  byte tempbyte = 0;

  if(Serial.available() > 0) {
    if((val = Serial.read()) == 2) {                  // check for header
    
  digitalWrite(13, HIGH);   // set the LED on
  delay(1000);              // wait for a second
  digitalWrite(13, LOW);    // set the LED off
  delay(1000);              // wait for a second
    
    
      bytesread = 0;
      while (bytesread < 12) {                        // read 10 digit code + 2 digit checksum
        if( Serial.available() > 0) {
          val = Serial.read();
          if((val == 0x0D)||(val == 0x0A)||(val == 0x03)||(val == 0x02)) { // if header or stop bytes before the 10 digit reading
            break;                                    // stop reading
          }

          // Do Ascii/Hex conversion:
          if ((val >= '0') && (val <= '9')) {
            val = val - '0';
          } else if ((val >= 'A') && (val <= 'F')) {
            val = 10 + val - 'A';
          }

          // Every two hex-digits, add byte to code:
          if (bytesread & 1 == 1) {
            // make some space for this hex-digit by
            // shifting the previous hex-digit with 4 bits to the left:
            code[bytesread >> 1] = (val | (tempbyte << 4));

            if (bytesread >> 1 != 5) {                // If we're at the checksum byte,
              checksum ^= code[bytesread >> 1];       // Calculate the checksum... (XOR)
            };
          } else {
            tempbyte = val;                           // Store the first hex digit first...
          };

          bytesread++;                                // ready to read next digit
        }
      }

      // Output to Serial:

      if (bytesread == 12) {                          // if 12 digit read is complete
        Serial.print("5-byte code: ");
        for (i=0; i<5; i++) {
          if (code[i] < 16) Serial.print("0");
          Serial.print(code[i], HEX);
          Serial.print(" ");
        }
        Serial.println();

        Serial.print("Checksum: ");
        Serial.print(code[5], HEX);
        Serial.println(code[5] == checksum ? " -- passed." : " -- error.");
        Serial.println();
      }

      bytesread = 0;
    }
  }
}