RFID PN532 Variable Type

Projects General Guidance
1

Hey all,

I'm using the PN532 RFID Library. I'm trying this snippet of code in the Arduino IDE to read a value off of the card. I can see it in the Serial Monitor, but I can't work out how to store it in a variable, I've tried different variations of int, char, uint8_t,uint16_t and they all resolve to 0.

nfc.PrintChar(data, 4);

This is the function being referenced in the C++ Library:

/**************************************************************************/
Adafruit_PN532::PrintChar(const byte *data, const uint32_t numBytes) {
  uint32_t szPos;
   for (szPos = 0; szPos < numBytes; szPos++) {
    if (data[szPos] <= 0x1F)
      PN532DEBUGPRINT.print(F("."));
    else
      PN532DEBUGPRINT.print((char)data[szPos]);
  }
  PN532DEBUGPRINT.println();
}
2

so data is a pointer to a byte buffer
the PrintChar() method receives that pointer and a number of byte to print as ASCII characters. Not all values of the ASCII table have a visual representation that's why the code checks if the byte is less or equal than 0x1F because as you can see in the table those are control codes.

image
image1834×1260 610 KB

(you could argue that >= 07F it's also not always printable)

so you would get a dot for those otherwise you get the ASCII representation.

so the question now is what do you want to get? a c-string equivalent to what would be printed out, a String, a shorter byte buffer? what do you want to do with it? does it hold an int or something you need to parse?

3

Thanks for your detailed reply, that helps to clear up how the function works.

I have stored a 4 digit number onto the tags. I have three tags : 1001,1002,1003.

The aim is to read which number it is, and then run it through a switch statement to then do different actions, based on what tag has been read.

Thanks!

4

so when you call nfc.PrintChar(data, 4); you see 1001 for example and you would want to get that number

You could create a small function that gets you that number, something like

long readTagID(const byte *data, const uint32_t numBytes) {
  char tempBuffer[numBytes + 1];       // create a temporary buffer enough bytes including  for a trailing null char
  memcpy(tempBuffer, data, numBytes);  // copy the bytes
  tempBuffer[numBytes] = '\0';         // add the trailing null char
  return atol(tempBuffer);             // parse the content as a long number and return it
}

and you would use it like this

long theTagID = readTagID(testData, 4); // testData being the payload you got from the tag

example here

5

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