Help: RFID and serLCD

Projects General Guidance
11 
  if(dat >= 0x30 && dat <= 0x39)
  {
    return (dat - 0x30);
  }else if(dat >= 0x41 && dat <= 0x46)

OK, so you are comparing an incoming digit to be in the range 0 to 9, or A to F?

And you don't think this is more readable? When it mentions 0-9 and A-F and not some hex equivalent?

  if(dat >= '0' && dat <= '9')
  {
    return (dat - '0');
  }else if(dat >= 'A' && dat <= 'F')

Oh well, whatever works for you I guess.

I relatively, or with a bit of maths, know the value used.

With my method you don't need the maths.