title: Arduino: Convert hex string to uint64 or decimal string
body: <p>I want to convert a hex string such as "43a2be2a42380" to their decimal representation using a uint64 variable. I need that because i'm implementing a RFID reader acting as a keyboard and the keypresses need to be the decimal digit.</p>
  
  <p>I have seen other answers (<a href="https://stackoverflow.com/questions/31830143/convert-hex-string-to-decimal-in-arduino">convert HEX string to Decimal in arduino</a>) and implementing a solution using strtoul but it only works for 32 bit integers and strtoull is not available.</p>
  
  <pre><code>uint64_t res = 0;
  String datatosend = String("43a2be2a42380");
  char charBuf[datatosend.length() + 1];
  datatosend.toCharArray(charBuf, datatosend.length() + 1) ;
  res = strtoul(charBuf, NULL, 16);
  </code></pre>
  
  <p>What can i do to get the decimal number of a big hex string / byte array using an Arduino?</p>