convert String to unsigned long long
avr-libc lacks strtoull() that I would normally use for such a conversion. You will have to do it manually.

Fortunately it's quite simple - just start with y=0 and, for each character in the string, multiply y by 10 then add the numeric value of the character in the string:

String x = "68976543210";
unsigned long long y = 0;

for (int i = 0; i &lt; x.length(); i++) {
    char c = x.charAt(i);
   if (c &lt; '0' || c &gt; '9') break;
   y *= 10;
   y += (c - '0');
}


Note that the Arduino Print API has no method for printing a 64-bit integer.