char *ulltoa(uint64_t value, char *buf, int radix) {
	char tmp[64 + 1];
	char *p1 = tmp, *p2;
	static const char xlat[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
	if(radix < 2 || radix > 36)	return nullptr;
	do {*p1++ = xlat[value % (unsigned)radix];} while((value /= (unsigned)radix));
	for(p2 = buf; p1 != tmp; *p2++ = *--p1) ;
	*p2 = '\0';
	return buf;
}

void setup() {
  char buffer[65];
  Serial.begin(115200);
  uint64_t x = 1234567890123456789ull;
  Serial.println(ulltoa(x, buffer, 10));

  x = 0xFF00FF01FF02FF03;
  Serial.print("0x"); Serial.println(ulltoa(x, buffer, 16));
}

void loop() {}