void setup() {
  uint64_t a = 0x9283;
  char* str = ui64toa(a);
  Serial.begin(115200);
  Serial.println(str);
  free(str);
}

void loop() {
  // put your main code here, to run repeatedly:

}

char* ui64toa(uint64_t value) {
  if (value == 0) {
    char* str = malloc(2);
    if (str == NULL) {
      Serial.println("malloc failed for ui64toa()");
      return NULL;
    }
    str[1] = '\0'; str[0] = '0';
    return str;
  }
  int i;
  for (i = 0; i <= 20; ++i) {
    if (value < pow(10, i))
      break;
  }
  char* str = malloc(i + 1);
  if (str == NULL) {
    Serial.println("malloc failed for ui64toa()");
    return NULL;
  }
  str[i] = (char) 0; // add null terminator
  for (--i; i >=0; --i) {
    str[i] = (char) ((value % 10) + 48);
    value = value / 10;
  }
  return str;
}