check for missing parameters periodically

def fetch_check(self, master):
        '''check for missing parameters periodically'''
        if self.param_period.trigger():
            if master is None:
                return
            if len(self.mav_param_set) == 0:
                master.param_fetch_all()
            elif self.mav_param_count != 0 and len(self.mav_param_set) != self.mav_param_count:
                if master.time_since('PARAM_VALUE') >= 1:
                    diff = set(range(self.mav_param_count)).difference(self.mav_param_set)
                    count = 0
                    while len(diff) > 0 and count < 10:
                        idx = diff.pop()
                        master.param_fetch_one(idx)
                        count += 1