// 優化效率
#define ddtick   __asm__("nop\n\t")
#if F_CPU == 20000000L
#define dd5tick  __asm__("nop\n\t""nop\n\t""nop\n\t""nop\n\t""nop\n\t")
#define delayOneUs()  { dd5tick; dd5tick; dd5tick; dd5tick; }
#endif
#if F_CPU == 16000000L
#define dd4tick  __asm__("nop\n\t""nop\n\t""nop\n\t""nop\n\t")
#define delayOneUs()  { dd4tick; dd4tick; dd4tick; dd4tick; }
#endif
#define delay2Us()    { delayOneUs(); delayOneUs(); }
#define delay5Us()    { delay2Us(); delay2Us(); delayOneUs(); }

int ping() {
	pinMode(PING_PIN, OUTPUT);
	digitalWrite(PING_PIN, LOW);  delay2Us();
	digitalWrite(PING_PIN, HIGH); delay5Us();
	digitalWrite(PING_PIN, LOW);
	pinMode(PING_PIN, INPUT);
	return pulseIn(PING_PIN, HIGH) / 29 / 2;
}