3.3.2 pulseIn(pin,state,timeout)

pulseIn函数用于读取引脚脉冲的时间长度，脉冲可以是HIGH或LOW。如果是HIGH，函数将先等引脚变为高电平，然后开始计时，一直到变为低电平为止。返回脉冲持续的时间长短, 单位为ms。如果超时还没有读到的话, 将返回0。

pulseIn函数返回值类型为无符号长整型（unsigned long），3个参数分别表示脉冲输入的引脚、脉冲响应的状态（高脉冲或低脉冲）和超时时间。函数原型在wiring_pulse.c中，如下：

unsigned long pulseIn(uint8_t pin, uint8_t state, unsigned long timeout)
{     
      uint8_t bit = digitalPinToBitMask(pin);
      uint8_t port = digitalPinToPort(pin);
      uint8_t stateMask = (state ? bit : 0);
      unsigned long width = 0;
      // keep initialization out of time critical area
     
      unsigned long numloops = 0;
      unsigned long maxloops = microsecondsToClockCycles(timeout) / 16;
     
      // wait for any previous pulse to end
      while ((*portInputRegister(port) & bit) == stateMask)
                if (numloops++ == maxloops)
                return 0;
      // wait for the pulse to start
      while ((*portInputRegister(port) & bit) != stateMask)
                if (numloops++ == maxloops)
                return 0;
      // wait for the pulse to stop
      while ((*portInputRegister(port) & bit) == stateMask)
                width++;
     
      return clockCyclesToMicroseconds(width * 10 + 16);
}

可以在开发环境的下列实例程序中找到pulseIn函数的应用：

Memsic2125.pde、Ping.pde