数字-侦察状态改变

监测按钮的状态变化（边沿监测）
如果你让按键工作，你经常会想做一些要按下很多次按钮的才有响应的动作。这时你需要知道按钮的从闭合到断开的变化状态，然后记录这个状态发生了多少次。这叫状态变化检测或者边沿检测。在这个教程里我们学习怎样检查状态变化，然后我们把相关信息发送到串口监视器里，并记录4次LED灯的开和关。
硬件要求

• Arduino or Genuino 开发板
• 即时按钮或者开关
• 10k ohm 电阻
• 连接线
• 面包板

电路

• 连接3根线到开发板。最开始两根，红和黑，连接到面包板上的两个长垂直行来提供5V电源电压和地。第三根线从数字引脚pin2连接到按钮的一个引脚。按钮的同一个引脚连接下拉电阻（10k ohm）到地。按钮的另一个引脚连接到5V电源。
  
• 按钮或者开关连接电路的两点。按钮是断开的（未按），按钮两个引脚是没有接通的，所以这个引脚连接到地（通过一个下拉电阻），读取为低电平或者0。当如果按钮是闭合的（未按），按钮两个引脚是接通的，所以这个引脚连接到5V，读取为高电平，或者1。
  
• 如果你没有连接到数字I/O口到任何地方，LED灯可能会不规则闪烁。这是因为输入引脚处于悬浮状态——它没有固定连接到电源或者地，并且它会随机在高电平和低电平之间切换。这是你需要下拉电阻的原因。
  

原理图

样例代码
下面的代码连续读取按钮的状态。然后通过循环来对比这个按钮的状态和它上一段时间的状态。如果当前按键的状态和之前的状态不一样并且当前状态是高电平，那么这个按键刚从关变为开。然后程序增加按键按下的计数器。
这个程序也检查按键按下次数的值，并且如果它是4的倍数，它会打开pin13的LED灯，否则这个LED灯饰熄灭的。

/*
  State change detection (edge detection)
 Often, you don't need to know the state of a digital input all the time,
 but you just need to know when the input changes from one state to another.
 For example, you want to know when a button goes from OFF to ON.  This is called
 state change detection, or edge detection.
 This example shows how to detect when a button or button changes from off to on
 and on to off.
 The circuit:
 * pushbutton attached to pin 2 from +5V
 * 10K resistor attached to pin 2 from ground
 * LED attached from pin 13 to ground (or use the built-in LED on
   most Arduino boards)
 created  27 Sep 2005
 modified 30 Aug 2011
 by Tom Igoe
This example code is in the public domain.
 http://www.arduino.cc/en/Tutorial/ButtonStateChange
 */
// this constant won't change:
const int  buttonPin = 2;    // the pin that the pushbutton is attached to
const int ledPin = 13;       // the pin that the LED is attached to
// Variables will change:
int buttonPushCounter = 0;   // counter for the number of button presses
int buttonState = 0;         // current state of the button
int lastButtonState = 0;     // previous state of the button
void setup() {
  // initialize the button pin as a input:
  pinMode(buttonPin, INPUT);
  // initialize the LED as an output:
  pinMode(ledPin, OUTPUT);
  // initialize serial communication:
  Serial.begin(9600);
}
void loop() {
  // read the pushbutton input pin:
  buttonState = digitalRead(buttonPin);
  // compare the buttonState to its previous state
  if (buttonState != lastButtonState) {
    // if the state has changed, increment the counter
    if (buttonState == HIGH) {
      // if the current state is HIGH then the button
      // wend from off to on:
      buttonPushCounter++;
      Serial.println("on");
      Serial.print("number of button pushes:  ");
      Serial.println(buttonPushCounter);
    } else {
      // if the current state is LOW then the button
      // wend from on to off:
      Serial.println("off");
    }
    // Delay a little bit to avoid bouncing
    delay(50);
  }
  // save the current state as the last state,
  //for next time through the loop
  lastButtonState = buttonState;
  // turns on the LED every four button pushes by
  // checking the modulo of the button push counter.
  // the modulo function gives you the remainder of
  // the division of two numbers:
  if (buttonPushCounter % 4 == 0) {
    digitalWrite(ledPin, HIGH);
  } else {
    digitalWrite(ledPin, LOW);
  }
}

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