Oct 15, 2015

测试Arduino中digital函数的执行时间

好久没有玩Arduino了，也没记录过多少关于Arduino的东西，现在有了时间，就写点东西吧，因为Arduino封装了基本功能的函数，方便很多，但好多函数内部可能添加了我们不知道的代码，我很自然地想到一个问题，运行时间是怎么样的呢，影响多大呢？测试一下，看看什么情况。

测试digitalWrite和digitalRead函数

sketch代码如下

程序中会使板上led以2秒为周期闪烁：

int ledState = 0;
long exeTime   = 0;
long dletaTime = 0;
String timeInfo = "Delta Time(write):";

void setup() {
    pinMode(13, OUTPUT);
    pinMode(12, INPUT);
    Serial.begin(9600);
}

void loop() {
    if(micros() - exeTime >= 1000000){
        // 测试digitalWrite的执行时间
        exeTime = micros();
        digitalWrite(13, ledState);
        dletaTime = micros() - exeTime;
        timeInfo = "Delta Time: Write->" + String(dletaTime) + "us";
        // 测试digitalRead的执行时间
        exeTime = micros();
        digitalRead(12);
        dletaTime = micros() - exeTime;
        timeInfo += ", Read->" + String(dletaTime) + "us";
        Serial.println(timeInfo);
        ledState = HIGH - ledState;
    }
}

结果及分析

结果如下：

Delta Time: Write->8us, Read->8us
Delta Time: Write->8us, Read->8us
Delta Time: Write->8us, Read->4us
Delta Time: Write->8us, Read->8us
Delta Time: Write->4us, Read->4us
Delta Time: Write->8us, Read->4us
Delta Time: Write->8us, Read->8us
Delta Time: Write->4us, Read->4us
Delta Time: Write->8us, Read->8us
Delta Time: Write->8us, Read->8us
...

分析

如上结果可以看到digitalWrite和digitalRead函数的执行时间大约8us，有时也会是4us，不太清楚这是怎么回事儿，不知道会不会是micros的时间误差导致的，不过看时间的话，这两个函数不会太影响运行时间的要求。

测试digitalWrite与寄存器直接赋值

sketch代码如下

int ledState = 0;
long exeTime   = 0;
long dletaTime = 0;
String timeInfo = "Delta Time(write):";

void setup() {
    pinMode(13, OUTPUT);
    pinMode(12, INPUT);
    Serial.begin(9600);
}

void loop() {
    if(micros() - exeTime >= 2000000){
        // 测试digitalWrite的执行时间
        exeTime = micros();
        digitalWrite(13, ledState);
        dletaTime = micros() - exeTime;
        timeInfo = "Delta Time: Write" + String(dletaTime) + "us";
        // 测试直接配置寄存器的执行时间
        delay(1000); 
        exeTime = micros();
        PORTB = 0x00;
        dletaTime = micros() - exeTime;
        timeInfo += ", Reg->" + String(dletaTime) + "us";
        Serial.println(timeInfo);
    }
}

结果及分析

结果如下：

Delta Time: Write8us, Reg->4us
Delta Time: Write4us, Reg->4us
Delta Time: Write8us, Reg->4us
Delta Time: Write8us, Reg->4us
Delta Time: Write8us, Reg->4us
Delta Time: Write8us, Reg->4us
Delta Time: Write8us, Reg->4us
...

分析：

根据结果可以看到，直接寄存器复制占用4us，虽比调用函数的速度快，但也差别不大，所以不用太担心调用digital函数会影响执行效率。

对比两种实现LED闪烁的方式

实现：

方式1：利用delay延时做到闪烁，每次loop打印Arduino运行的时间。

int ledState = 0;
long exeTime   = 0;
String timeInfo = "Time(write):";

void setup() {
    pinMode(13, OUTPUT);
    Serial.begin(9600);
    exeTime = millis();
}

void loop() {
    exeTime = millis();
    timeInfo = "time:" + String(exeTime) + "ms";
    digitalWrite(13, ledState);
    ledState = 1 - ledState;
    Serial.println(timeInfo);
    delay(1000);
}

方式2：利用时间判断，做到精确1000ms循环，每个loop打印运行时间；

int ledState = 0;
long exeTime   = 0;
String timeInfo = "Time(write):";

void setup() {
    pinMode(13, OUTPUT);
    Serial.begin(9600);
    exeTime = millis();
}

void loop() {
    if(millis() - exeTime >= 1000){
        exeTime = millis();
        timeInfo = "time:" + String(exeTime) + "ms";

        digitalWrite(13, ledState);
        ledState = 1 - ledState;

        Serial.println(timeInfo);
    }
}

对比结果

方式1

time:0ms
time:999ms
time:1999ms
time:3000ms
time:4001ms
time:5002ms
time:6002ms
time:7003ms
time:8003ms
time:9004ms
time:10004ms
time:11004ms
time:12005ms
time:13005ms
time:14006ms
time:15006ms

方式2

time:1000ms
time:2000ms
time:3000ms
time:4000ms
time:5000ms
time:6000ms
time:7000ms
time:8000ms
time:9000ms
time:10000m
time:11000ms
time:12000ms
time:13000ms
time:14000ms
time:15000ms
time:16000ms