How to capture millisecond in arduino - arduino

Question

It is possible to serial print how milliseconds every output high. Let say i write an code analogRead. If analogread bigger than 600, then digitalwrite 13, high. If output pin 13 high, then capture how millisecond until the pin 13 goto low.
I try millis() but it cannot reset back to zero... It there anyway to reset millis() to zero?

Answer 1

You need a variable to keep track of your millis() count between events, kind of like a stop watch: you need to remember you started counting, so you can measure the difference when you stop.
Here's a quick sketch to illustrate the idea. Code isn't tested on an Arduino, but you should be able to understand the concept from the comments:
long lastTime;
int ledState = 0;
void setup() {
Serial.begin(115200);
pinMode(13,OUTPUT);
}
void loop() {
int analogValue = analogRead(0);
int newLedState = (analogValue > 600);//need to check, hoping boolean will evaluat to 1 when true
if(ledState == 0 && newLedState == 1){//if the led was of, but will be turned on, start the stop watch
lastTime = millis();//store the current time
ledState = newLedState;
}
if(ledState == 1 && newLedState == 0){//if the led was on, but will be turned off, get the difference between the last time we started counting time
long difference = millis() - lastTime; //get the current time, but subtract the last stored time
ledState = newLedState;
Serial.println(difference);
}
digitalWrite(13,ledState);
}

Answer 2

Okay so I think I understand what you want here. You want to capture the time difference between 2 pins changing their electrical state. To do this you can use pulseIn().
Using your example in your question we get...
if (13 == HIGH) {
millis = pulseIn(13, LOW);
}
That will give you the duration between the pin going high and then going low, stored in the variable millis. It's good practice to store it in a long.