What is the work of pulseIn?

Question

I have code for an ultrasonic sensor which I found from a site. Here is the code:

#define trigPin 12
#define echoPin 13

void setup() {
  Serial.begin (9600);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
}

void loop() {
  int duration, distance;
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(1000);
  digitalWrite(trigPin, LOW);
  duration = pulseIn(echoPin, HIGH);
  distance = (duration/2) / 29.1;
  if (distance >= 200 || distance <= 0){
    Serial.println("Out of range");
  }
  else {
    Serial.print(distance);
    Serial.println(" cm");
  }
  delay(500);
}

However, I don't understand the work of pulseIn() function. I mean, I want to know when the time count starts and when it ends. For example, in this code, does the time start at digitalWrite(trigPin, HIGH); or does the time start at the pulseIn() function?

If it's the second one, when it stops, then how does the time give us the distance of an obstacle when I am already delaying 1000 microseconds after I send a ping in air?

Answer 1

From the docs:

Reads a pulse (either HIGH or LOW) on a pin. For example, if value is HIGH, pulseIn() waits for the pin to go HIGH, starts timing, then waits for the pin to go LOW and stops timing. Returns the length of the pulse in microseconds. _pulsein

So, in this case, pulseIn(echoPin, HIGH) starts counting the number of microseconds until echoPin goes HIGH and stores that in duration.

It starts and ends on that line, it's what is known as a blocking function. It will really sit there until echoPin goes high and tells you how long it took (unless you specify a timeout).

That also means that any delays you have before or after the pulseIn call don't affect it in any way.

The way you get distance from this time is by the following equation:

distance = (duration/2) / 29.1;

You divide by two because it goes out and back so the time would be double that of a one-way travel. The 29.1 is the speed of sound (which is 343.5 m/s => 1 / 0.03435 = 29.1). So note the result is in CM's, not inches. You could probably figure it out by looking at the data sheet of the sensor or just get a lot of samples relating duration to distance (you would measure the distance manually) and get an equation that is very similar.

Answer 2

Indeed it would be a problem if you started to measure the pulse length 1000 microseconds after it starts. However, this is not how the HC-SR04 sensor works:

• the sensor is triggered by the falling edge of TRIG, at digitalWrite(trigPin, LOW);

• the ECHO pulse starts about 0.3 ms after the trigger

That's why a delay of 1 ms doesn't affect the measurement result. pulseIn(echoPin, HIGH) will actually wait for the ECHO pin go HIGH, then start measuring the pulse length until it goes LOW again. As such, the duration of the TRIG pulse can be reduced to 10 microseconds (minimum TRIG duration for HC-SR04), to make the measurements faster.

Answer 3

It is important to know that you do not measure the time from trigger start - by setting the Trigger pin HIGH - up to the Echo signal.

The Sensor HC-SR04 starts the measurement by receiving a HIGH on the Trigger input and then sends, a short time later, the time coded as the length of the HIGH level on the Echo pin.

If you use pulseIn() 2 ms, i.e. 2000μs, after triggering, it should work fine.

Answer 4

Don't get confused, as the ultrasonic module has a particular way of working. First you set a pulse in the trigpin. When it ends, the module sends 8 bursts of 40 kHz pulses (and thats actually what is used to measure distance, not your pulse in the trigpin, which goes nowhere). At exactly the moment the first burst is sent, the echo pin sets itself in HIGH. When this is happening, the program is in the line of pulseIn, and since echopin is in HIGH it starts timing (because pulseIN(echopin,HIGH) waits for echopin to be HIGH for start timing). When the first pulse of the 40 kHz bounces in the object and gets back to the receiver, the echopin sets itself in LOW. Then the pulseIn function stops the time and returns it. Then the program continues running. This module is a little bit tricky in order to learn the way of working of pulseIn.