Generating and reading a PWM signal at the same time on Arduino Uno R3?

Question

The code below should generate a PWM signal on digital pin 3. But know I want to see if it is even working. I was thinking about a Event triggering method like ISR. But somehow it does not work. I guess a ISR needs to a Input Pin, e.g. it would work, when I would've a PWM signal coming from somewhere.

Is it even possible to generate a PWM signal and read it to see some results on the serial monitor?

edited code:

byte const PWM_PIN = 3;

volatile unsigned long pwm_value = 0;
volatile unsigned long prev_time = 0;

void setup() {
  pinMode(PWM_PIN, OUTPUT);
  Serial.begin(9600);
  attachInterrupt(digitalPinToInterrupt(PWM_PIN), rising, RISING);
}

void loop() {
  digitalWrite(PWM_PIN, HIGH);
  delay(400);
  digitalWrite(PWM_PIN, LOW);
  delay(1600);
}

void rising() {
  attachInterrupt(digitalPinToInterrupt(PWM_PIN), falling, FALLING);
  prev_time = millis();
}

void falling() {
  attachInterrupt(digitalPinToInterrupt(PWM_PIN), rising, RISING);
  pwm_value = millis()-prev_time;
  Serial.println(pwm_value);
}

Answer 1

You are declaring the variables prev_time and pwm_value as int, which has (on a standard Arduino) 16 bits. Since you use the signed variant, you get a range of values from -32768 to 32767. But micros() is returning an unsigned long, which has 4 bytes (32 bits) and is not signed. With int there are only 32.767 ms (since it represents microseconds here), without letting the variable overflow. Even the full range would give you only about 65 ms. It simply cannot hold a value like 1000000. If you assign this value to an int, it will directly overflow (which - I think - would give you the value modulo 2^16), effectively loosing big parts of the information. You cannot get meaningful results with this in such a high time scale.

When measuring time (with millis() or micros()) you should always be sure to use unsigned long as type to prevent these problems.

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Apart from the variable type, why do you use micros() here? You are measuring times in the hundreds of milliseconds, so why not use millis()? This would keep the values lower.

Answer 2

1. You aren’t generating a correct PWM, because Serial.println introduces a delay. The right way for generating PWM is to use analogWrite function.

2. You can’t use attachInterrupt function for detecting rising and falling edge of an output pin.

3. In order to detect the rising and falling edge of a PWM signal, you have to use the timer interrupt of PWM module. For doing this, You have to write/read directly ATmega registers without using Arduino libraries. What version of Arduino are you using?