Electronic – arduino – How to measure PWM on Oscilloscope

arduinooscilloscope

I’m measuring the voltage level from an Arduino Uno PWM pin using an oscilloscope. I’m using analogWrite(pin, voltage) in the loop with pinMode(pin,OUTPUT) in the setup.

I set the trigger to edge (rising), auto mode, but all I see is noise. Tried zooming in/out and moving up and down on the screen to find the signal at 3.3 V but couldn’t find it.

What am I doing wrong?

Best Answer

Here is Sparkfun's example code:

int PWM_out_pin = 9; // Must be one of 3, 5, 6, 9, 10, or 11
// for Arduino Uno
void setup() {
pinMode(PWM_out_pin, OUTPUT);
}
void loop() {
byte PWM_out_level;
PWM_out_level = ... // Code logic to set output level
analogWrite( PWM_out_pin, PWM_out_level);
}

Use code similar to that (you need to have that loop structure, and I suggest, as a test, setting the output level to 127 so you get close to a square wave).

Make sure you use one of the valid pins and have your probe connected to that pin and the ground lead connected to GND of the Arduino.

If you still see nothing, try the autoset function on your oscilloscope, or just set the sweep to about 1-2ms/division and the sensitivity to 0.5 or 1V/division. Make sure your probe and scope are set to the proper sensitivity (usually 10:1 is best) and make sure the probe is not switched to ground if that is an option. Most scopes have a cal output that you can hook the probe to, and it has a similar frequency to the Arduino PWM (use that to check your scope).

Some oscilloscopes will give very strange readings if you set the sweep too slow in comparison to the signal (due to lacking an anti-aliasing filter).

Related Solutions

Electronic – arduino – AttatchInterrupt Constantly interrupts

As you've determined that kind of receiver will have a PWM output to control a servo, the Wikipedia Servo Control article has some good information about it. If you wanted to use it anyway the normal neutral position will be 1.5mS so you could use something like the following code that uses the pulseIn function to treat a pulse of 1.75mS or longer as being on:

#define MY_PIN 0

void setup() {
    Serial.begin(9600);
    pinMode(MY_PIN, INPUT);
}

void loop() {
   if (pulseIn(MY_PIN, HIGH) >= 1750)
      Serial.println("TX on");        
}

Note that the typical period between pulses is 20mS so your code will be stalled in pulseIn for around that period. If that's a problem you could go back to using the interrupt and set it for CHANGE and keep track of the time between when the line goes high and low again. It would be something like the following untested code:

#define MY_PIN 0 // Make sure this matches the interrupt line
unsigned long start_micros = 0;
unsigned long last_duration = 0;

void setup() {
    Serial.begin(9600);
    pinMode(MY_PIN, INPUT);
    attachInterrupt(0 , PWM_handler, CHANGE);
}

void loop() {
   if (last_duration >= 1750)
      Serial.println("TX on");        
}

void PWM_hander()
{
    if (digitalRead(MY_PIN) == HIGH)
        start_micros = micros();
    else
        last_duration = micros() - start_micros;
}

Electrical – Arduino – Pulses get unstable when updating PWM duty-cycle

Could be because your variable 'val' does get different values. Because usually when you have a closed loop system the LSB's toggle even though the final value stabilizes. Even if yours is not a closed loop system I think by just logging 'val' first and checking the values must give a clue. Because if 'val' changes then its correct that your duty also changes with it.

Because in your second case you have the same code but it does not jitter which means that the arithmetic operations are not causing the jitter. I therefore think if you log the data (print the data continuously and see the value of 'val') it would be helpful or at least directing you in finding the right solution.

Best Answer

Related Solutions

Electronic – arduino – AttatchInterrupt Constantly interrupts

Electrical – Arduino – Pulses get unstable when updating PWM duty-cycle

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