Serial monitor output not in sync with Oscilloscope reading

[dannykwc]

PWMTryWMaster1.ino (1.6 KB)
Good day. I'm an intern who has been assigned with a project to design a test kit for a PWM fan using Master and Slave Arduinos. Now, I've written the code for master Arduino with some help from the Internet. When I'm testing the output from the PWMPin 9 using a oscilloscope, it is outputting PWM signals with the desired intervals. But, when I'm displaying the value in Serial Monitor its lagging by one dutycycle. I.e: Oscilloscope shows 40% on time, but serial monitor shows 20%. Pardon my messy coding, I'm still new with Arduino. Any help would be appreciated. Thank you.

const int PWMPin=9;
int count=0;
unsigned long startTime;
int RPM;

void setup (){
  pinMode(PWMPin, OUTPUT);
  analogWrite (PWMPin, 0);
  Serial.begin(115200);
  attachInterrupt (digitalPinToInterrupt (2), counter, RISING);
}
void loop (){
  for (int pwm=0; pwm<=255; pwm+=51){
    analogWrite(PWMPin, pwm);
    delay(2000);
    startTime= millis();
    count=0;
    while ((millis()-startTime)<1000){
  }
  RPM=(count*60)/2;
  Serial.print("DutyCycle=");
  Serial.print(map(pwm,0,255,0,100));
  Serial.print("%\t");
  Serial.print("RPM= ");
  Serial.println(RPM); 
}}
void counter(){
  count++;
}

[LarryD]

Can you confirm the signal coming into pin 2 is a logic level signal and not open collector ?

Do not use delay( ) or while( ) in sketches unless you are absolutely sure the blocking code will not effect your sketch.

[dannykwc]

Hi, the Pin 2 is supposed to be connected to the Tachometer/Sense Wire of 4-wire fan to read the RPM.
I don't have the fan with me right now. Once I have it, I'll check and let you know.

As for, the while and delay(100), I tried removing them both, but it is still same.

[LarryD]

Have not gone line by line thru your sketch.

---

This is a two second delay:

@dannykwc

delay(2000);

This is a one second delay:

@dannykwc

while ((millis()-startTime)<1000){ }

[dannykwc]

Should I try after removing the 2 seconds delay along with one second delay?
But, then the reading in both oscilloscope and serial monitor will be too fast to check.

[dannykwc]

I have edited the code to much shorter version for your checking. The result is same.

[LarryD]

So you are freezing all normal code execution for 3 seconds, for what reason ?

---

If you want to trigger the scope to a line of code, first toggle a free Arduino pin and use it/that as a trigger for the scope.

Is this a digital storage scope ?

---

What do you want to see on the scope and at the same exact time, what do you want to happen on the serial monitor .

[LarryD]

@dannykwc

void setup (){
  pinMode(PWMPin, OUTPUT);
  analogWrite (PWMPin, 0);
  Serial.begin(115200);
  attachInterrupt (digitalPinToInterrupt (2), counter, RISING);
}

Try:

void setup ()
{
  pinMode(PWMPin, OUTPUT);
  pinMode(2, INPUT_PULLUP);  //  <————<<<<
  analogWrite (PWMPin, 0);
  Serial.begin(115200);
  attachInterrupt (digitalPinToInterrupt (2), counter, RISING);
}

Edit

Is this a 5v or 12v fan ?

BTW
Always show us a good schematic of your proposed circuit.
Show us a good image of your ‘actual’ wiring.
Give links to components.

[dannykwc]

i. The delay is for the fan to move to desired speed before outputting the RPM reading back to pin 2. I'm still new to Arduino, so I might be doing this the wrong way.

iii. The scope is PC Oscilloscope branded Hantek.
iv. When the scope displays 20% duty cycle , the serial monitor should do the same. Here its lagging, instead of 20% its displaying 0%, and when the reading is 40%, it displaying 20%

[dannykwc]

This a 24V fan. I have not connected the fan yet as my supervisor have not handed it to me. 24V Fan

[LarryD]

If I have the correct data sheet your sensor output is open collector so it will need the pull-up to 5v.

[dannykwc]

Yes that is correct datasheet. I drew a 10k pull-up resistor in schematic.
Just wanted to confirm, does that mean the sketch I uploaded is not causing the problem? If yes, then I will get back once the fan is handed out to me and after I retested. To see whether it happens then. Thanks again for replying