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Measure square wave frequency

Former Member

Hello,

 

I'm working with the Arduino Due ans I used the following code:

 

// period of pulse accumulation and serial output, milliseconds

#define MainPeriod 100

long previousMillis = 0; // will store last time of the cycle end

volatile unsigned long duration=0; // accumulates pulse width

volatile unsigned int pulsecount=0;

volatile unsigned long previousMicros=0;

 

void setup()

{

  Serial.begin(19200);

  attachInterrupt(6, myinthandler, RISING);

}

 

void loop()

{

  unsigned long currentMillis = millis();

  if (currentMillis - previousMillis >= MainPeriod)

  {

    previousMillis = currentMillis;  

    // need to bufferize to avoid glitches

    unsigned long _duration = duration;

    unsigned long _pulsecount = pulsecount;

    duration = 0; // clear counters

    pulsecount = 0;

    float Freq = 1e6 / float(_duration);

    Freq *= _pulsecount; // calculate F

    // output time and frequency data to RS232

    Serial.print(currentMillis);

    Serial.print(" "); // separator!

    Serial.print(Freq);

    Serial.print(" ");

    Serial.print(_pulsecount);

    Serial.print(" ");

    Serial.println(_duration);

  }

}

 

void myinthandler() // interrupt handler

{

  unsigned long currentMicros = micros();

  duration += currentMicros - previousMicros;

  previousMicros = currentMicros;

  pulsecount++;

}

 

to measure the frequency.

 

I can measure frequencies higher than 1.5kHz accurately, but I can't seem to measure lower frequencies.

 

How can I measure lower frequencies, say 400Hz and above?

 

Thanks.

Parents 

  • // mills() reference http://arduino.cc/en/Reference/millis
// attachinterrupt() reference http://arduino.cc/en/Reference/attachInterrupt
// Due pin 0,1,2 corresponding to pin interrupt 0,1,2 respectively
// All the best from RAVI BUTANI
// this code measures frequency only accurate for >100Hz
// for less frequency minor modifications needed


unsigned long time_stamp;
unsigned int count0 = 0;
unsigned int count1 = 0;
unsigned int count2 = 0;


unsigned int output0 = 0;
unsigned int output1 = 0;
unsigned int output2 = 0;


void setup()
{
  Serial.begin(9600);
  time_stamp = millis();
  attachInterrupt(0, sensor0, RISING); // sensor 1 on mega pin no 2 Due pin 0
  attachInterrupt(1, sensor1, RISING); // sensor 2 on mega pin no 3 Due Pin 1
  attachInterrupt(2, sensor2, RISING); // sensor 3 on mega pin no 21 Due Pin 2
}


void loop()
{
  if(millis() - time_stamp > 50)
  {
    detachInterrupt(0); // disable interrups
    detachInterrupt(1);
    detachInterrupt(2);
    output0 = count0 * 20;
    output1 = count1 * 20;
    output2 = count2 * 20;
    Serial.print("Sensor0 Reading = ");
    Serial.println(output0);
    Serial.print("Sensor1 Reading = ");
    Serial.println(output1);
    Serial.print("Sensor2 Reading = ");
    Serial.println(output2);
    delay(40);
    count0 = 0;
    count1 = 0;
    count2 = 0;
    time_stamp = millis();
    attachInterrupt(0, sensor0, RISING); // we have done with this mesasurenment,enable interrups for next cycle
    attachInterrupt(1, sensor1, RISING);
    attachInterrupt(2, sensor2, RISING);
  }
}


void sensor0()
{
  count0++;
}


void sensor1()
{
  count1++;
}


void sensor2()
{
  count2++;
}

  • in reply to ravi_butani

    Thanks for your reply.

     

    I tried that, it measures 1500Hz and higher quite accurately but anything below it and it fails.

     

    Do you know what else can be done to fix this?

     

    Thanks again.

  • in reply to Former Member

    did you checkout the spark fun link I provided, what was the result

     

    Thanks

  • in reply to Robert Peter Oakes

    Thanks for that, but as far as I know this library is not compatible with the Due, right?

  • in reply to Former Member

    So in short, yes you can do it, I have done it before, a while back. I wont be able to investigate until the weekend though

  • in reply to Former Member

    Hey Matt, its obvious that if you want to measure low freq accurately you need long time window ...means your measurement time increases... in my code i hv kept time window of 50ms...

    you can consider more time window for accurate low freq measurements..

    modify these lines..

    if(millis() - time_stamp > 50)// 50ms window

    output0 = count0 * 20;

    all three outputx lines

     

    to

    if(millis() - time_stamp > 500)// 500ms window

    output0 = count0 * 2;

  • in reply to ravi_butani

    I'll try that later and report back, thanks!

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