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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

  • When you measure frequency you need to adopt different strategies if your pulse frequency is not much faster than the gate period and of course your clock frequency needs to be much higher than the pulse frequency. If you are counting input pulses per gate period then the best you can do with 1 second gate is 0.5% resolution at 200Hz.

    If you count clock pulses per input pulse then an 8MHz clock gives you 0.0025% resolution at 200Hz but only 0.05% resolution at 4kHz.

    You'll get better results if you count clock pulses per n input pulses and adjust n to keep the gate period constant, and even better results if you generate a new count after every input pulse but then calculate the frequency over the last n input pulses and average the results.

     

    The timer in the AVR offers input capture - I'm not sure which Arduino you have but if based on a atmega328 you have 16 bit resolution at  up to 16 MHz  - you will need to able to service an interrupt at the pulse frequency and read the timer capture register on each rising or falling edge. This will probably need you to code in C at register level but you should be able to measure 200Hz to 4kHz with 1part in 4 - 8million resolution over that range (using a 1 second gate time and variable n (see above)). The accuracy will depend on the accuracy of the processor clock.

     

    MK

Reply

  • When you measure frequency you need to adopt different strategies if your pulse frequency is not much faster than the gate period and of course your clock frequency needs to be much higher than the pulse frequency. If you are counting input pulses per gate period then the best you can do with 1 second gate is 0.5% resolution at 200Hz.

    If you count clock pulses per input pulse then an 8MHz clock gives you 0.0025% resolution at 200Hz but only 0.05% resolution at 4kHz.

    You'll get better results if you count clock pulses per n input pulses and adjust n to keep the gate period constant, and even better results if you generate a new count after every input pulse but then calculate the frequency over the last n input pulses and average the results.

     

    The timer in the AVR offers input capture - I'm not sure which Arduino you have but if based on a atmega328 you have 16 bit resolution at  up to 16 MHz  - you will need to able to service an interrupt at the pulse frequency and read the timer capture register on each rising or falling edge. This will probably need you to code in C at register level but you should be able to measure 200Hz to 4kHz with 1part in 4 - 8million resolution over that range (using a 1 second gate time and variable n (see above)). The accuracy will depend on the accuracy of the processor clock.

     

    MK

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