Arduino dual audio frequency counter and pulse width measuring sketch.

## dualFrequencyCounter.ino
/* DualFrequencyCounter */

// is a bit sketchy, but seems accurate enough.

// Relies on
// - Adafruit's SPI_VFD library
// - one of those fancy Samsung 20x2 VFDs
// - and the usual digitalWriteFast lib which I know you've already got.

// checked with a DSO Nano oscilloscope, not that they're all that great

// audio should probably be buffered, limited to 0-5v
// and fed through a comparator before input into pin 2 and 3

#include <SPI_VFD.h>
#include <digitalWriteFast.h>

#define TIMERLOAD 0
#define AVGLENGTH 21
#define SCREENROW0 0
#define SCREENROW1 1
#define CHANGERISE0 0
#define CHANGERISE1 1
#define CHANGEFALL0 2
#define CHANGEFALL1 3

SPI_VFD vfd(13, 11, 12);  // init vfd: data, clock, strobe

volatile unsigned long  lastTimeRise0, lastTimeFall0, thisTimeRise0, thisTimeFall0;
volatile unsigned long  lastTimeRise1, lastTimeFall1, thisTimeRise1, thisTimeFall1;
volatile unsigned long int timerOverflowCount = 0;

byte avgCount0, avgCount1 = 0;
unsigned long avgTotal0[AVGLENGTH], avgTotal1[AVGLENGTH];
unsigned long pulseWidthArray0[AVGLENGTH], pulseWidthArray1[AVGLENGTH];
unsigned long pulseWidthHighArray0[AVGLENGTH], pulseWidthHighArray1[AVGLENGTH];
volatile byte change = 0;

void setup() {
  //Serial.begin(115200);  // for testing

  // set up interrupts
  attachInterrupt(0, changingInt0, CHANGE);
  attachInterrupt(1, changingInt1, CHANGE);

  // say helloooooo
  vfd.begin(20, 2);
  vfd.print("frequency counter");
  delay(1000);
  vfd.clear();

  // initialize timer1
  noInterrupts();           // disable all interrupts
  TCCR1A = 0;
  TCCR1B = 0;

  TCNT1 = TIMERLOAD;            // preload timer 65536-16MHz/8/32Hz
  TCCR1B |= (1 << CS10);    // 8 prescaler CS11  // 1 prescaler CS10
  TIMSK1 |= (1 << TOIE1);   // enable timer overflow interrupt
  interrupts();             // enable all interrupts
}


void printRow(unsigned long ticks, const byte screenRow) {
  float hz = getHertz(ticks);
  if(hz>0) {
    String spacer = "";
    if(int(hz)==1000) { spacer="  "; } else
    if(int(hz)<10) { spacer = "    "; } else
    if(int(hz)<100) { spacer = "   "; } else
    if(int(hz)<1000) { spacer = "  "; } else
    if(int(hz)<10000) { spacer = " "; }

    vfd.setCursor(0, screenRow);
    vfd.print(spacer);
    vfd.print(hz, 2);
    vfd.print("hz     ");
  }
}

void loop() {
  unsigned long ticks0 = thisTimeRise0 - lastTimeRise0;
  unsigned long ticks1 = thisTimeRise1 - lastTimeRise1;

  unsigned long pulseHigh0 = thisTimeRise0 - thisTimeFall0;
  unsigned long pulseHigh1 = thisTimeRise1 - thisTimeFall1;
  unsigned long medianPulseWidthHigh;

  // chuck out any ridiculously long or short periods
  if (bitRead(change, CHANGERISE0)==1 && ticks0>400 && ticks0<4000000000) {
    if(avgCount0 < AVGLENGTH) {
      avgTotal0[avgCount0] = ticks0;    // add current period onto our array for averaging
      pulseWidthHighArray0[avgCount0] = pulseHigh0;
      avgCount0++;
    } else {
      // when array full up,  print median average
      unsigned long medianTicks = median(avgCount0, avgTotal0);
      printRow(medianTicks, SCREENROW0);
      medianPulseWidthHigh = median(avgCount0, pulseWidthHighArray0);
      printPulseWidth(medianPulseWidthHigh, medianTicks, SCREENROW0);
      avgCount0 = 0;
    }
   change &= ~(1 << CHANGERISE0);
  }

  if (bitRead(change, CHANGERISE1)==1 && ticks1>400 && ticks1<4000000000) {
    if(avgCount1 < AVGLENGTH) {
      avgTotal1[avgCount1] = ticks1;
      pulseWidthHighArray1[avgCount1] = pulseHigh1;
      avgCount1++;
    } else {
      unsigned long medianTicks = median(avgCount1, avgTotal1);
      printRow(medianTicks, SCREENROW1);
      medianPulseWidthHigh = median(avgCount1, pulseWidthHighArray1);
      printPulseWidth(medianPulseWidthHigh, medianTicks, 1);
      avgCount1 = 0;
    }
    change &= ~(1 << CHANGERISE1);
  }
}

// has to use a floating point thing as one of the operands otherwise it'll end up 1 or 0
float getPulseWidth(unsigned long pulseWidthHigh, unsigned long pulseWidth) {
  return ((double)pulseWidthHigh / pulseWidth) * 100;
}

void printPulseWidth(unsigned long pulseWidthHigh, unsigned long pulseWidth, int screenRow) {
   vfd.setCursor(15, screenRow);
   float pwPercent = getPulseWidth(pulseWidthHigh, pulseWidth);
   if(pwPercent<100) {
     vfd.print(pwPercent, 1);
     vfd.print("%");
   }
}


float getHertz(unsigned long ticks) {
  return 1/((0.0625 * ticks) / 1000000); // (x uS in a tick, * number of ticks) / microsec in a second  = period
}


ISR(TIMER1_OVF_vect) {        // interrupt service routine that wraps a user defined function supplied by attachInterrupt
  timerOverflowCount++;
}

void changingInt0() {
    if(digitalReadFast(2)) {
      lastTimeRise0 = thisTimeRise0;
      thisTimeRise0 = (timerOverflowCount * 65535) + TCNT1;
      change |= 1 << CHANGERISE0;
    }  else {
      lastTimeFall0 = thisTimeFall0;
      thisTimeFall0 = (timerOverflowCount * 65535) + TCNT1;
      change |= 1 << CHANGEFALL0;
    }
}

void changingInt1() {
    if(digitalReadFast(3)) {
      lastTimeRise1 = thisTimeRise1;
      thisTimeRise1 = (timerOverflowCount * 65535) + TCNT1;
      change |= 1 << CHANGERISE1;
    }  else {
      lastTimeFall1 = thisTimeFall1;
      thisTimeFall1 = (timerOverflowCount * 65535) + TCNT1;
      change |= 1 << CHANGEFALL1;
    }
}


unsigned long int median(unsigned long int n, unsigned long int x[]) {
    unsigned long temp;
    int i, j;
    // the following two loops sort the array x in ascending order
    for(i=0; i<n-1; i++) {
        for(j=i+1; j<n; j++) {
            if(x[j] < x[i]) {
                // swap elements
                temp = x[i];
                x[i] = x[j];
                x[j] = temp;
            }
        }
    }

    if(n%2==0) {
        // if there is an even number of elements, return mean of the two elements in the middle
        return((x[n/2] + x[n/2 - 1]) / 2.0);
    } else {
        // else return the element in the middle
        return x[n/2];
    }
}