Measuring inductances using an arduino

I just read this excellent article about measuring inductances:

http://reibot.org/2011/07/19/measuring-inductance/

The technique consists in measuring the resonance frequency of a LC circuit. Since this frequency is:

f = 1.0/(2.0*M_PI*sqrt(L*C))

By measuring f with the arduino and knowing C you can easily get L.

I modified the code so its much more precise, the original code is using 'pulse' to read half an oscillation. My code instead measures one whole oscillation by using an AVR's hw feature called Input Compare, this feaure starts and stops a timer that runs at 16Mhz.

Enjoy!

//measuring inductance using the higher precision input compare  
//based on http://reibot.org/2011/07/19/measuring-inductance/  
//capture Flag  
volatile uint8_t Flag;
void InitTimer1(void)
{
    cli();
    //Set Initial Timer value  
    TCCR1A = 0;
    TCCR1B = 0;
    // Input Capture Noise Canceller  
    TCCR1B |= (1 << ICNC1);
    //First capture on rising edge  
    TCCR1B |= (1 << ICES1);
    //Start timer without prescaller  
    TCCR1B &= ~ ((1 << CS12) | (1 << CS11) | (1 << CS10));
    TCCR1B |= (1 << CS10);
    //Enable input capture and overflow interrupts  
    TIMSK1 = 0;
    TIMSK1 |= (1 << ICIE1) | (1 << TOIE1);
    Flag = 0;
    sei();
}
volatile uint16_t Capt;
volatile uint16_t T1Ovs2;
//capture ISR  
ISR(TIMER1_CAPT_vect)
{
    if (Flag == 0)
    {
        //reset overflows  
        TCNT1 = 0;
        T1Ovs2 = 0;
    }
    else if (Flag == 1)
    {
        Capt = ICR1;
        //disable capture and overflow interrupts
        TIMSK1 &= ~ ((1 << ICIE1) | (1 << TOIE1));
        //clear pending interrupts  
        TIFR1 = (1 << ICF1) | (1 << TOV1);
    }
    //increment Flag  
    Flag++;
}
//Overflow ISR  
ISR(TIMER1_OVF_vect)
{
    T1Ovs2++; //increment overflow counter  
}
//13 is the input to the circuit (connects to 150ohm resistor)
//8 is the comparator/op-amp output.   
//reibot.org for guide  
double pulse, freq, inductance;
//insert capacitance here. Currently using 2uF  
double capacitance = 2E-6;
void setup()
{
    Serial.begin(115200);
    pinMode(8, INPUT);
    digitalWrite(8, LOW);
    pinMode(13, OUTPUT);
    Serial.println("Why hello!");
    delay(200);
}
void loop()
{
    digitalWrite(13, HIGH);
    delay(10); //give some time to charge inductor.   
    digitalWrite(13, LOW);
    delayMicroseconds(15); //make sure resonance is measured  
    InitTimer1();
    delay(100); // wait for ICR to read once cycle  
    pulse = (T1Ovs2 * 65536 + Capt) / 16.0;
    if (pulse > 0.1)
    {
        freq = 1.E6 / pulse;
        inductance = 1. / (capacitance * freq * freq * 4. * 3.14159 * 3.14159);
        inductance *= 1E6;
        Serial.print("High for uS:");
        Serial.print(pulse);
        Serial.print("\tfrequency Hz:");
        Serial.print(freq);
        Serial.print("\tinductance uH:");
        Serial.println(inductance);
        delay(20);
    }
}