File content as of revision 0:863c3f4f452f:

#include "mbed.h"

//////////////////////////////////////////
// GPa@quicknet.nl
/////////////////////////////////////////
//
// An unkwown ceramic capacitor is connected to 
// pin21 via a 82 kilo ohm resister, the other 
// capacitor terminal is connected to ground
//
// A ceramic capacitor is used becouse it is non polarized 
// 
//                                 ||
//     pin21-----RRRRRRR-----------||------- pin1 (GND)        
//                            |    || 
//                82k ohm     |    ?? Farad
//                            |
//     pin20-------------------
//
// Mbed pins used
// Pin 20...... analog input
// Pin 21...... digital out
//////////////////////////////////////////
// How does it work
// measure voltage on pin 20
// If (not zero Volt) then wait
// else pin 21 goes high 3.3 Volt
// wait 2 micro seconds (the capacitor is now charging)
// measure voltage on pin 20
// the Voltage increase in 2 micro seconds is a measure
// for the size of the capacitor
// a small capacitor will charge fast a big one slow
//
// Cap = current * time / measured voltage change
// current = voltage over the resistor / resistor value
// voltage max is 3.3 volt !!!
//
// The measuring results are send via Teraterm to the PC screen
// Note 1)
// The tolerance range of a ceramic capacitor
// lies between 5 and 100%, not very accure at all.
// Note 2)
// Choose the charging time such that the capacitor 
// reaches about 3 Volt, lower you get less accurate results
// higher the 3.3 is not acceptable to the analogin
//
// Cap sizes are
// m = 10^-3  (mili)
// u = 10^-6  (micro)
// n = 10^-9  (nano)
// p = 10^-12 (pico)
// f = 10^-15 (femto)
/////////////////////////////////////////

AnalogIn ain(p20);
DigitalOut red(p21);
Serial pc(USBTX, USBRX); // tx, rx

int main() {
    int i, cnt;
    double volt_0;       // first measurement
    double volt_1;       // second measurement
    double volt_delta;   // delta voltage
    double current;      // max current trough resistor
    double cx;           // capacitance to be calculated;
    double mmtime =20;   // measure time in micro seconds


    pc.baud(9600);
    pc.format(8,Serial::None,1);

    for (i=0; i<2250; i +=1)  {
        // starting at zero volt
        red.write(0);
        wait_ms(20);        // deterime the number of measurements

        // start measurement, wait for 0.0 volt on input
        volt_0 = 1;
        cnt=0;
        while (volt_0 > 0.0001) {   // wait for 0.0 Volt
            wait_us(2);  
            cnt+=1;                 // wait
            volt_0 = ain.read();
        }
        red.write(1);               // output high
        wait_us(mmtime);            // measuring time
        volt_1 = ain.read();
        red.write(0);               // output low

        // calculations
        volt_0 *= 3.3;
        volt_1 *= 3.3;

        volt_delta = abs(volt_1 - volt_0);
        current = (3.3- volt_delta/2) / 82000;   // 82k resistor
        cx = current* mmtime/(1000 * 1000 * volt_delta);
        //pc.printf("Volt_0  %f, volt_1 %f, current %f  ", volt_0, volt_1, current);
        //pc.printf("Delta voltage %f, time %f ", volt_delta, mmtime);
        pc.printf("cnt= %3d, Delta= %5.4f Volt ", cnt, volt_delta);
        if (volt_1 > 3.1)
            pc.printf("Out of range \n\r");
        else {
            if (cx < 1 and cx >= 0.001) {
                cx = cx *1000;
                pc.printf("Cap. is %5.4f milli Farad    ", cx);
            } else if (cx < 0.001 and cx >= 0.000001) {
                cx = cx *1000 *1000;
                pc.printf("Cap. is %5.4f micro Farad     ", cx);
            } else if (cx < 0.000001 and cx >= 0.000000001) {
                cx = cx *1000 *1000 *1000;
                pc.printf("Cap. is %4.2f nano Farad      ", cx);
            }
            if (cx <  0.000000001 and cx >=  0.000000000001) {
                cx = cx *1000 *1000 *1000 *1000;
                pc.printf("Cap. is %4.0f pico Farad       ", cx);
            } else if (cx < 0.000000000001)
                pc.printf("Cx = %f Out of range too small! ", cx);
            pc.printf("\r");
            // pc.printf("\n");
        }
    }
}