code from https://www.olimex.com/forum/index.php?topic=3632.0

ACS712-example.ino

float watt_average = 0;
float measuredvalue = 0;
float measuredvalue1 = 0;
float measuredvalue2 = 0;
float digstepV = 0.0080566; // IS MULTIPLIED BY 10
float voltage = 230;  // the net voltage
int tel = 1;

char var;
float longaverage = 0;

float longaverage2 = 0;


int AnalogPin = 20; //  = a5 the pin that is being used on the Olimexino-STM32  a0 = 15 a1 = 16 see the latest schematic
// https://www.olimex.com/Products/Duino/STM32/OLIMEXINO-STM32/resources/OLIMEXINO-STM32_Rev_D.pdf


void setup()
{
    // Declare the sensorPin as INPUT_ANALOG:
    pinMode(AnalogPin, INPUT_ANALOG);
}


void measure_power()
{
    int n = 0;
    float totalsum = 0;
    watt_average = 0;

    while(n < 1000) // preform 1000 measurements and look if somebody or something is interested
    {
        // measure AC current in watts
        // The ACS712 chip 20A has a linear curve of 100 mv per ampere between -20 till 20 ampere from 0,5 volt till 4,5 volt by a supply-voltage of 5 volt
        // The Olimexino_stm32 works only with 3.3 volts. With a resistor divider (potentiometer between 5V and ground trim the output till 3.3V and you have the correct divider)
        // on the output of the ACS712 is fed into the ADC of the Olimexino-stm32, as long there is no current flowing the division is nearly linear so take at least 4,7K as value.   
        // the Olimexino stm32 has a 12 bit DAC digital analog converter. 2^12 is 4096 the zero-crossing is at 2048
        // If the measured value is less than 2048 the relevant voltage = 2048 min the measured value (example 2048 - 490 = 1558)
        // If the measured value is more than 2048 the relevant voltage = the measured value min 2048 ( example 3567 - 2048 = 1519)
        // The result is only positive values. 3.3 Volts divided by 4096 = 0.00080566 volt per bit step. 
        // multiplication of the measured value with 0.00080566 gives the voltage of the output from the ACS712 in V every volt = 10 amp
        // so if we measure a value of 156 * 0.00080566 = 0.12568296 volt * 1000 = 125.68296 Mv every milivolt = 0.01 amp = 1.2568 amp P = 230 * 1.2568 = 289 Watt
        // multiplying by 1000 and then dividing by 100 is the same a one multiplication by 10 so we can change the  digstepV constant to 0.0080566
        // According to the Law of Ohm P=I*V The voltage of the net in Europe is 230 Volt rms  http://www.researchgate.net/post/Is_the_single_phase_230V_that_we_get_Vrms_or_Vpp
        // By preforming many measurements in rapid succession(1000)and store every measurement we get a clear picture of the real power that is being used.
        // after 1000 measurements we calculate the mean value and look is there is any interest in the value. If not we start a new cycle
        // the cycles go so fast that a mean of the mean seams even more appropriate the clock runs a 72MHZ 
        // Millivolts (mV)    Volts (V)
        //      0 mV        0 V
        //      1 mV            0.001 V
        //      10 mV            0.01 V
        //      100 mV            0.1 V       
        //      1000 mV    1 V
        //


        measuredvalue = analogRead(AnalogPin);

        if(measuredvalue < 2048){
            measuredvalue = 2048 - measuredvalue;
        }

        else if(measuredvalue > 2048){
            measuredvalue = measuredvalue - 2048;
        }
        else if( measuredvalue == 2048){
            measuredvalue = 0;
        }

        measuredvalue1 = measuredvalue * digstepV; // multiply the measurement with the constant of the bit-step voltage
        measuredvalue2 = measuredvalue1  * voltage; // P=I*V law of Ohm NB 230 is already the rms value of the electricity net 
        totalsum += measuredvalue2;    // sum all the values together so we can calculate the mean value after 1000 measurements
        n++;                    // Add one to the counter
    }

    watt_average = totalsum / n;  // The mean wattage = sum of measurements divided by number of measurements
}

void loop()

{

    measure_power();

    longaverage += watt_average;
    tel++;

    if (SerialUSB.available() > 0){
        var = SerialUSB.read();
        // if there is some request on the serial port just print the result of the last measurement and start a new one.
        // if there is no interest start a new measurement as well.     
        SerialUSB.print("  watt_average  ");
        SerialUSB.print(watt_average);
        SerialUSB.print("    tel = ");       
        SerialUSB.print(tel);
        longaverage2 = longaverage/tel;
        SerialUSB.print("   long average = ");     
        SerialUSB.println(longaverage2);
        longaverage = 0;


        tel = 1;     
    }

    delay(100);
}