read temperature from DHT11, using pi4j

Question

I'm trying to read temperature data from a DHT11 temperature sensor, using pi4j. I followed the code written in c and python in this site: http://www.uugear.com/portfolio/dht11-h ... or-module/ But it's not working. when I test the instruction 'dht11Pin.getState()' it's always in HIGH state, never changing. Is there anything wrong in my code?

Below is my code:

import java.util.concurrent.TimeUnit;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import com.pi4j.component.ObserveableComponentBase;
import com.pi4j.io.gpio.GpioController;
import com.pi4j.io.gpio.GpioFactory;
import com.pi4j.io.gpio.GpioPinDigitalMultipurpose;
import com.pi4j.io.gpio.Pin;
import com.pi4j.io.gpio.PinMode;
import com.pi4j.io.gpio.PinPullResistance;
import com.pi4j.io.gpio.PinState;
import com.pi4j.io.gpio.RaspiPin;

public class DHT11 extends ObserveableComponentBase {

private static final Pin DEFAULT_PIN = RaspiPin.GPIO_04;
private static final int MAXTIMINGS = 85;
private int[] dht11_dat = { 0, 0, 0, 0, 0 };
private GpioPinDigitalMultipurpose dht11Pin;
private static final Logger LOGGER = LogManager.getLogger(DHT11.class
        .getName());

public DHT11() {
    final GpioController gpio = GpioFactory.getInstance();
    dht11Pin = gpio.provisionDigitalMultipurposePin(DEFAULT_PIN,
            PinMode.DIGITAL_INPUT, PinPullResistance.PULL_UP);
}

public DHT11(int pin) {
    final GpioController gpio = GpioFactory.getInstance();
    dht11Pin = gpio.provisionDigitalMultipurposePin(LibPins.getPin(pin),
            PinMode.DIGITAL_INPUT, PinPullResistance.PULL_UP);
}

public double getTemperature() {
    PinState laststate = PinState.HIGH;
    int j = 0;
    dht11_dat[0] = dht11_dat[1] = dht11_dat[2] = dht11_dat[3] = dht11_dat[4] = 0;
    StringBuilder value = new StringBuilder();
    try {

        dht11Pin.setMode(PinMode.DIGITAL_OUTPUT);
        dht11Pin.low();
        Thread.sleep(18);
        dht11Pin.high();
        TimeUnit.MICROSECONDS.sleep(40);
        dht11Pin.setMode(PinMode.DIGITAL_INPUT);

        for (int i = 0; i < MAXTIMINGS; i++) {
            int counter = 0;
            while (dht11Pin.getState() == laststate) {
                counter++;
                TimeUnit.MICROSECONDS.sleep(1);
                if (counter == 255) {
                    break;
                }
            }

            laststate = dht11Pin.getState();

            if (counter == 255) {
                break;
            }

            /* ignore first 3 transitions */
            if ((i >= 4) && (i % 2 == 0)) {
                /* shove each bit into the storage bytes */
                dht11_dat[j / 8] <<= 1;
                if (counter > 16) {
                    dht11_dat[j / 8] |= 1;
                }
                j++;
            }
        }
        // check we read 40 bits (8bit x 5 ) + verify checksum in the last
        // byte
        if ((j >= 40) && checkParity()) {
            value.append(dht11_dat[2]).append(".").append(dht11_dat[3]);
            LOGGER.info("temperature value readed: " + value.toString());
        }

    } catch (InterruptedException e) {

        LOGGER.error("InterruptedException: " + e.getMessage(), e);
    }
    if (value.toString().isEmpty()) {
        value.append(-1);
    }
    return Double.parseDouble(value.toString());
}

private boolean checkParity() {
    return (dht11_dat[4] == ((dht11_dat[0] + dht11_dat[1] + dht11_dat[2] + dht11_dat[3]) & 0xFF));
}

}

Answer 1

I started with the original poster's java code, and replaced the com.pi4j.io.gpio package references with the com.pi4j.wiringpi package. I had recently installed the newest pi4j package and wiringpi version on my Raspberry Pi.

Using that package the Java code below works approximately the same as the c version of this program. I am getting about 80% - 85% accurate responses with a DHT-11. Which is about the same as I was getting using wiringPi in c.

package gpio;
import com.pi4j.wiringpi.Gpio;
import com.pi4j.wiringpi.GpioUtil;

public class DHT11 {
    private static final int    MAXTIMINGS  = 85;
    private final int[]         dht11_dat   = { 0, 0, 0, 0, 0 };

    public DHT11() {

        // setup wiringPi
        if (Gpio.wiringPiSetup() == -1) {
            System.out.println(" ==>> GPIO SETUP FAILED");
            return;
        }

        GpioUtil.export(3, GpioUtil.DIRECTION_OUT);
    }

    public void getTemperature(final int pin) {
        int laststate = Gpio.HIGH;
        int j = 0;
        dht11_dat[0] = dht11_dat[1] = dht11_dat[2] = dht11_dat[3] = dht11_dat[4] = 0;

        Gpio.pinMode(pin, Gpio.OUTPUT);
        Gpio.digitalWrite(pin, Gpio.LOW);
        Gpio.delay(18);

        Gpio.digitalWrite(pin, Gpio.HIGH);
        Gpio.pinMode(pin, Gpio.INPUT);

        for (int i = 0; i < MAXTIMINGS; i++) {
            int counter = 0;
            while (Gpio.digitalRead(pin) == laststate) {
                counter++;
                Gpio.delayMicroseconds(1);
                if (counter == 255) {
                    break;
                }
            }

            laststate = Gpio.digitalRead(pin);

            if (counter == 255) {
                break;
            }

            /* ignore first 3 transitions */
            if (i >= 4 && i % 2 == 0) {
                /* shove each bit into the storage bytes */
                dht11_dat[j / 8] <<= 1;
                if (counter > 16) {
                    dht11_dat[j / 8] |= 1;
                }
                j++;
            }
        }
        // check we read 40 bits (8bit x 5 ) + verify checksum in the last
        // byte
        if (j >= 40 && checkParity()) {
            float h = (float) ((dht11_dat[0] << 8) + dht11_dat[1]) / 10;
            if (h > 100) {
                h = dht11_dat[0]; // for DHT11
            }
            float c = (float) (((dht11_dat[2] & 0x7F) << 8) + dht11_dat[3]) / 10;
            if (c > 125) {
                c = dht11_dat[2]; // for DHT11
            }
            if ((dht11_dat[2] & 0x80) != 0) {
                c = -c;
            }
            final float f = c * 1.8f + 32;
            System.out.println("Humidity = " + h + " Temperature = " + c + "(" + f + "f)");
        } else {
            System.out.println("Data not good, skip");
        }

    }

    private boolean checkParity() {
        return dht11_dat[4] == (dht11_dat[0] + dht11_dat[1] + dht11_dat[2] + dht11_dat[3] & 0xFF);
    }

    public static void main(final String ars[]) throws Exception {

        final DHT11 dht = new DHT11();

        for (int i = 0; i < 10; i++) {
            Thread.sleep(2000);
            dht.getTemperature(21);
        }

        System.out.println("Done!!");

    }
}