In this article we're going to investigate how to write a device driver for Android Things. In particular, we're going to extend the functionality offered in Google's contrib driver package for the BMX280 temperature and ambient pressure to allow for humidity measurements as well.

We're also going to modify the driver so that it allows different I2C addresses, since the contrib driver hardcodes that to 0x77.

The full code for this project is available from this repo.

[TOC]

Hardware requirements

For this article we're going to be using:

The problem

The need for this exercise comes from the fact that Google's contrib drivers for this ambient sensor is meant to be used on both the BMP280 and BME280 (hence they call it the BMX280). The difference between the two sensors is that the BME also has the ability to measure humidity (besides temperature and ambient pressure), a feature that is not supported in Google's driver.

Furthermore, the driver hardcodes the address of the sensor to 0x77, although the chip itself can also support 0x76, and some escape boards (such as this one from Amazon) come with this latter address (and no way to change it). Our modifications will allow for an arbitrary address.

Connecting things up

We're going to use the I2C protocol for communicating with the sensor. In order to determine which pins to connect, we're going to be referencing both the Raspberry Pi pin-out diagram and the labels on the escape board itself.

We need to connect 4 wires:

  • the pin labeled VCC on the board connects to any 3.3V pin on the Raspberry (marked with orange in the diagram) such as pin 1.
  • the pin labeled GND on the board connects to any Ground pin on the Raspberry (marked with black in the diagram) such as pin 6.
  • the pin labeled SDI (or SDA on some boards) connects to the I2C1 (SDA) pin on the Raspberry - pin 3.
  • the pin labeled SCK (or SCL on some boards) connects to the I2C2 (SCL) pin on the Raspberry - pin 5.

Once that is setup we're ready to connect to the device.

Testing connectivity

Let's go back to Android Studio and test our handiwork. Let's set up a new project (like we did in this article) and name the main activity MainActivity. First thing, we're going to open the device and try to read its device ID. Looking at the BME280 datasheet page 25, we find that:

Register 0xD0 “id”: The “id” register contains the chip identification number chip_id[7:0], which is 0x60. This number can be read as soon as the device finished the power-on-reset.

This means that if we read register number 0xD0 we should be getting back a response of 0x60 for the BME280. Let's code that up:

public class MainActivity extends Activity {

    private static final String TAG = "MainActivity";
    private static final int ADDRESS = 0x76;
    private final PeripheralManagerService managerService = new PeripheralManagerService();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        printDeviceId();
    }

    private void printDeviceId() {
        List<String> deviceList = managerService.getI2cBusList();
        if (deviceList.isEmpty()) {
            Log.i(TAG, "No I2C bus available on this device.");
        } else {
            Log.i(TAG, "List of available devices: " + deviceList);
        }
        I2cDevice device = null;
        try {
            device = managerService.openI2cDevice(deviceList.get(0), ADDRESS);
            Log.d(TAG, "Device ID byte: 0x" + Integer.toHexString(device.readRegByte(0xD0)));
        } catch (IOException|RuntimeException e) {
            Log.e(TAG, e.getMessage(), e);
        } finally {
            try {
                device.close();
            } catch (Exception ex) {
                Log.d(TAG, "Error closing device");
            }
        }
    }
}

Note: I'm assuming the device address is 0x76, as is in the break-out board I purchased. However, do note that the BME chip itself supports either 0x77 or 0x76, with some boards even allowing selecting between these two addresses via a jumper. In case your board documentation does not state which address to use, the best thing is to change the private static final int ADDRESS = 0x76 line and test out both of them and see which works. In case none of them does, check the wiring scheme again.

Here is the output if everything works correctly:

02-22 12:57:51.884 9730-9730/com.example.pitepmerature W/System: ClassLoader referenced unknown path: /data/app/com.example.pitepmerature-1/lib/arm  
02-22 12:57:51.977 9730-9730/com.example.pitepmerature I/MainActivity: List of available devices: [I2C1]  
02-22 12:57:51.981 9730-9730/com.example.pitepmerature D/MainActivity: Device ID byte: 0x60  

Reading temperature and pressure

Now that we determined the sensor is wired up correctly, let's proceed to bring in the contrib driver and read a few temperature and pressure measurements (remember, the driver does not support humidity readings).

Let's start by adding the library to the build.gradle:

dependencies {  
// ...
    compile 'com.google.android.things.contrib:driver-bmx280:0.1'
}

Next, we will extend the Bmx280 class with our own Bme280 class. For now, we only do this in order to expose a constructor that supports another address that 0x77, but later on we will be adding more functionality to it.

Hack alert: we will be putting the Bme280 class into the package com.google.android.things.contrib.driver.bmx280 in order to be able to use one of the "package protected" constructors for the Bmx280. Due to the architecture of the driver, the only alternative would be to copy the entire code into our codebase. Hopefully, Google will change this in the future.

package com.google.android.things.contrib.driver.bmx280;  
//...
public class Bme280 extends Bmx280 {

    public Bme280(I2cDevice device) throws IOException {
        super(device);
    }
}

Now we can modify our MainActivity to use the driver:

public class MainActivity extends Activity {

    private static final String TAG = "MainActivity";
    private static final int ADDRESS = 0x76;
    private final PeripheralManagerService managerService = new PeripheralManagerService();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        printDeviceId();
        readSample();
    }

    private void readSample() {
        try (Bme280 bmxDriver = new Bme280(managerService.openI2cDevice(managerService.getI2cBusList().get(0), ADDRESS))){
            bmxDriver.setTemperatureOversampling(Bmx280.OVERSAMPLING_1X);
            bmxDriver.setPressureOversampling(Bmx280.OVERSAMPLING_1X);

            bmxDriver.setMode(Bme280.MODE_NORMAL);
            for(int i = 0 ; i < 5 ; i++) {
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                }
                Log.d(TAG, "Temperature: " + bmxDriver.readTemperature());
                Log.d(TAG, "Pressure: " + bmxDriver.readPressure());
            }


        } catch (IOException e) {
            Log.e(TAG, "Error during IO", e);
            // error reading temperature
        }
    }

//...
}

Lets run it:

02-22 13:34:17.945 7196-7196/com.example.pitepmerature I/MainActivity: List of available devices: [I2C1]  
02-22 13:34:17.950 7196-7196/com.example.pitepmerature D/MainActivity: Device ID byte: 0x60  
02-22 13:34:20.034 7196-7196/com.example.pitepmerature D/MainActivity: Temperature: 24.499886  
02-22 13:34:20.037 7196-7196/com.example.pitepmerature D/MainActivity: Pressure: 1003.63416  
02-22 13:34:22.065 7196-7196/com.example.pitepmerature D/MainActivity: Temperature: 24.51503  
02-22 13:34:22.068 7196-7196/com.example.pitepmerature D/MainActivity: Pressure: 1003.60455  
02-22 13:34:24.094 7196-7196/com.example.pitepmerature D/MainActivity: Temperature: 24.520079  
02-22 13:34:24.098 7196-7196/com.example.pitepmerature D/MainActivity: Pressure: 1003.58563  
02-22 13:34:26.124 7196-7196/com.example.pitepmerature D/MainActivity: Temperature: 24.525127  
02-22 13:34:26.127 7196-7196/com.example.pitepmerature D/MainActivity: Pressure: 1003.70374  
02-22 13:34:28.154 7196-7196/com.example.pitepmerature D/MainActivity: Temperature: 24.525127  
02-22 13:34:28.157 7196-7196/com.example.pitepmerature D/MainActivity: Pressure: 1003.53925  

Adding humidity readings to the driver

The trickiest part of writing this driver is undoubtedly getting the calibration calculations right, so let's get that out of the way first.

Calibration formula

Let's have a look at the datasheet. On page 49 we find Compensation formulas in double precision floating point. We're going to be using those, since they offer the best precision at the cost of processing power, of which our Raspberry Pi has plenty. Sadly, the provided code is in C, so we'll have to translate it to Java. Let's have a look at the code for the humidity compensation:

// Returns humidity in %rH as as double. Output value of “46.332” represents 46.332 %rH
double bme280_compensate_H_double(BME280_S32_t adc_H);  
{
    double var_H;
    var_H = (((double)t_fine) – 76800.0);
    var_H = (adc_H – (((double)dig_H4) * 64.0 + ((double)dig_H5) / 16384.0 * var_H)) *
        (((double)dig_H2) / 65536.0 * (1.0 + ((double)dig_H6) / 67108864.0 * var_H *
        (1.0 + ((double)dig_H3) / 67108864.0 * var_H)));
    var_H = var_H * (1.0 – ((double)dig_H1) * var_H / 524288.0);
    if (var_H > 100.0)
        var_H = 100.0;
    else if (var_H < 0.0)
        var_H = 0.0;
    return var_H;
}

If you haven't yet ran away in abject terror at the sight of that piece of code, let's look at what it does. It takes a 32 bytes signed raw reading from the sensor and uses some fancy formulas (containing plenty of magic numbers) to refine it into a more precise reading. It does also use the last compensated temperature reading (t_fine) as well as a few parameters (aka trimming parameters: dig_H1 through dig_H6) which we will have to read from the chip.

Let's look at how the function would look like in Java:

    // Compensation formula from the BME280 datasheet.
    // https://cdn-shop.adafruit.com/datasheets/BST-BME280_DS001-10.pdf
    float compensateHumidity(int adc_H, float temp)
    {
        int dig_H1 = mHumCalibrationData[0];
        int dig_H2 = mHumCalibrationData[1];
        int dig_H3 = mHumCalibrationData[2];
        int dig_H4 = mHumCalibrationData[3];
        int dig_H5 = mHumCalibrationData[4];
        int dig_H6 = mHumCalibrationData[5];

        float var_H;
        var_H = (temp - 76800f);
        var_H = (adc_H - (((float)dig_H4) * 64f + ((float)dig_H5) / 16384f * var_H)) *
        (((float)dig_H2) / 65536f * (1f + ((float)dig_H6) / 67108864f * var_H *
                (1f + ((float)dig_H3) / 67108864f * var_H)));
        var_H = var_H * (1f - ((float)dig_H1) * var_H / 524288f);
        if (var_H > 100)
            var_H = 100f;
        else if (var_H < 0)
            var_H = 0f;
        return var_H;
    }

Note I have changed the temperature reading to be passed as a parameter to the function and that we have to actually populate the mHumCalibrationData array with the real data.

Reading the trimming parameters

To read the parameters we turn to the datasheet document again, this time to pages 22 and 23, Trimming parameter readout chapter. The last 6 lines of the table there describe where to read those parameters from.

Register Address        Register content            Data type

0xA1                    dig_H1 [7:0]                unsigned char  
0xE1 / 0xE2             dig_H2 [7:0] / [15:8]       signed short  
0xE3                    dig_H3 [7:0]                unsigned char  
0xE4 / 0xE5[3:0]        dig_H4 [11:4] / [3:0]       signed short  
0xE5[7:4] / 0xE6        dig_H5 [3:0] / [11:4]       signed short  
0xE7                    dig_H6                      signed char  

As you can see, that requires a bit of bit-fu to get all the bits in the right order. This sort of thing is a bit more low-level than what most Android programmers are used to so please correct me if I got things wrong. This is the code I came up with:

    private static final int BME280_REG_HUM_CALIB_1 = 0xA1;
    private static final int BME280_REG_HUM_CALIB_2 = 0xE1;
    private static final int BME280_REG_HUM_CALIB_3 = 0xE3;
    private static final int BME280_REG_HUM_CALIB_4 = 0xE4;
    private static final int BME280_REG_HUM_CALIB_5 = 0xE5;
    private static final int BME280_REG_HUM_CALIB_6 = 0xE6;
    private static final int BME280_REG_HUM_CALIB_7 = 0xE7;
//...
    private void readHumidityCalibrationData() throws IOException {
        mHumCalibrationData[0] = mDevice.readRegByte(BME280_REG_HUM_CALIB_1) & 0xFF;
        mHumCalibrationData[1] = mDevice.readRegWord(BME280_REG_HUM_CALIB_2);
        mHumCalibrationData[2] = mDevice.readRegByte(BME280_REG_HUM_CALIB_3) & 0xFF;

        int E4 = mDevice.readRegByte(BME280_REG_HUM_CALIB_4) & 0xFF;
        int E5 = mDevice.readRegByte(BME280_REG_HUM_CALIB_5) & 0xFF;
        int E6 = mDevice.readRegByte(BME280_REG_HUM_CALIB_6) & 0xFF;
        int E7 = mDevice.readRegByte(BME280_REG_HUM_CALIB_7);

        mHumCalibrationData[3] = (E4 << 4) | (E5 & 0x0F);
        mHumCalibrationData[4] = ((E5 & 0xF0) << 4) | E6;
        mHumCalibrationData[5] = E7;

        Log.d(TAG, Arrays.toString(mHumCalibrationData));
    }

That last line outputs values such as:

02-22 13:34:17.984 7196-7196/com.example.pitepmerature D/Bme280: [75, 356, 0, 333, 0, 30]  

Setting the oversampling mode

Before we can actually read samples from the BME we have to first set up it's oversampling mode. All we need to do is write the value to the appropriate register, which we get from the datasheet page 26:

    private static final int BME280_REG_CTRL_HUM = 0xF2;
//...
    public void setHumidityOversampling(@Oversampling int oversampling) throws IOException {
        mDevice.writeRegByte(BME280_REG_CTRL_HUM, (byte)(oversampling));
        mHumidityOversampling = oversampling;
    }

Reading a humidity measurement

Let's now look at the datasheet, page 29. It states that Register 0xFD…0xFE “hum” (_msb, _lsb) contains the humidity reading.

    private final byte[] mBuffer = new byte[2]; // for reading sensor values
//...
    public float readHumidity() throws IOException, IllegalStateException {
        if (mHumidityOversampling == OVERSAMPLING_SKIPPED) {
            throw new IllegalStateException("temperature oversampling is skipped");
        }
        int rawHum = readSample(BME280_REG_HUM);
        return compensateHumidity(rawHum, readTemperature());
    }

    private int readSample(int address) throws IOException, IllegalStateException {
        if (mDevice == null) {
            throw new IllegalStateException("I2C device is already closed");
        }
        synchronized (mBuffer) {
            mDevice.readRegBuffer(address, mBuffer, 2);
            // msb[7:0] lsb[7:0]
            int msb = mBuffer[0] & 0xff;
            int lsb = mBuffer[1] & 0xff;
            return msb << 8 | lsb;
        }
    }

Finishing touches

Make sure everything is in place and that the constructor reads the calibration data before any actual measurements are made.

public class Bme280 extends Bmx280 {  
    private static final String TAG = "Bme280";

    private static final int BME280_REG_HUM_CALIB_1 = 0xA1;
    private static final int BME280_REG_HUM_CALIB_2 = 0xE1;
    private static final int BME280_REG_HUM_CALIB_3 = 0xE3;
    private static final int BME280_REG_HUM_CALIB_4 = 0xE4;
    private static final int BME280_REG_HUM_CALIB_5 = 0xE5;
    private static final int BME280_REG_HUM_CALIB_6 = 0xE6;
    private static final int BME280_REG_HUM_CALIB_7 = 0xE7;

    private static final int BME280_REG_CTRL_HUM = 0xF2;
    private static final int BME280_REG_HUM = 0xFD;

    private I2cDevice mDevice;
    private final byte[] mBuffer = new byte[2];

    private int mHumidityOversampling;

    public Bme280(I2cDevice device) throws IOException {
        super(device);
        mDevice = device;
        readHumidityCalibrationData();
    }
//...
}

Now let's go back to MainActivity and add the calls to read the humidity as well:

    private void readSample() {
        try (Bme280 bmxDriver = new Bme280(managerService.openI2cDevice(managerService.getI2cBusList().get(0), ADDRESS))){
            bmxDriver.setTemperatureOversampling(Bmx280.OVERSAMPLING_1X);
            bmxDriver.setPressureOversampling(Bmx280.OVERSAMPLING_1X);
            bmxDriver.setHumidityOversampling(Bmx280.OVERSAMPLING_1X);

            bmxDriver.setMode(Bme280.MODE_NORMAL);
            for(int i = 0 ; i < 5 ; i++) {
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                }
                Log.d(TAG, "Temperature: " + bmxDriver.readTemperature());
                Log.d(TAG, "Pressure: " + bmxDriver.readPressure());
                Log.d(TAG, "Humidity: " + bmxDriver.readHumidity());
            }


        } catch (IOException e) {
            Log.e(TAG, "Error during IO", e);
            // error reading temperature
        }
    }

Run the app again. Here is a sample output:

02-22 15:06:51.392 17989-17989/com.example.pitepmerature I/MainActivity: List of available devices: [I2C1]  
02-22 15:06:51.397 17989-17989/com.example.pitepmerature D/MainActivity: Device ID byte: 0x60  
02-22 15:06:51.434 17989-17989/com.example.pitepmerature D/Bme280: [75, 356, 0, 333, 0, 30]  
02-22 15:06:53.474 17989-17989/com.example.pitepmerature D/MainActivity: Temperature: 23.581142  
02-22 15:06:53.478 17989-17989/com.example.pitepmerature D/MainActivity: Pressure: 1002.87085  
02-22 15:06:53.481 17989-17989/com.example.pitepmerature D/MainActivity: Humidity: 59.578003  
02-22 15:06:55.515 17989-17989/com.example.pitepmerature D/MainActivity: Temperature: 23.581142  
02-22 15:06:55.518 17989-17989/com.example.pitepmerature D/MainActivity: Pressure: 1002.80756  
02-22 15:06:55.521 17989-17989/com.example.pitepmerature D/MainActivity: Humidity: 59.578003  
02-22 15:06:57.544 17989-17989/com.example.pitepmerature D/MainActivity: Temperature: 23.581142  
02-22 15:06:57.548 17989-17989/com.example.pitepmerature D/MainActivity: Pressure: 1002.88983  
02-22 15:06:57.550 17989-17989/com.example.pitepmerature D/MainActivity: Humidity: 59.578003  
02-22 15:06:59.574 17989-17989/com.example.pitepmerature D/MainActivity: Temperature: 23.581142  
02-22 15:06:59.577 17989-17989/com.example.pitepmerature D/MainActivity: Pressure: 1002.8624  
02-22 15:06:59.580 17989-17989/com.example.pitepmerature D/MainActivity: Humidity: 59.578003  
02-22 15:07:01.614 17989-17989/com.example.pitepmerature D/MainActivity: Temperature: 23.591238  
02-22 15:07:01.617 17989-17989/com.example.pitepmerature D/MainActivity: Pressure: 1002.7972  
02-22 15:07:01.620 17989-17989/com.example.pitepmerature D/MainActivity: Humidity: 59.578003  

Updating the user driver

Sadly, the user driver class (Bmx280SensorDriver) cannot be extended since the only constructor assumes the address to be 0x77. The only workaround I found is simply to copy over the code of the class into our own class and add the code to it. Let's name the new class Bme280SensorDriver. We will follow the same pattern the original authors used for temperature and pressure, but remove the constructor that hardcodes the address and add the following instead:

public class Bme280SensorDriver implements AutoCloseable {  
//...
    private HumidityUserDriver mHumidityUserDriver;

    private HumidityUserDriver mHumidityUserDriver;

    public Bme280SensorDriver(I2cDevice device) throws IOException {
        mDevice = new Bme280(device);
    }

    public void registerHumiditySensor() {
        if (mDevice == null) {
            throw new IllegalStateException("cannot register closed driver");
        }

        if (mHumidityUserDriver == null) {
            mHumidityUserDriver = new HumidityUserDriver();
            UserDriverManager.getManager().registerSensor(mHumidityUserDriver.getUserSensor());
        }
    }

    public void unregisterHumiditySensor() {
        if (mHumidityUserDriver != null) {
            UserDriverManager.getManager().unregisterSensor(mHumidityUserDriver.getUserSensor());
            mHumidityUserDriver = null;
        }
    }

    private void maybeSleep() throws IOException {
        if ((mTemperatureUserDriver == null || !mTemperatureUserDriver.isEnabled()) &&
                (mPressureUserDriver == null || !mPressureUserDriver.isEnabled()) &&
                (mHumidityUserDriver == null || !mHumidityUserDriver.isEnabled())
                ) {
            mDevice.setMode(Bmx280.MODE_SLEEP);
        } else {
            mDevice.setMode(Bmx280.MODE_NORMAL);
        }
    }

    private class HumidityUserDriver extends UserSensorDriver {
        // DRIVER parameters
        // documented at https://source.android.com/devices/sensors/hal-interface.html#sensor_t
        private static final float DRIVER_MAX_RANGE = 100f;
        private static final float DRIVER_RESOLUTION = 0.00008f;
        private static final float DRIVER_POWER = 280f / 1000.f;
        private static final int DRIVER_VERSION = 1;
        private static final String DRIVER_REQUIRED_PERMISSION = "";

        private boolean mEnabled;
        private UserSensor mUserSensor;

        private UserSensor getUserSensor() {
            if (mUserSensor == null) {
                mUserSensor = UserSensor.builder()
                        .setType(Sensor.TYPE_RELATIVE_HUMIDITY)
                        .setName(DRIVER_NAME)
                        .setVendor(DRIVER_VENDOR)
                        .setVersion(DRIVER_VERSION)
                        .setMaxRange(DRIVER_MAX_RANGE)
                        .setResolution(DRIVER_RESOLUTION)
                        .setPower(DRIVER_POWER)
                        .setMinDelay(DRIVER_MIN_DELAY_US)
                        .setRequiredPermission(DRIVER_REQUIRED_PERMISSION)
                        .setMaxDelay(DRIVER_MAX_DELAY_US)
                        .setUuid(UUID.randomUUID())
                        .setDriver(this)
                        .build();
            }
            return mUserSensor;
        }

        @Override
        public UserSensorReading read() throws IOException {
            return new UserSensorReading(new float[]{mDevice.readHumidity()});
        }

        @Override
        public void setEnabled(boolean enabled) throws IOException {
            mEnabled = enabled;
            mDevice.setHumidityOversampling(
                    enabled ? Bmx280.OVERSAMPLING_1X : Bmx280.OVERSAMPLING_SKIPPED);
            maybeSleep();
        }

        private boolean isEnabled() {
            return mEnabled;
        }
}

Bringing it all together

We're now finally ready to have our MainActivity print out the readings via the our new driver instead of polling the device every two seconds. First, let's comment out the readSample(); line and add the following to the onCreate method:

        mListener = new SensorEventListener() {
            @Override
            public void onSensorChanged(SensorEvent sensorEvent) {
                String type = "";
                if(sensorEvent.sensor.getType() == Sensor.TYPE_AMBIENT_TEMPERATURE) {
                    type = "Temp: ";
                } else if(sensorEvent.sensor.getType() == Sensor.TYPE_PRESSURE) {
                    type = "Pressure: ";
                } else if(sensorEvent.sensor.getType() == Sensor.TYPE_RELATIVE_HUMIDITY) {
                    type = "Humidity: ";
                }
                Log.d(TAG, type + sensorEvent.values[0]);
            }

            @Override
            public void onAccuracyChanged(Sensor sensor, int i) {

            }
        };

The listener's onSensorChanged method is what gets called when a new value is available. A true app would process the data here (for example it might post it to a back-end service) but for now we're just going to print it on the console. The code that registers the listener should be placed in the onCreate method as well:

        mSensorManager.registerDynamicSensorCallback(new SensorManager.DynamicSensorCallback() {
            @Override
            public void onDynamicSensorConnected(Sensor sensor) {
                if (sensor.getType() == Sensor.TYPE_AMBIENT_TEMPERATURE ||
                        sensor.getType() == Sensor.TYPE_PRESSURE ||
                        sensor.getType() == Sensor.TYPE_RELATIVE_HUMIDITY
                        ) {
                    mSensorManager.registerListener(mListener, sensor,
                            SensorManager.SENSOR_DELAY_NORMAL);
                }
            }
        });

        try {
            I2cDevice device = managerService.openI2cDevice(managerService.getI2cBusList().get(0), ADDRESS);
            mSensorDriver = new Bme280SensorDriver(device);
            mSensorDriver.registerTemperatureSensor();
            mSensorDriver.registerPressureSensor();
            mSensorDriver.registerHumiditySensor();
        } catch (IOException e) {
            Log.e(TAG, e.getMessage(), e);
        }

Finally, let's clean up in the onStop method:

    @Override
    protected void onStop() {
        super.onStop();
        try {
            ((SensorManager)getSystemService(Context.SENSOR_SERVICE)).unregisterListener(mListener);

            mSensorDriver.unregisterTemperatureSensor();
            mSensorDriver.unregisterPressureSensor();
            mSensorDriver.unregisterHumiditySensor();
            mSensorDriver.close();
        } catch (Exception e) {
            // error closing sensor
        }
    }

Running the code should now present you with a lot more furiously changing output than before:

02-22 19:10:47.702 6760-6760/? I/MainActivity: List of available devices: [I2C1]  
02-22 19:10:47.706 6760-6760/? D/MainActivity: Device ID byte: 0x60  
02-22 19:10:47.711 6760-6760/? I/SensorManager: DYNS Register dynamic sensor callback  
02-22 19:10:47.740 6760-6760/? D/Bme280: [75, 356, 0, 333, 0, 30]  
02-22 19:10:48.120 6760-6760/? I/SensorManager: DYNS received DYNAMIC_SENSOR_CHANED broadcast  
02-22 19:10:48.121 6760-6760/? I/SensorManager: DYNS native SensorManager.getDynamicSensorList return 3 sensors  
02-22 19:10:48.122 6760-6760/? I/SensorManager: DYNS dynamic sensor list cached should be updated  
02-22 19:10:48.122 6760-6760/? I/SensorManager: DYNS received DYNAMIC_SENSOR_CHANED broadcast  
02-22 19:10:48.124 6760-6760/? I/SensorManager: DYNS native SensorManager.getDynamicSensorList return 3 sensors  
02-22 19:10:48.124 6760-6760/? I/SensorManager: DYNS received DYNAMIC_SENSOR_CHANED broadcast  
02-22 19:10:48.125 6760-6760/? I/SensorManager: DYNS native SensorManager.getDynamicSensorList return 3 sensors  
02-22 19:10:48.323 6760-6760/? D/MainActivity: Temp: 23.884022  
02-22 19:10:48.324 6760-6760/? D/MainActivity: Pressure: 678.6115  
02-22 19:10:48.337 6760-6760/? D/MainActivity: Humidity: 59.578007  
02-22 19:10:48.354 6760-6760/? D/MainActivity: Temp: 23.904217  
02-22 19:10:48.364 6760-6760/? D/MainActivity: Pressure: 1002.95685  
02-22 19:10:48.398 6760-6760/? D/MainActivity: Temp: 23.934504  
02-22 19:10:48.406 6760-6760/? D/MainActivity: Pressure: 1002.95276  
02-22 19:10:48.415 6760-6760/? D/MainActivity: Humidity: 59.578014  
02-22 19:10:48.442 6760-6760/? D/MainActivity: Temp: 23.964792  
02-22 19:10:48.451 6760-6760/? D/MainActivity: Pressure: 1002.92114  
..............

Conclusion

In this article we've shown how it is possible to write low-level drivers for hardware devices in Android Things. While the implementation details are a bit too low-level for most Android developers, it should feel familiar for the IoT hardware interface programmers. In any case, if the platform does become popular one would expect these drivers to be available for most usual hardware.

The full code for this project is available from this repo.