GPIO OneWire DHT11 reader

View the code on Github

This sample shows how to read from the DHT11 from a Universal Windows Application. The DHT11 is a low cost temperature and humidity sensor that uses a single wire to interface to the host controller. This wire is used by the host to request a sample from the DHT11 and by the DHT11 to transmit data back to the host.

The DHT11 is right on the edge performance-wise of what the GPIO APIs can handle. If there is background activity such as network, USB, filesystem, or graphics activity, it can prevent the sample from successfully sampling from the DHT11.

For a description of the protocol used by the DHT11, see this article. The datasheet is here.

Screenshot

Requirements

Minimum supported build 10.0.10556
Supported Hardware Raspberry Pi 2 or 3
Dragonboard 410C

Hardware Setup

You will need the following hardware to run this demo:

  • A DHT11 or DHT22 sensor
  • A couple of female-to-female connector wires

Connect the components as shown in the following diagram:

Schematic Wiring Diagram

Running the Demo

  1. Clone the ms-iot/samples git repository and open GpioOneWire/GpioOneWire.vcxproj in Visual Studio 2017.
  2. Right click on the project in the solution explorer, and click Properties.
  3. In the project properties dialog, select the Debugging tab.
  4. Enter the IP address of your device in the Machine Name field.
  5. Set Authentication Type to Universal (Unencrypted Protocol)
  6. Hit F5 to build, deploy, and debug the project. You should see temperature and humidity samples updated on the screen every 2 seconds.

How it works

The logic that interacts with the DHT11 is contained in the Dht11::Sample() method. Since the 1s and 0s that the DHT11 sends back are encoded as pulse widths, we need a way to precisely measure the time difference between falling edges. We use QueryPerformanceCounter() for this purpose. The units of QueryPerformanceCounter are platform-dependent, so we must call QueryPerformanceFrequency() to determine the resolution of the counter.

A difference of 76 microseconds between falling edges denotes a ‘0’, while a difference of 120 microseconds between falling edges denotes a ‘1’. We choose 110 microseconds as a reasonable threshold above which we will consider bits to be 1s, while we will consider pulses shorter than this threshold to be 0s. We convert 110 microseconds to QueryPerformanceCounter (QPC) units to be used later.

HRESULT GpioOneWire::Dht11::Sample (GpioOneWire::Dht11Reading& Reading)
{
    Reading = Dht11Reading();

    LARGE_INTEGER qpf;
    QueryPerformanceFrequency(&qpf);

    // This is the threshold used to determine whether a bit is a '0' or a '1'.
    // A '0' has a pulse time of 76 microseconds, while a '1' has a
    // pulse time of 120 microseconds. 110 is chosen as a reasonable threshold.
    // We convert the value to QPF units for later use.
    const unsigned int oneThreshold = static_cast<unsigned int>(
        110LL * qpf.QuadPart / 1000000LL);

Next, we send the sequence required to activate the sensor. The GPIO signal is normally pulled high while the device is idle, and we must pull it low for 18 milliseconds to request a sample. We latch a low value to the pin and set it as an output, driving the GPIO pin low.

    // Latch low value onto pin
    this->pin->Write(GpioPinValue::Low);

    // Set pin as output
    this->pin->SetDriveMode(GpioPinDriveMode::Output);

    // Wait for at least 18 ms
    Sleep(SAMPLE_HOLD_LOW_MILLIS);

We then revert the pin to an input which causes it to go high, and wait for the DHT11 to pull the pin low, then high again.

    // Set pin back to input
    this->pin->SetDriveMode(this->inputDriveMode);

    GpioPinValue previousValue = this->pin->Read();

    // catch the first rising edge
    const ULONG initialRisingEdgeTimeoutMillis = 1;
    ULONGLONG endTickCount = GetTickCount64() + initialRisingEdgeTimeoutMillis;
    for (;;) {
        if (GetTickCount64() > endTickCount) {
            return HRESULT_FROM_WIN32(ERROR_TIMEOUT);
        }

        GpioPinValue value = this->pin->Read();
        if (value != previousValue) {
            // rising edgue?
            if (value == GpioPinValue::High) {
                break;
            }
            previousValue = value;
        }
    }

After receiving the first rising edge, we catch all of the falling edges and measure the time difference between them to determine whether the bit is a 0 or 1.

    LARGE_INTEGER prevTime = { 0 };

    const ULONG sampleTimeoutMillis = 10;
    endTickCount = GetTickCount64() + sampleTimeoutMillis;

    // capture every falling edge until all bits are received or
    // timeout occurs
    for (unsigned int i = 0; i < (Reading.bits.size() + 1);) {
        if (GetTickCount64() > endTickCount) {
            return HRESULT_FROM_WIN32(ERROR_TIMEOUT);
        }

        GpioPinValue value = this->pin->Read();
        if ((previousValue == GpioPinValue::High) && (value == GpioPinValue::Low)) {
            // A falling edge was detected
            LARGE_INTEGER now;
            QueryPerformanceCounter(&now);

            if (i != 0) {
                unsigned int difference = static_cast<unsigned int>(
                    now.QuadPart - prevTime.QuadPart);
                Reading.bits[Reading.bits.size() - i] =
                    difference > oneThreshold;
            }

            prevTime = now;
            ++i;
        }

        previousValue = value;
    }

After all bits have been received, we validate the checksum to make sure the received data is valid. The data is returned through the Reading reference parameter.

    if (!Reading.IsValid()) {
        // checksum mismatch
        return HRESULT_FROM_WIN32(ERROR_INVALID_DATA);
    }

    return S_OK;
}

Sample code

Language: C++

Tags: c++, intermediate, gpio

Verified to work with:
Windows 10 IoT Core: Version 10.0.15063.0
Visual Studio 2017
Windows SDK: Version 15063
(Included with Visual Studio)


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