Andy Pomfret / DHT11

Simple library for the DHT11 temperature and humidity sensor. Forked from an existing Mbed DHT11 project.

Dependents:   UoY-DHT11-test

Simple DHT11 temperature and humidity library.

Example usage

#include "mbed.h"
#include "DHT11.h"

DHT11 dht(D8); // Change pin name here if required

main()
{
    printf("T:%d, H:%d\r\n", dht.readTemperature(), dht.readHumidity());
}

The sensor may be read as often as desired, but temperature and humidity values are cached and will only be updated if they are more than 2 seconds old. This is the underlying sensor update rate.

Please note that this project has been modified only enough to make it work for its intended purpose. Various parts of this project still need work, and the source code should not be seen as an example of best practice.

DHT11.cpp

Committer:
s_inoue_mbed
Date:
2014-09-10
Revision:
7:50f5c8efd967
Parent:
6:257e2ab66d0f
Child:
8:160047ca45bf

File content as of revision 7:50f5c8efd967:

/*
 * Library for the use of the DHT11, a temperature and humidity sensor
 * Shigenori Inoue, September 10, 2014
 */

#include "DHT11.h"

// Constructor
DHT11::DHT11(PinName pin) : io(pin, PIN_INPUT, OpenDrain, 1), io_irq(pin)
{
    io_irq.rise(this, &DHT11::pos_edge);
    io_irq.fall(this, &DHT11::neg_edge);
    io_irq.disable_irq();
    init();    
    first_time = true;
}

// Destructor
DHT11::~DHT11(void) {}

// Reading the data bits from the DHT11
int DHT11::readData()
{
    // Initialize
    init();

    // Checking the measurement frequency
    if (t.read_ms() < 2000 & first_time == false) {
        t.reset();
        return TOO_FAST_READ;
    }

    // Checking the data bus
    if (io == 0) {
        io.input();
        return BUS_BUSY;
    }

    // Sending start signal
    io.output();
    t.reset();
    t.start();
    do {
        io = 0;
    } while (t.read_ms() < 18);
    io = 1;
    io.input();

    // Waiting for the start of the response signal
    t.reset();
    t.start();
    do {
        if (t.read_us() > 100) {
            io.input();
            return NOT_PRESENT;
        }
    } while (io == 1);

    // Wainting for the start of the ready signal
    t.reset();
    t.start();
    do {
        if (t.read_us() > 100) {
            io.input();
            return NOT_READY;
        }
    } while (io == 0);

    // Wainting for the end of the ready signal
    t.reset();
    t.start();
    do {} while (io == 1);

    // Starting the pulse width sensing

    io_irq.enable_irq();

    do {
        wait_us(100);
        if (wdt > 50) {
            return WATCHDOG_ERR;
        }
        wdt++;
    } while (eod == false);

    // Calculating the check sum
    chksum = ((data & 0xff00000000) >> 32)
             + ((data & 0x00ff000000) >> 24)
             + ((data & 0x0000ff0000) >> 16)
             + ((data & 0x000000ff00) >> 8);
    if (chksum != (data & 0x00000000ff)) {
        io.input();
        io = 1;
        return CHKSUM_ERR;
    } else {
        t.reset();
        first_time = false;
        return OK;
    }
}

// Extracting humidity data from the received data
int DHT11::readHumidity()
{
    if (err == OK) {
        return (data & 0xff00000000) >> 32;
    } else {
        return 0xffffffff;
    }
}

// Extracting temperature data from the received data
int DHT11::readTemperature()
{
    if (err == OK) {
        return (data & 0x0000ff0000) >> 16;
    } else {
        return 0xffffffff;
    }
}

// Initialization of variables
void DHT11::init(void)
{
    t_pulse_us = 0;
    data = 0;
    chksum = 0;
    cnt = 0;
    wdt = 0;
    err = OK;
    eod = false;
}

void DHT11::pos_edge(void)
{
    // Disabling the interruptions
    io_irq.disable_irq();

    // Initializing the Timer
    t.reset();

    // Enabling the interruptions
    io_irq.enable_irq();
}

void DHT11::neg_edge(void)
{
    // Disabling the interruptions
    io_irq.disable_irq();

    // Reading the length of
    t_pulse_us = t.read_us();

    // Detecting 0 if the pulse width ranges from 26 us to 28 us
    if (20 <= t_pulse_us && t_pulse_us <= 30) {
        // Shifting the data buffer and not adding 1 (because this bit is zero)
        data = data << 1;

        // Counting up the bits
        cnt++;
    }

    // Detecting 1 if the pulse width ranges from 68 us to 72 us
    else if (60 <= t_pulse_us && t_pulse_us <= 80) {
        // Shifting the data buffer and adding 1 (because this bit is one)
        data = data << 1;
        data++;

        // Counting up the bits
        cnt++;
    }

    // Detecting the end of Data
    if (cnt < 40) {
        // Enabling the interruptions
        io_irq.enable_irq();
    } else {
        eod = true;
    }
}