File content as of revision 2:df22ddf10d75:

/*
 *  DHT Library for  Digital-output Humidity and Temperature sensors
 *
 *  Works with DHT11, DHT22
 *             SEN11301P,  Grove - Temperature&Humidity Sensor     (Seeed Studio)
 *             SEN51035P,  Grove - Temperature&Humidity Sensor Pro (Seeed Studio)
 *             AM2302   ,  temperature-humidity sensor
 *             HM2303   ,  Digital-output humidity and temperature sensor
 *
 *  Copyright (C) Wim De Roeve
 *                based on DHT22 sensor library by HO WING KIT
 *                Arduino DHT11 library
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documnetation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to  whom the Software is
 * furished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS OR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

#include "DHT.h"

#define DHT_DATA_BIT_COUNT 40

DHT::DHT(PinName pin, eType DHTtype)
{
    _pin = pin;
    _DHTtype = DHTtype;
    _firsttime = true;
}

DHT::~DHT()
{
    
}

eError DHT::stall(DigitalInOut &io, int const level, int const max_time)
{
    int cnt = 0;
    while (level == io) {
        if (cnt > max_time) {
            return ERROR_NO_PATIENCE;
        }
        cnt++;
        wait_us(1);
    }
    return ERROR_NONE;
}

eError DHT::readData()
{
    uint8_t i = 0, j = 0, b = 0, data_valid = 0;
    uint32_t bit_value[DHT_DATA_BIT_COUNT] = {0};

    eError err = ERROR_NONE;
    time_t currentTime = time(NULL);

    DigitalInOut DHT_io(_pin);

    // IO must be in hi state to start
    if (ERROR_NONE != stall(DHT_io, 0, 250)) {
        return BUS_BUSY;
    }

    // start the transfer
    DHT_io.output();
    DHT_io = 0;
    // only 500uS for DHT22 but 18ms for DHT11
    (_DHTtype == 11) ? wait_ms(18) : wait(1);
    DHT_io = 1;
    wait_us(30);
    DHT_io.input();
    // wait till the sensor grabs the bus
    if (ERROR_NONE != stall(DHT_io, 1, 40)) {
        return ERROR_NOT_PRESENT;
    }
    // sensor should signal low 80us and then hi 80us
    if (ERROR_NONE != stall(DHT_io, 0, 100)) {
        return ERROR_SYNC_TIMEOUT;
    }
    if (ERROR_NONE != stall(DHT_io, 1, 100)) {
        return ERROR_NO_PATIENCE;
    }
    // capture the data
    for (i = 0; i < 5; i++) {
        for (j = 0; j < 8; j++) {
            if (ERROR_NONE != stall(DHT_io, 0, 75)) {
                return ERROR_DATA_TIMEOUT;
            }
            // logic 0 is 28us max, 1 is 70us
            wait_us(40);
            bit_value[i*8+j] = DHT_io;
            if (ERROR_NONE != stall(DHT_io, 1, 50)) {
                return ERROR_DATA_TIMEOUT;
            }
        }
    }
    // store the data
    for (i = 0; i < 5; i++) {
        b=0;
        for (j=0; j<8; j++) {
            if (bit_value[i*8+j] == 1) {
                b |= (1 << (7-j));
            }
        }
        DHT_data[i]=b;
    }

    // uncomment to see the checksum error if it exists
    //printf(" 0x%02x + 0x%02x + 0x%02x + 0x%02x = 0x%02x \n", DHT_data[0], DHT_data[1], DHT_data[2], DHT_data[3], DHT_data[4]);
    data_valid = DHT_data[0] + DHT_data[1] + DHT_data[2] + DHT_data[3];
    if (DHT_data[4] == data_valid) {
        _lastReadTime = currentTime;
        _lastTemperature = CalcTemperature();
        _lastHumidity = CalcHumidity();

    } else {
        err = ERROR_CHECKSUM;
    }

    return err;

}

float DHT::CalcTemperature()
{
    int v;

    switch (_DHTtype) {
        case DHT11:
            v = DHT_data[2];
            return float(v);
        case DHT22:
            v = DHT_data[2] & 0x7F;
            v *= 256;
            v += DHT_data[3];
            v /= 10;
            if (DHT_data[2] & 0x80)
                v *= -1;
            return float(v);
    }
    return 0;
}

float DHT::ReadHumidity()
{
    return _lastHumidity;
}

float DHT::ConvertCelciustoFarenheit(float const celsius)
{
    return celsius * 9 / 5 + 32;
}

float DHT::ConvertCelciustoKelvin(float const celsius)
{
    return celsius + 273.15;
}

// dewPoint function NOAA
// reference: http://wahiduddin.net/calc/density_algorithms.htm
float DHT::CalcdewPoint(float const celsius, float const humidity)
{
    float A0= 373.15/(273.15 + celsius);
    float SUM = -7.90298 * (A0-1);
    SUM += 5.02808 * log10(A0);
    SUM += -1.3816e-7 * (pow(10, (11.344*(1-1/A0)))-1) ;
    SUM += 8.1328e-3 * (pow(10,(-3.49149*(A0-1)))-1) ;
    SUM += log10(1013.246);
    float VP = pow(10, SUM-3) * humidity;
    float T = log(VP/0.61078);   // temp var
    return (241.88 * T) / (17.558-T);
}

// delta max = 0.6544 wrt dewPoint()
// 5x faster than dewPoint()
// reference: http://en.wikipedia.org/wiki/Dew_point
float DHT::CalcdewPointFast(float const celsius, float const humidity)
{
    float a = 17.271;
    float b = 237.7;
    float temp = (a * celsius) / (b + celsius) + log(humidity/100);
    float Td = (b * temp) / (a - temp);
    return Td;
}

float DHT::ReadTemperature(eScale Scale)
{
    if (Scale == FARENHEIT)
        return ConvertCelciustoFarenheit(_lastTemperature);
    else if (Scale == KELVIN)
        return ConvertCelciustoKelvin(_lastTemperature);
    else
        return _lastTemperature;
}

float DHT::CalcHumidity()
{
    int v;

    switch (_DHTtype) {
        case DHT11:
            v = DHT_data[0];
            return float(v);
        case DHT22:
            v = DHT_data[0];
            v *= 256;
            v += DHT_data[1];
            v /= 10;
            return float(v);
    }
    return 0;
}