dht22.c

// Ref: https://github.com/technion/lol_dht22/blob/master/dht22.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>

#include <wiringx.h>

#include <dht22.h>

void dht22_setup()
{
    printf("wiringXValidGPIO(%d)\n",DHTPIN);
    if (wiringXValidGPIO(DHTPIN) != 0)
    {
        printf("Invalid GPIO %d\n", DHTPIN);
    }
}

uint8_t sizecvt(const int read)
{
    /* digitalRead() and friends from wiringpi are defined as returning a value
    < 256. However, they are returned as int() types. This is a safety function */

    if (read > 255 || read < 0)
    {
        printf("Invalid data from wiringPi library\n");
        exit(EXIT_FAILURE);
    }
    return (uint8_t)read;
}

int read_dht22_dat()
{
    uint8_t laststate = HIGH;
    uint8_t counter = 0;
    uint8_t j = 0, i;

    dht22_dat[0] = dht22_dat[1] = dht22_dat[2] = dht22_dat[3] = dht22_dat[4] = 0;

    // pull pin down for 18 milliseconds
    pinMode(DHTPIN, PINMODE_OUTPUT);
    digitalWrite(DHTPIN, HIGH);
    delayMicroseconds(500000);
    digitalWrite(DHTPIN, LOW);
    delayMicroseconds(20000);
    // prepare to read the pin
    pinMode(DHTPIN, PINMODE_INPUT);

    // detect change and read data
    for (i = 0; i < MAXTIMINGS; i++)
    {
        counter = 0;
        while (sizecvt(digitalRead(DHTPIN)) == laststate)
        {
            counter++;
            delayMicroseconds(2);
            if (counter == 255)
                break;
        }
        laststate = sizecvt(digitalRead(DHTPIN));
        if (counter == 255)
            break;

        // ignore first 3 transitions
        if ((i >= 4) && (i % 2 == 0))
        {
            // shove each bit into the storage bytes
            dht22_dat[j / 8] <<= 1;
            if (counter > 16)
                dht22_dat[j / 8] |= 1;
            j++;
        }
    }

    // check we read 40 bits (8bit x 5 ) + verify checksum in the last byte
    // print it out if data is good
    if ((j >= 40) &&
        (dht22_dat[4] == ((dht22_dat[0] + dht22_dat[1] + dht22_dat[2] + dht22_dat[3]) & 0xFF)))
    {
        float t, h;
        h = (float)dht22_dat[0] * 256 + (float)dht22_dat[1];
        h /= 10;
        t = (float)(dht22_dat[2] & 0x7F) * 256 + (float)dht22_dat[3];
        t /= 10.0;
        if ((dht22_dat[2] & 0x80) != 0)
            t *= -1;

        printf("Humidity = %.2f %% Temperature = %.2f *C \n", h, t);
        return 1;
    }
    else
    {
        printf("Data not good, skip\n");
        return 0;
    }
}