dht11/dht11.c

/*****************************************************************************
 * Copyright (C) 2014
 *   
 * Michiel Broek <mbroek at mbse dot eu>
 *
 * This file is part of the mbsePi-apps
 *
 * This is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License as published by the
 * Free Software Foundation; either version 2, or (at your option) any
 * later version.
 *
 * mbsePi-apps is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with EC-65K; see the file COPYING.  If not, write to the Free
 * Software Foundation, 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
 *****************************************************************************/

#include "dht11.h"

#ifdef HAVE_WIRINGPI_H

#define MAXTIMINGS 100

int		dht11_pin = -1;
int		dht11_temperature = -1;
int		dht11_humidity = -1;
int		dht11_valid = false;
int		dht11_t_offset = 0;
int		dht11_h_offset = 0;


static uint8_t sizecvt(const int read_value) {
    /* 
     * 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_value > 255 || read_value < 0) {
	syslog(LOG_NOTICE, "invalid data from wiringPi library");
    }
		  
    return (uint8_t)read_value;
}


/*
 * DHT11 sensor read. This should be used in a thread loop.
 */
void dht11Read(void) {
    int         	tries = 5;
    unsigned short	got_correct_data = 0;

    if (dht11_pin == -1)
	return;

    while (tries && !got_correct_data) {
	uint8_t	laststate = HIGH;
	uint8_t	counter = 0;
	uint8_t	j = 0, i = 0;
	int	dht11_dat[5] = {0,0,0,0,0};

	/*
	 * Select output mode to send the start signal.
	 */
	pinMode(dht11_pin, OUTPUT);                 
	digitalWrite(dht11_pin, HIGH);
	usleep(1000);

	/*
	 * Low for at least 18 milliseconds
	 */
	digitalWrite(dht11_pin, LOW);
	usleep(20000);
	digitalWrite(dht11_pin, HIGH);
	pinMode(dht11_pin, INPUT);

	/*
	 * Detect change and read data
	 */
	for (i=0; i<MAXTIMINGS; i++) {
	    counter = 0;
	    delayMicroseconds(10);
	    while (sizecvt(digitalRead(dht11_pin)) == laststate) {
		counter++;
		delayMicroseconds(1);
		if (counter == 255) {
		    break;
		}
	    }
	    laststate = sizecvt(digitalRead(dht11_pin));

	    if (counter == 255) 
		break;

	    /*
	     * ignore first 3 transitions
	     */
	    if ((i >= 4) && (i%2 == 0)) {

		// shove each bit into the storage bytes
		dht11_dat[(int)((double)j/8)] <<= 1;
		if (counter > 16)
		    dht11_dat[(int)((double)j/8)] |= 1;
		j++;
	    }
	}

	/*
	 * If there is no sensor, j = 0
	 */
	if ((counter == 255) && (j == 0)) {
	    if (dht11_temperature != -1) {
		syslog(LOG_NOTICE, "dht11 sensor disappeared");
	    } else {
		syslog(LOG_NOTICE, "dht11 sensor not present");
	    }
	    dht11_temperature = -1;
	    dht11_humidity = -1;
	    dht11_valid = false;
	    return;
	} 

	/*
	 * check we read 40 bits (8bit x 5 ) + verify checksum in the last byte
	 * print it out if data is good
	 */
	if ((j >= 40) && (dht11_dat[4] == ((dht11_dat[0] + dht11_dat[1] + dht11_dat[2] + dht11_dat[3]) & 0xFF))) {
		got_correct_data = 1;

		int h = dht11_dat[0] + dht11_dat[1];
		int t = (dht11_dat[2] & 0x7F) + dht11_dat[3];
		t += dht11_t_offset;
		h += dht11_h_offset;

		if ((dht11_dat[2] & 0x80) != 0) 
		    t *= -1;

		dht11_temperature = t;
		dht11_humidity = h;
		dht11_valid = true;
	} else {
		tries--;
		if (tries == 0)
		    syslog(LOG_INFO, "dht11 data checksum was wrong 5 times");
		usleep(100000);
	}
    }
}


void dht11Init(int pin, int t_offset, int h_offset) {
    dht11_pin = pin;
    dht11_t_offset = t_offset;
    dht11_h_offset = h_offset;
}


int main(int argc, char *argv[]) {
  
    int PIN = 3;
     
    if (wiringPiSetup() == -1)
	return 0;

    dht11Init(PIN, 1, 6);
    dht11Read();
    if (dht11_valid) {
	fprintf(stdout, "DHT11: temperature %d degrees, humidity %d%%\n", dht11_temperature, dht11_humidity);
    } else {
	fprintf(stdout, "DHT11: no valid data or sensor not connected\n");
    }

    exit(0);
}

#else

int main(int argc, char *argv[]) {
    fprintf(stderr, "This program does nothing without the wiringPi library\n");
    return 0;
}

#endif