dht22.c

/*
 *  dht.c:
 *      Author: Juergen Wolf-Hofer
 *      based on / adapted from http://www.uugear.com/portfolio/read-dht1122-temperature-humidity-sensor-from-raspberry-pi/
 *	reads temperature and humidity from DHT11 or DHT22 sensor and outputs according to selected mode
 */

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

// CONSTANTS 
#define MAX_TIMINGS	85
#define DEBUG 0
#define WAIT_TIME 2000

// GLOBAL VARIABLES
uint8_t dht_pin = 3;  // default GPIO 22 (wiringPi 3)
char mode = 'c';      // valid modes are c, f, h

int data[5] = { 0, 0, 0, 0, 0 };
float temp_cels = -1;
float temp_fahr = -1;
float humidity  = -1;

// FUNCTION DECLARATIONS
int init();
void printUsage();
int read_dht_data();

// FUNCTION DEFINITIONS
int read_dht_data() {
	uint8_t laststate = HIGH;
	uint8_t counter	= 0;
	uint8_t j = 0;
	uint8_t i;

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

	/* pull pin down for 18 milliseconds */
	pinMode(dht_pin, OUTPUT);
	digitalWrite(dht_pin, LOW);
	delay(18);

	/* prepare to read the pin */
	pinMode(dht_pin, INPUT);

	/* detect change and read data */
	for ( i = 0; i < MAX_TIMINGS; i++ ) {
		counter = 0;
		while ( digitalRead( dht_pin ) == laststate ) {
			counter++;
			delayMicroseconds( 1 );
			if ( counter == 255 ) {
				break;
			}
		}
		laststate = digitalRead( dht_pin );

		if ( counter == 255 )
			break;

		/* ignore first 3 transitions */
		if ( (i >= 4) && (i % 2 == 0) ) {
			/* shove each bit into the storage bytes */
			data[j / 8] <<= 1;
			if ( counter > 16 )
				data[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) && (data[4] == ( (data[0] + data[1] + data[2] + data[3]) & 0xFF) ) ) {
		float h = (float)((data[0] << 8) + data[1]) / 10;
		if ( h > 100 ) {
			h = data[0];	// for DHT11
		}
		float c = (float)(((data[2] & 0x7F) << 8) + data[3]) / 10;
		if ( c > 125 ) {
			c = data[2];	// for DHT11
		}
		if ( data[2] & 0x80 ) {
			c = -c;
		}
		temp_cels = c;
		temp_fahr = c * 1.8f + 32;
		humidity = h;
		if (DEBUG) printf( "read_dht_data() Humidity = %.1f %% Temperature = %.1f *C (%.1f *F)\n", humidity, temp_cels, temp_fahr );
		return 0; // OK
	} else {
		if (DEBUG) printf( "read_dht_data() Data not good, skip\n" );
		temp_cels = temp_fahr = humidity = -1;
		return 1; // NOK
	}
}

void printUsage() {
	fprintf(stdout, "Usage: dht22 t|h pin\n"
                        "             c   .. temperature celsius\n"
                        "             f   .. temperature fahrenheit\n"
                        "             h   .. humidity\n"
                        "             pin .. GPIO pin (wiringPi numbering)\n");
}

int init() {
	if (wiringPiSetup() == -1) {
		fprintf(stderr, "Failed to initialize wiringPi\n");
		exit(1);
		return 1;
	}
	return 0;
}

int main(int argc, char *argv[]) {
	int done = 0;

	if (argc!=3) {
		printUsage(); 
		exit(1);
		return 1;
	} else {
		mode = argv[1][0];		
		dht_pin = atoi(argv[2]);
		if (mode!='c' && mode!='f' && mode!='h') {
			printUsage();
			exit(1);
			return 1;
		}
	}
	if (DEBUG) fprintf(stdout, "Reading DHT22 ... mode=%c sensorPIN=%i\n", mode, dht_pin);

	init();

	while (!done) {
		done = !read_dht_data();
		delay(WAIT_TIME); 
	}

	switch(mode) {
		case 'c':
			fprintf(stdout, "%.1f\n", temp_cels);
			break;
		case 'f':
			fprintf(stdout, "%.1f\n", temp_fahr);
			break;
		case 'h':
			fprintf(stdout, "%.1f\n", humidity);
			break;
		default:
			fprintf(stderr, "invalid mode '%c', should not happen\n", mode);
	}
	
	if (DEBUG) printf( "main() Humidity = %.1f %% Temperature = %.1f *C (%.1f *F)\n", humidity, temp_cels, temp_fahr );

	return(0);
}