dht-sensor.cpp

#include <stdio.h>
#include <string.h>
#include <sys/time.h>
#include <bcm2835.h>
#include <unistd.h>

#define BCM2708_PERI_BASE   0x20000000
#define GPIO_BASE           (BCM2708_PERI_BASE + 0x200000)
#define MAXTIMINGS          100
#define DHT11               11
#define DHT22               22
#define AM2302              22

int initialized = 0;
unsigned long long last_read[32] = {};
float last_temperature[32] = {};
float last_humidity[32] = {};

unsigned long long getTime()
{
    struct timeval tv;
    gettimeofday(&tv, NULL);
    unsigned long long time = (unsigned long long)(tv.tv_sec) * 1000 +
                              (unsigned long long)(tv.tv_usec) / 1000;
    return time;
}

long readDHT(int type, int pin, float &temperature, float &humidity)
{
    int bitCount = 0;
    int timeout;
    int bits[MAXTIMINGS];
    int data[MAXTIMINGS / 8] = {};

    #ifdef VERBOSE
    #ifdef DBG_CONSOLE
    FILE *pTrace = stdout;
    #else
    FILE *pTrace = fopen("dht-sensor.log", "a");
    if (pTrace == NULL)
    {
        puts("WARNING: unable to initialize trace file, it will be redirected to stdout.");
        pTrace = stdout;
    }
    #endif
    #endif

    #ifdef VERBOSE
    fprintf(pTrace, "start sensor read (type=%d, pin=%d).\n", type, pin);
    #endif

    // throttle sensor reading - if last read was less than 2s then return same
    unsigned long long now = getTime();
    if ((last_read[pin] > 0) && (now - last_read[pin] < 3000))
    {
        #ifdef VERBOSE
        fprintf(pTrace, "*** too early to read again pin %d: %llu\n", pin, now - last_read[pin]);
        #endif
        temperature = last_temperature[pin];
        humidity = last_humidity[pin];
        return 0;
    }

    // request sensor data
    bcm2835_gpio_fsel(pin, BCM2835_GPIO_FSEL_OUTP);
    bcm2835_gpio_write(pin, HIGH);
    usleep(10000);
    bcm2835_gpio_write(pin, LOW);
    usleep(type == 11 ? 2500 : 800);
    bcm2835_gpio_write(pin, HIGH);
    bcm2835_gpio_fsel(pin, BCM2835_GPIO_FSEL_INPT);

    // wait for sensor response
    for (timeout = 0; timeout < 1000000 && bcm2835_gpio_lev(pin) == LOW; ++timeout);
    if (timeout >= 100000)
    {
        #ifdef VERBOSE
        fprintf(pTrace, "*** timeout #1\n");
        #ifdef DBG_CONSOLE
        fflush(pTrace);
        #else
        fclose(pTrace);
        #endif
        #endif
        return -3;
    }

    for (timeout = 0; timeout < 1000000 && bcm2835_gpio_lev(pin) == HIGH; ++timeout);
    if (timeout >= 100000)
    {
        #ifdef VERBOSE
        fprintf(pTrace, "*** timeout #2\n");
        #ifdef DBG_CONSOLE
        fflush(pTrace);
        #else
        fclose(pTrace);
        #endif
        #endif
        return -3;
    }

    // read data
    for (bitCount = 0; bitCount < MAXTIMINGS; ++bitCount)
    {
        for (timeout = 0; bcm2835_gpio_lev(pin) == LOW && timeout < 50000; ++timeout);
        for (timeout = 0; bcm2835_gpio_lev(pin) == HIGH && timeout < 50000; ++timeout);
        bits[bitCount] = timeout;
        if (timeout >= 50000) break;
    }

    // data decoding, get widest pulse
    int peak = bits[1];
    #ifdef VERBOSE
    fprintf(pTrace, "init peak: %d\n", bits[1]);
    #endif
    for (int i = 2; i < bitCount; ++i)
    {
        if (peak < bits[i])
        {
            peak = bits[i];
            #ifdef VERBOSE
            fprintf(pTrace, "update peak: %d (%d)\n", i, bits[i]);
            #endif
        }
    }

    // convert pulses to bits
    #ifdef VERBOSE
    fprintf(pTrace, "bitCount=%d, peak=%d:\n", bitCount, peak);
    #endif
    int k = 0;
    for (int i = 1; i < bitCount; ++i)
    {
        data[k] <<= 1;
        if ((2 * bits[i] - peak) > 0)
        {
            data[k] |= 1;
            #ifdef VERBOSE
            fprintf(pTrace, "1 (%03d) ", bits[i]);
        }
        else
        {
            fprintf(pTrace, "0 (%03d) ", bits[i]);
            #endif
        }
        if (i % 8 == 0)
        {
            k++;
            #ifdef VERBOSE
            fprintf(pTrace, "\n");
            #endif
        }
    }

    // crc checking
    #ifdef VERBOSE
    int crc = ((data[0] + data[1] + data[2] + data[3]) & 0xff);
    fprintf(pTrace, "\n=> %x %x %x %x (%x/%x) : %s\n",
        data[0], data[1], data[2], data[3], data[4],
        crc, (data[4] == crc) ? "OK" : "ERR");
    #endif

    // checks if received data is valid:
    // - minimum of 5 bytes (40 bits)
    // - has to be whole bytes
    // - checksum must match
    if ((bitCount > 40) && ((bitCount - 1) % 8 == 0) &&
        (data[4] == ((data[0] + data[1] + data[2] + data[3]) & 0xff)))
    {
        #ifdef VERBOSE
        fprintf(pTrace, "sensor type = %d, ", type);
        #endif

        if (type == DHT11)
        {
            #ifdef VERBOSE
            printf("temperature = %d, humidity = %d\n", data[2], data[0]);
            #endif
            temperature = data[2];
            humidity = data[0];
        }
        else if (type == DHT22)
        {
            float f, h;
            h = data[0] * 256 + data[1];
            h /= 10.0;

            f = (data[2] & 0x7F) * 256 + data[3];
            f /= 10.0;
            if (data[2] & 0x80) f *= -1;

            #ifdef VERBOSE
            fprintf(pTrace, "temperature = %.1f, humidity = %.1f\n", f, h);
            #endif
            temperature = f;
            humidity = h;
        }
        else
        {
            #ifdef VERBOSE
            fprintf(pTrace, "*** unknown sensor data type\n");
            #ifdef DBG_CONSOLE
            fflush(pTrace);
            #else
            fclose(pTrace);
            #endif
            #endif
            return -2;
        }
    }
    else
    {
        #ifdef VERBOSE
        fprintf(pTrace, "*** unexpected data: bits=%d: %d != %d + %d + %d + %d\n",
        bitCount, data[4], data[0], data[1], data[2], data[3]);
        #ifdef DBG_CONSOLE
        fflush(pTrace);
        #else
        fclose(pTrace);
        #endif
        #endif
        return -1;
    }

    #ifdef VERBOSE
    fprintf(pTrace, "*** obtained readout successfully.\n");
    #ifdef DBG_CONSOLE
    fflush(pTrace);
    #else
    fclose(pTrace);
    #endif
    #endif

    // update last readout
    last_read[pin] = now;
    last_temperature[pin] = temperature;
    last_humidity[pin] = humidity;

    return 0;
}

int initialize()
{
    if (!bcm2835_init())
    {
        #ifdef VERBOSE
        puts("BCM2835 initialization failed.");
        #endif
        return 1;
    }
    else
    {
        #ifdef VERBOSE
        puts("BCM2835 initialized.");
        #endif
        initialized = 1;
        memset(last_read, 0, sizeof(unsigned long long)*32);
        memset(last_temperature, 0, sizeof(float)*32);
        memset(last_humidity, 0, sizeof(float)*32);
        return 0;
    }
}