hal/arduino/HalMultiSensor/SensorDHT.cpp

/* DHT library
MIT license
written by Adafruit Industries
*/
// Modified by Ziver Koc
//

#include "SensorDHT.h"


void SensorDHT::setup()
{
    // set up the pins!
    pinMode(_pin, INPUT_PULLUP);
    _maxcycles = microsecondsToClockCycles(1000);  // 1 millisecond timeout for
                                                   // reading pulses from DHT sensor.
}


// Expect the signal line to be at the specified level for a period of time and
// return a count of loop cycles spent at that level (this cycle count can be
// used to compare the relative time of two pulses).  If more than a millisecond
// ellapses without the level changing then the call fails with a 0 response.
// This is adapted from Arduino's pulseInLong function (which is only available
// in the very latest IDE versions):
//   https://github.com/arduino/Arduino/blob/master/hardware/arduino/avr/cores/arduino/wiring_pulse.c
uint32_t SensorDHT::expectPulse(bool level) {
    uint32_t count = 0;
    // On AVR platforms use direct GPIO port access as it's much faster and better
    // for catching pulses that are 10's of microseconds in length:
    #ifdef __AVR
    uint8_t portState = level ? _bit : 0;
    while ((*portInputRegister(_port) & _bit) == portState) {
        if (count++ >= _maxcycles) {
            return 0; // Exceeded timeout, fail.
        }
    }
    // Otherwise fall back to using digitalRead (this seems to be necessary on ESP8266
    // right now, perhaps bugs in direct port access functions?).
    #else
    while (digitalRead(_pin) == level) {
        if (count++ >= _maxcycles) {
            return 0; // Exceeded timeout, fail.
        }
    }
    #endif

    return count;
}


void SensorDHT::read(TemperatureData& retData)
{
    // Reset 40 bits of received data to zero.
    static uint8_t data[5];
    data[0] = data[1] = data[2] = data[3] = data[4] = 0;

    // Send start signal.  See DHT datasheet for full signal diagram:
    //   http://www.adafruit.com/datasheets/Digital%20humidity%20and%20temperature%20sensor%20AM2302.pdf

    // Go into high impedence state to let pull-up raise data line level and
    // start the reading process.
    digitalWrite(_pin, HIGH);
    delay(250);

    // First set data line low for 20 milliseconds.
    pinMode(_pin, OUTPUT);
    digitalWrite(_pin, LOW);
    delay(20);

    uint32_t cycles[80];
    {
        // Turn off interrupts temporarily because the next sections are timing critical
        // and we don't want any interruptions.
        noInterrupts();

        // End the start signal by setting data line high for 40 microseconds.
        digitalWrite(_pin, HIGH);
        delayMicroseconds(40);

        // Now start reading the data line to get the value from the DHT sensor.
        pinMode(_pin, INPUT_PULLUP);
        delayMicroseconds(10);  // Delay a bit to let sensor pull data line low.

        // First expect a low signal for ~80 microseconds followed by a high signal
        // for ~80 microseconds again.
        if (expectPulse(LOW) == 0) {
            DEBUG("DHT:Timeout waiting for start signal low pulse.");
            return;
        }
        if (expectPulse(HIGH) == 0) {
            DEBUG("DHT:Timeout waiting for start signal high pulse.");
            return;
        }

        // Now read the 40 bits sent by the sensor.  Each bit is sent as a 50
        // microsecond low pulse followed by a variable length high pulse.  If the
        // high pulse is ~28 microseconds then it's a 0 and if it's ~70 microseconds
        // then it's a 1.  We measure the cycle count of the initial 50us low pulse
        // and use that to compare to the cycle count of the high pulse to determine
        // if the bit is a 0 (high state cycle count < low state cycle count), or a
        // 1 (high state cycle count > low state cycle count). Note that for speed all
        // the pulses are read into a array and then examined in a later step.
        for (int i=0; i<80; i+=2) {
            cycles[i]   = expectPulse(LOW);
            cycles[i+1] = expectPulse(HIGH);
        }

        interrupts();
    } // Timing critical code is now complete.

    // Inspect pulses and determine which ones are 0 (high state cycle count < low
    // state cycle count), or 1 (high state cycle count > low state cycle count).
    for (int i=0; i<40; ++i) {
        uint32_t lowCycles  = cycles[2*i];
        uint32_t highCycles = cycles[2*i+1];
        if ((lowCycles == 0) || (highCycles == 0)) {
            DEBUG("DHT:Timeout waiting for pulse.");
            return;
        }
        data[i/8] <<= 1;
        // Now compare the low and high cycle times to see if the bit is a 0 or 1.
        if (highCycles > lowCycles) {
            // High cycles are greater than 50us low cycle count, must be a 1.
            data[i/8] |= 1;
        }
        // Else high cycles are less than (or equal to, a weird case) the 50us low
        // cycle count so this must be a zero.  Nothing needs to be changed in the
        // stored data.
    }


    // Check we read 40 bits and that the checksum matches.
    if (data[4] == ((data[0] + data[1] + data[2] + data[3]) & 0xFF)) {
        switch (_type) {
        case DHT11:
          retData.temperature = data[2];
          retData.humidity = data[0];
          break;
        case DHT22:
        case DHT21:
            retData.temperature = data[2] & 0x7F;
            retData.temperature *= 256;
            retData.temperature += data[3];
            retData.temperature *= 0.1;
            if (data[2] & 0x80) {
                retData.temperature *= -1;
            }
            retData.humidity = data[0];
            retData.humidity *= 256;
            retData.humidity += data[1];
            retData.humidity *= 0.1;
            break;
        }
    }
    else {
        DEBUG("DHT:Checksum failure!");
    }
}