DHT.h

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#ifndef DHT_H
#define DHT_H

#include "GPIO.h"

template<BOARD::pin_t DATA_PIN, uint8_t DEVICE>
class DHT {
public:
  DHT() :
    m_humidity(0),
    m_temperature(0)
  {
    m_data.open_drain();
  }

  float humidity()
  {
    return (m_humidity);
  }

  float temperature()
  {
    return (m_temperature);
  }

  int read(float& humidity, float& temperature)
  {
    // Issue start signal and wait for device to respond
    m_data.output();
    delay(START_SIGNAL);
    m_data.input();
    int retry = 16;
    do {
      delayMicroseconds(PULLUP);
    } while (m_data && --retry);
    if (retry == 0 || m_data.pulse() < THRESHOLD) return (-2);

    // Read data from the device. Each bit is pulse width coded;
    // low for 50 us, and high 26-28 us for zero(0), and 70 us for one(1).
    uint8_t chsum = 0;
    uint8_t d[5];
    for (int i = 0; i < 5; i++) {
      uint8_t v = 0;
      for (int j = 0; j < 8; j++) {
        uint8_t width = m_data.pulse();
        v = (v << 1) | (width > THRESHOLD);
      }
      d[i] = v;
      if (i < 4) chsum += v;
    }

    // Validate check sum
    if (chsum != d[4]) return (-1);

    // Check type of device and convert data
    if (DEVICE == 11) {
      humidity = d[0];
      temperature = d[2];
    }
    else {
      humidity = ((d[0] << 8) | d[1]) / 10.0;
      temperature = (((d[2] & 0x7f) << 8) | d[3]) / 10.0;
      if (d[2] & 0x80) temperature *= -1;
    }

    // Check for value changes
    int res = (humidity != m_humidity) + (temperature != m_temperature);
    if (res > 0) {
      m_humidity = humidity;
      m_temperature = temperature;
    }

    // Return number of changed values: 0, 1 och 2
    return (res);
  }

protected:
  static const uint16_t START_SIGNAL = 18;

  static const uint16_t PULLUP = 4;

  static const int THRESHOLD = 60;

  GPIO<DATA_PIN> m_data;

  float m_humidity;

  float m_temperature;
};

template<BOARD::pin_t DATA_PIN> class DHT11 : public DHT<DATA_PIN, 11> {};

template<BOARD::pin_t DATA_PIN> class DHT21 : public DHT<DATA_PIN, 21> {};

template<BOARD::pin_t DATA_PIN> class DHT22 : public DHT<DATA_PIN, 22> {};
#endif