/*!
* @file DHT.cpp
*
* @mainpage DHT series of low cost temperature/humidity sensors.
*
* @section intro_sec Introduction
*
* This is a library for DHT series of low cost temperature/humidity sensors.
*
* You must have Adafruit Unified Sensor Library library installed to use this
* class.
*
* Adafruit invests time and resources providing this open source code,
* please support Adafruit andopen-source hardware by purchasing products
* from Adafruit!
*
* @section author Author
*
* Written by Adafruit Industries.
*
* @section license License
*
* MIT license, all text above must be included in any redistribution
*/
#include "DHT.h"
#define MIN_INTERVAL 2000 /**< min interval value */
#define TIMEOUT \
UINT32_MAX /**< Used programmatically for timeout. \
Not a timeout duration. Type: uint32_t. */
/*!
* @brief Instantiates a new DHT class
* @param pin
* pin number that sensor is connected
* @param type
* type of sensor
* @param count
* number of sensors
*/
DHT::DHT(uint8_t pin, uint8_t type, uint8_t count) {
(void)count; // Workaround to avoid compiler warning.
_pin = pin;
_type = type;
#ifdef __AVR
_bit = digitalPinToBitMask(pin);
_port = digitalPinToPort(pin);
#endif
_maxcycles =
microsecondsToClockCycles(1000); // 1 millisecond timeout for
// reading pulses from DHT sensor.
// Note that count is now ignored as the DHT reading algorithm adjusts itself
// based on the speed of the processor.
}
/*!
* @brief Setup sensor pins and set pull timings
* @param usec
* Optionally pass pull-up time (in microseconds) before DHT reading
*starts. Default is 55 (see function declaration in DHT.h).
*/
void DHT::begin(uint8_t usec) {
// set up the pins!
pinMode(_pin, INPUT_PULLUP);
// Using this value makes sure that millis() - lastreadtime will be
// >= MIN_INTERVAL right away. Note that this assignment wraps around,
// but so will the subtraction.
_lastreadtime = millis() - MIN_INTERVAL;
DEBUG_PRINT("DHT max clock cycles: ");
DEBUG_PRINTLN(_maxcycles, DEC);
pullTime = usec;
}
/*!
* @brief Read temperature
* @param S
* Scale. Boolean value:
* - true = Fahrenheit
* - false = Celcius
* @param force
* true if in force mode
* @return Temperature value in selected scale
*/
float DHT::readTemperature(bool S, bool force) {
float f = NAN;
if (read(force)) {
switch (_type) {
case DHT11:
f = data[2];
if (data[3] & 0x80) {
f = -1 - f;
}
f += (data[3] & 0x0f) * 0.1;
if (S) {
f = convertCtoF(f);
}
break;
case DHT12:
f = data[2];
f += (data[3] & 0x0f) * 0.1;
if (data[2] & 0x80) {
f *= -1;
}
if (S) {
f = convertCtoF(f);
}
break;
case DHT22:
case DHT21:
f = ((word)(data[2] & 0x7F)) << 8 | data[3];
f *= 0.1;
if (data[2] & 0x80) {
f *= -1;
}
if (S) {
f = convertCtoF(f);
}
break;
}
}
return f;
}
/*!
* @brief Converts Celcius to Fahrenheit
* @param c
* value in Celcius
* @return float value in Fahrenheit
*/
float DHT::convertCtoF(float c) { return c * 1.8 + 32; }
/*!
* @brief Converts Fahrenheit to Celcius
* @param f
* value in Fahrenheit
* @return float value in Celcius
*/
float DHT::convertFtoC(float f) { return (f - 32) * 0.55555; }
/*!
* @brief Read Humidity
* @param force
* force read mode
* @return float value - humidity in percent
*/
float DHT::readHumidity(bool force) {
float f = NAN;
if (read(force)) {
switch (_type) {
case DHT11:
case DHT12:
f = data[0] + data[1] * 0.1;
break;
case DHT22:
case DHT21:
f = ((word)data[0]) << 8 | data[1];
f *= 0.1;
break;
}
}
return f;
}
/*!
* @brief Compute Heat Index
* Simplified version that reads temp and humidity from sensor
* @param isFahrenheit
* true if fahrenheit, false if celcius
*(default true)
* @return float heat index
*/
float DHT::computeHeatIndex(bool isFahrenheit) {
float hi = computeHeatIndex(readTemperature(isFahrenheit), readHumidity(),
isFahrenheit);
return hi;
}
/*!
* @brief Compute Heat Index
* Using both Rothfusz and Steadman's equations
* (http://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml)
* @param temperature
* temperature in selected scale
* @param percentHumidity
* humidity in percent
* @param isFahrenheit
* true if fahrenheit, false if celcius
* @return float heat index
*/
float DHT::computeHeatIndex(float temperature, float percentHumidity,
bool isFahrenheit) {
float hi;
if (!isFahrenheit)
temperature = convertCtoF(temperature);
hi = 0.5 * (temperature + 61.0 + ((temperature - 68.0) * 1.2) +
(percentHumidity * 0.094));
if (hi > 79) {
hi = -42.379 + 2.04901523 * temperature + 10.14333127 * percentHumidity +
-0.22475541 * temperature * percentHumidity +
-0.00683783 * pow(temperature, 2) +
-0.05481717 * pow(percentHumidity, 2) +
0.00122874 * pow(temperature, 2) * percentHumidity +
0.00085282 * temperature * pow(percentHumidity, 2) +
-0.00000199 * pow(temperature, 2) * pow(percentHumidity, 2);
if ((percentHumidity < 13) && (temperature >= 80.0) &&
(temperature <= 112.0))
hi -= ((13.0 - percentHumidity) * 0.25) *
sqrt((17.0 - abs(temperature - 95.0)) * 0.05882);
else if ((percentHumidity > 85.0) && (temperature >= 80.0) &&
(temperature <= 87.0))
hi += ((percentHumidity - 85.0) * 0.1) * ((87.0 - temperature) * 0.2);
}
return isFahrenheit ? hi : convertFtoC(hi);
}
/*!
* @brief Read value from sensor or return last one from less than two
*seconds.
* @param force
* true if using force mode
* @return float value
*/
bool DHT::read(bool force) {
// Check if sensor was read less than two seconds ago and return early
// to use last reading.
uint32_t currenttime = millis();
if (!force && ((currenttime - _lastreadtime) < MIN_INTERVAL)) {
return _lastresult; // return last correct measurement
}
_lastreadtime = currenttime;
// Reset 40 bits of received data to zero.
data[0] = data[1] = data[2] = data[3] = data[4] = 0;
#if defined(ESP8266)
yield(); // Handle WiFi / reset software watchdog
#endif
// 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.
pinMode(_pin, INPUT_PULLUP);
delay(1);
// First set data line low for a period according to sensor type
pinMode(_pin, OUTPUT);
digitalWrite(_pin, LOW);
switch (_type) {
case DHT22:
case DHT21:
delayMicroseconds(1100); // data sheet says "at least 1ms"
break;
case DHT11:
default:
delay(20); // data sheet says at least 18ms, 20ms just to be safe
break;
}
uint32_t cycles[80];
{
// End the start signal by setting data line high for 40 microseconds.
pinMode(_pin, INPUT_PULLUP);
// Delay a moment to let sensor pull data line low.
delayMicroseconds(pullTime);
// Now start reading the data line to get the value from the DHT sensor.
// Turn off interrupts temporarily because the next sections
// are timing critical and we don't want any interruptions.
InterruptLock lock;
// First expect a low signal for ~80 microseconds followed by a high signal
// for ~80 microseconds again.
if (expectPulse(LOW) == TIMEOUT) {
DEBUG_PRINTLN(F("DHT timeout waiting for start signal low pulse."));
_lastresult = false;
return _lastresult;
}
if (expectPulse(HIGH) == TIMEOUT) {
DEBUG_PRINTLN(F("DHT timeout waiting for start signal high pulse."));
_lastresult = false;
return _lastresult;
}
// 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);
}
} // 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 == TIMEOUT) || (highCycles == TIMEOUT)) {
DEBUG_PRINTLN(F("DHT timeout waiting for pulse."));
_lastresult = false;
return _lastresult;
}
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.
}
DEBUG_PRINTLN(F("Received from DHT:"));
DEBUG_PRINT(data[0], HEX);
DEBUG_PRINT(F(", "));
DEBUG_PRINT(data[1], HEX);
DEBUG_PRINT(F(", "));
DEBUG_PRINT(data[2], HEX);
DEBUG_PRINT(F(", "));
DEBUG_PRINT(data[3], HEX);
DEBUG_PRINT(F(", "));
DEBUG_PRINT(data[4], HEX);
DEBUG_PRINT(F(" =? "));
DEBUG_PRINTLN((data[0] + data[1] + data[2] + data[3]) & 0xFF, HEX);
// Check we read 40 bits and that the checksum matches.
if (data[4] == ((data[0] + data[1] + data[2] + data[3]) & 0xFF)) {
_lastresult = true;
return _lastresult;
} else {
DEBUG_PRINTLN(F("DHT checksum failure!"));
_lastresult = false;
return _lastresult;
}
}
// 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 DHT::expectPulse(bool level) {
// F_CPU is not be known at compile time on platforms such as STM32F103.
// The preprocessor seems to evaluate it to zero in that case.
#if (F_CPU > 16000000L) || (F_CPU == 0L)
uint32_t count = 0;
#else
uint16_t count = 0; // To work fast enough on slower AVR boards
#endif
// 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 TIMEOUT; // 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 TIMEOUT; // Exceeded timeout, fail.
}
}
#endif
return count;
}