osagie-ogbomo/dht20

Arduino library for DHT20 I2C temperature and humidity sensor.

DHT20

Arduino library for I2C DHT20 temperature and humidity sensor.

Description

The DHT20 is a humidity an temperature sensor.

The sensor has a fixed address of 0x38. It is not known if the address can be changed.

The library must be initiated by calling the begin() function, or begin(dataPin, clockPin) for ESP32 and similar platforms.

Thereafter one has to call the read() function to do the actual reading, and call getTemperature() and getHumidity() to get the measured values. Calling these latter again will return the same values until a new read() is done.

The read() call of this sensor is blocking for 80+ milliseconds (datasheet 7.4) so the library also has a asynchronous interface. See below.

Since 0.1.3 and 0.1.4 the performance of read() has been optimized, still blocking but less long for about 45 milliseconds.

0.2.0

In #8 a bug is described that the sensor "freezes". Cause is not well understood.

Two solutions / workarounds are found:

• call resetSensor() before EVERY read(). This is the preferred solution.
• use Wire.setClock(200000) 100 K and lower speeds freezes the sensor. With clock set to 200 K and above the sensor seems to work for longer periods. Tested several speeds on UNO, no pull ups, < 10 cm wire.

Note: setting the I2C clock possibly interferes with other devices on the I2C bus, so it is not a solution in the end.

The 0.2.0 version embeds the resetSensor() into requestData() to reset the sensor if needed in both synchronous and asynchronous calls. This keeps the API simple. The reads are 1-2 ms slower than 0.1.4. (< 50 ms). Still far below the 80 ms mentioned in the datasheet.

Connection

Always check datasheet!

Front view

          +--------------+
  VDD ----| 1            |
  SDA ----| 2    DHT20   |
  GND ----| 3            |
  SCL ----| 4            |
          +--------------+

Tested

Verified to work with Arduino UNO and ESP32 and ESP8266 (see #8) Please let me know if other platforms work (or not).

Interface

Constructor

• DHT20(TwoWire *wire = &Wire) constructor, using a specific Wire (I2C bus).
• bool begin(uint8_t dataPin, uint8_t clockPin) begin for ESP32 et al, to set I2C bus pins.
• bool begin() initializer for non ESP32. Returns true if connected.
• bool isConnected() returns true if the address of the DHT20 can be seen on the I2C bus.

Core

• int8_t read() read the sensor and store the values internally. Returns the status of the read which should be 0 == DHT20_OK.
• float getHumidity() returns last Humidity read. Multiple calls will return same value until a new read() is made.
• float getTemperature() returns last Temperature read. Multiple calls will return same value until a new read() is made.

Offset

• void setHumOffset(float offset) set an offset to calibrate the sensor (1st order).
• float getHumOffset() return current humidity offset, default 0.
• void setTempOffset(float offset) set an offset to calibrate the sensor (1st order).
• float getTempOffset() return current temperature offset, default 0.

Asynchronous interface

There are two timings that need to be considered (from datasheet):

• time between requests = 1000 ms.
• time between request and data ready = 80 ms.

The async interface allows one to continue processing after a requestData() has been made. Note there should be at least 1000 milliseconds between subsequent requests.

With bool isMeasuring() one can check if a new measurement is ready. Alternative is to delay for up to 80 ms. If so the sensor can be read with readData().

To interpret the read bits to temperature, humidity and status one needs to call convert() as last step.

• int requestData() signals the sensor to make a new measurement. Note there must be at least 1000 milliseconds between requests!
• int readData() does the actual reading of the data.
• int convert() converts the read bits to temperature and humidity.

See the example DHT20_async.ino

Status

• uint8_t readStatus() forced read of the status only. This function blocks a few milliseconds to optimize communication.
• bool isCalibrated() idem, wrapper around readStatus()
• bool isMeasuring() idem, wrapper around readStatus()
• bool isIdle() idem, wrapper around readStatus()
• int internalStatus() returns the internal status of the sensor. (for debug).

status bit	meaning
7	1 = measurement, 0 = idle
6 - 4	unknown
3	1 = calibrated, 0 = not
2 - 0	unknown

Experimental 0.1.4 resetSensor

Use with care!

• uint8_t resetSensor() if at startup the sensor does not return a status of 0x18, three registers 0x1B, 0x1C and 0x1E need to be reset. See datasheet 7.4 Sensor Reading Process, point 1. There is no documentation about the meaning of these registers. The code is based upon example code for the AHT20 (from manufacturer).

The call is needed to get the read() working well so it has been embedded into the read calls. (0.2.0)

Timing

• uint32_t lastRead() last time the sensor is read in milliseconds since start.
• uint32_t lastRequest() last time a request is made to make a measurement.

Return codes

name	value	notes
DHT20_OK	00	OK
DHT20_ERROR_CHECKSUM	-10	values might be OK if they are like recent previous ones.
DHT20_ERROR_CONNECT	-11	check connection
DHT20_MISSING_BYTES	-12	check connection
DHT20_ERROR_BYTES_ALL_ZERO	-13	check connection
DHT20_ERROR_READ_TIMEOUT	-14
DHT20_ERROR_LASTREAD	-15	wait 1 second between reads

Operation

See examples

Future

must

• improve documentation.
• investigate the bug from #8 further

should

could

• improve unit tests.
• investigate
  • sensor calibration (website aosong?)
• investigate optimizing timing in readStatus()
  • delay(1) ==> microSeconds(???).
• connected flag?
• add uint8_t getAddress() to return the address (convenience).

won't

• void setIgnoreChecksum(bool = false) ignore checksum flag speeds up communication a bit
• bool getIgnoreChecksum() get checksum flag. for completeness.