SHT2x

Arduino library for the SHT2x, HTU2x and Si70xx temperature and humidity sensors.

Description

This library is not tested extensively yet. It works for the Si7021 incl. the asynchronous interface. It should work for SHT20, SHT21 and SHT25 but these are not tested yet. The SHT2x family of sensors should work up to 400 KHz I2C.

Furthermore there are several other sensors that should be compatible (see table below), but have not been tested either.

Accuracy table

Sensor	Temperature	Humidity	Notes
SHT20	~0.3	±3.0
SHT21	~0.3	±3.0
SHT25	~0.3	±1.8
HTU20			to-do
HTU21			to-do
Si7013			to-do
Si7020			to-do
Si7021	~0.3	~2.0
GY21			to-do

All sensors in this family of sensors have address 0x40 (64 decimal). If you want to use more on one I2C bus one needs either an I2C multiplexer or one should switch sensors on/off like the select in SPI communication.

0.5.0 Breaking change

Version 0.5.0 introduced a breaking change. You cannot set the pins in begin() any more. This reduces the dependency of processor dependent Wire implementations. The user has to call Wire.begin() and can optionally set the Wire pins before calling begin().

Related

• https://github.com/RobTillaart/SHT31
• https://github.com/RobTillaart/SHT85
• https://github.com/RobTillaart/tinySHT2x

Interface

#include "SHT2x.h"

Constructors

All classes below are derived from SHT2x class.

• SHT2x(TwoWire *wire = &Wire) constructor base class. Optional set the wire interface for platforms with multiple I2C buses.
• SHT20(TwoWire *wire = &Wire) constructor.
• SHT21(TwoWire *wire = &Wire) constructor.
• SHT25(TwoWire *wire = &Wire) constructor.
• HTU20(TwoWire *wire = &Wire) constructor.
• HTU21(TwoWire *wire = &Wire) constructor.
• Si7013(TwoWire *wire = &Wire) constructor.
• Si7020(TwoWire *wire = &Wire) constructor.
• Si7021(TwoWire *wire = &Wire) constructor.

Base interface

• bool begin() calls reset() which can take up to 15 ms. Returns false if device address is nor reachable or device cannot be reset.
• bool read() Reads both the temperature and humidity.
  Initial release has a blocking delay.
• bool isConnected() check if sensor is reachable over I2C. Returns false if not connected.
• uint16_t getStatus() returns a 2 bit status. See Status fields below.
• uint32_t lastRead() in milliSeconds since start of program.
• bool reset() resets the sensor, soft reset, no hard reset supported.

Temperature and humidity

• float getHumidity() computes the relative humidity in % based off the latest raw reading, and returns it.
• float getTemperature() computes the temperature in °C based off the latest raw reading, and returns it.
• uint16_t getRawHumidity() returns the raw two-byte representation of humidity directly from the sensor.
• uint16_t getRawTemperature() returns the raw two-byte representation of temperature directly from the sensor.

Note that the temperature and humidity values are recalculated on every call to getHumidity() and getTemperature(). If you're worried about the extra cycles, you should make sure to cache these values or only request them after you've performed a new read().

Note: The raw temperature and raw humidity are ideal to minimize storage or to minimize communication.

The Si70xx sensors also support reading the temperature that was established with the last measurement and that includes the humidity measurement. It can be used after a call to getHumidity() (base interface) or requestHumidity() (asynchronous interface) without requiring a separate acquisition. It can also be used in combination with a temperature measurement and will return the same value as getTemperature(). Several calling sequences have be tested and the results can be found in a comment of pull request #27.

• bool readCachedTemperature() calculates the humidity from raw measurement.

Asynchronous interface

Experimental since 0.2.2 this interface can change in the future Discussion see #16

• bool requestTemperature() starts new temperature request. Overrules optional existing / running request.
• bool requestHumidity() starts new humidity request. Overrules optional existing / running request.
• bool reqTempReady() returns true if temperature request is ready.
• bool reqHumReady() returns true if humidity request is ready.
• bool requestReady() checks if either temperature or humidity is ready.
• bool readTemperature() calculates the temperature from raw measurement.
• bool readHumidity() calculates the humidity from raw measurement.
• uint32_t lastRequest() timestamp of last request.
• uint8_t getRequestType() get current request type.

Value	Symbolic	Description	Notes
0x00	SHT2x_REQ_NONE	no request pending
0x01	SHT2x_REQ_TEMPERATURE	temperature request pending
0x02	SHT2x_REQ_HUMIDITY	humidity request pending

TODO elaborate documentation.

Error interface

• int getError() returns last set error flag and clears it. Be sure to clear the error flag by calling getError() before calling any command as the error flag could be from a previous command.

Value	Symbolic	Description	Notes
0x00	SHT2x_OK	no error
0x81	SHT2x_ERR_WRITECMD	I2C write failed
0x82	SHT2x_ERR_READBYTES	I2C read failed
0x83	SHT2x_ERR_HEATER_OFF	Could not switch off heater
0x84	SHT2x_ERR_NOT_CONNECT	Could not connect
0x85	SHT2x_ERR_CRC_TEMP	CRC error in temperature
0x86	SHT2x_ERR_CRC_HUM	CRC error in humidity
0x87	SHT2x_ERR_CRC_STATUS	CRC error in status field	not used
0x88	SHT2x_ERR_HEATER_COOLDOWN	Heater need to cool down
0x88	SHT2x_ERR_HEATER_ON	Could not switch on heater

Note: the HTU20 / HTU21 classes share the same error codes.

Heater interface

WARNING: Do not use heater for long periods.
Datasheet SHT2x does not mention max time so the maximum time of the SHT3x series is assumed here.

Use the heater for max 180 seconds, and let it cool down 180 seconds = 3 minutes. THe library guards the cool down time by preventing switching the heater on within 180 seconds of the last switch off. Note: this guarding is not reboot persistent.

WARNING: The user is responsible to switch the heater off manually!

The class does NOT do this automatically.
Switch off the heater by directly calling heatOff() or indirectly by calling isHeaterOn().

• void setHeatTimeout(uint8_t seconds) Set the time out of the heat cycle. This value is truncated to max 180 seconds.
• uint8_t getHeatTimeout() returns the value set.
• bool heatOn() switches heat cycle on if not already on. Returns false if fails, setting error to SHT2x_ERR_HEATER_COOLDOWN or to SHT2x_ERR_HEATER_ON.
• bool heatOff() switches heat cycle off. Returns false if fails, setting error to SHT2x_ERR_HEATER_OFF.
• bool isHeaterOn() is the sensor still in heating cycle?

Electronic ID

• needs more testing

• only tested with a Si7021 sensor

• uint32_t getEIDA() returns unique ID part A.

• uint32_t getEIDB() returns unique ID part B.

• uint8_t getFirmwareVersion() returns firmware version.

Status fields

From HTU20 datasheet (read for details).

bits	value	Symbolic	Description
00	0	SHT2x_STATUS_OPEN_CIRCUIT	open circuit
01	1	SHT2x_STATUS_TEMPERATURE	temperature reading
10	2	SHT2x_STATUS_HUMIDITY	humidity reading
11	3	SHT2x_STATUS_CLOSED_CIRCUIT	closed circuit

Resolution

Warning experimental

• needs more testing as the results are not in line with the datasheet.

• only tested on a HTUxx and a Si7021 sensor.

• tested with SHT2X_resolution.ino

• void setResolution(uint8_t res) res = 0..3, other values return false.

• uint8_t getResolution() returns resolution set 0..3.

Datasheet SHT20 Table 8: resolution

RES	Humidity	Temperature
0	12 bit	14 bit
1	08 bit	12 bit
2	10 bit	13 bit
3	11 bit	11 bit

Datasheet SHT20 Table 7: timing versus results of real measurements. ( #11 )

RES	HUM	TEMP	TOTAL	REAL
0	29	85	114	116
1	4	22	26	113
2	9	43	52	084
3	15	11	26	102

Timing in milliseconds.

Battery

• bool batteryOK() returns true if VCC > 2.5 Volts.

Future

Must

• improve documentation
  • reorganize interface
  • async documentation
• clean up code.

Should

• test test test
  • get hardware
• add examples
  • test resolutions
  • performance different resolutions
  • test battery

Could

• fix TODO in code (.cpp and .h) and documentation
• update unit tests
• add type info in derived classes?

Wont

• add getSerialNumber() getEIDA() and getEIDB() covers this

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Thank you,