Arduino library for the SHT2x, HTU2x and Si70xx temperature and humidity sensors.
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.
- https://github.com/RobTillaart/SHT31
- https://github.com/RobTillaart/SHT85
- https://github.com/RobTillaart/tinySHT2x
#include "SHT2x.h"
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.
- bool begin(int dataPin, int clockPin) begin function for ESP8266 & ESP32; returns false if device address is incorrect or device cannot be reset.
- bool begin() calls reset() which can take up to 15 ms.
- 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.
- 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.
Experimental since 0.2.2 this interface can change in the future Discussion see RobTillaart#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.
- 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.
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?
-
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.
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 |
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. ( RobTillaart#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.
- bool batteryOK() returns true if VCC > 2.5 Volts.
- improve documentation
- reorganize interface
- async documentation
- clean up code.
- add crc8 check (need sensor to test)
- improve error handling (all code paths)
- investigate blocking delay() in read
- add offset for temperature and humidity
- fix reset(): clear state of resolution, heater and error
- add readCachedTemperature()
- test test test
- get hardware
- add examples
- test resolutions
- performance different resolutions
- test battery
- fix TODO in code (.cpp and .h) and documentation
- update unit tests
- add type info in derived classes?
- add getSerialNumber() getEIDA() and getEIDB() covers this
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