SHT31

Arduino library for the SHT31 temperature and humidity sensor.

Relates to the SHT85 library - https://github.com/RobTillaart/SHT85

Description

The SHT3x family of sensors should work up to 1 MHz I2C

This library should also work for SHT30 and SHT35 but these are not tested yet.

Accuracy table

SENSOR	Temperature	Humidity
SHT30	~0.3	2.0
SHT31	~0.3	1.5
SHT35	~0.2	1.5
SHT85	~0.2	1.5

An elaborated library for the SHT31 sensor can be found here

https://github.com/hawesg/SHT31D_Particle_Photon_ClosedCube

A derived class for using the SHT31 sensor with SoftWire (soft I2C) can be found here

https://github.com/RobTillaart/SHT31_SW

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().

Interface

#include "SHT31.h"

Constructor

SHT31(uint8_t address = SHT_DEFAULT_ADDRESS, TwoWire *wire = &Wire) constructor. Optional select address and the I2C bus (Wire, Wire1 etc).
bool begin() Returns false if device address is incorrect or device cannot be reset.
bool isConnected() check sensor is reachable over I2C. Returns false if not connected.
uint8_t getAddress() returns address set in the constructor.

Read

bool read(bool fast = true) blocks 4 (fast) or 15 (slow) milliseconds + actual read + math. Does read both the temperature and humidity.
uint16_t readStatus() details see datasheet and Status fields below.
uint32_t lastRead() in milliSeconds since start of program.
bool reset(bool hard = false) resets the sensor, soft reset by default. Returns false if it fails.
float getHumidity() computes the relative humidity in % based on the latest raw reading, and returns it.
float getTemperature() computes the temperature in °C based on the latest raw reading, and returns it.
float getFahrenheit() computes the temperature in °F based on 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 reading.

Error interface

int getError() returns last set error flag and clear 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.

Error	Symbolic	Description
0x00	SHT31_OK	no error
0x81	SHT31_ERR_WRITECMD	I2C write failed
0x82	SHT31_ERR_READBYTES	I2C read failed
0x83	SHT31_ERR_HEATER_OFF	Could not switch off heater
0x84	SHT31_ERR_NOT_CONNECT	Could not connect
0x85	SHT31_ERR_CRC_TEMP	CRC error in temperature
0x86	SHT31_ERR_CRC_HUM	CRC error in humidity
0x87	SHT31_ERR_CRC_STATUS	CRC error in status field
0x88	SHT31_ERR_HEATER_COOLDOWN	Heater need to cool down
0x88	SHT31_ERR_HEATER_ON	Could not switch on heater

Heater interface

WARNING: Do not use heater for long periods.

Use the heater for max 180 seconds, and let it cool down 180 seconds = 3 minutes. Version 0.3.3 and up 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
bool heatOn() switches heat cycle on if not already on. Returns false if fails, setting error to SHT31_ERR_HEATER_COOLDOWN or to SHT31_ERR_HEATER_ON.
bool heatOff() switches heat cycle off. Returns false if fails, setting error to SHT31_ERR_HEATER_OFF.
bool isHeaterOn() is the sensor still in heating cycle? replaces heatUp(). Will switch the heater off if max heating time has passed.
bool heatUp() will be obsolete in the future. replaced by isHeaterOn()

Async interface

See async example for usage

bool requestData() requests a new measurement. Returns false if this fails.
bool dataReady() checks if enough time has passed to read the data. (15 milliseconds)
bool readData(bool fast = true) fast = true skips the CRC check. Returns false if reading fails or in case of a CRC failure.

Status fields

BIT	Description	value	notes
15	Alert pending status	0	no pending alerts
		1	at least one pending alert - default
14	Reserved	0
13	Heater status	0	Heater OFF - default
		1	Heater ON
12	Reserved	0
11	Humidity tracking alert	0	no alert - default
		1	alert
10	Temperature tracking alert	0	no alert - default
		1	alert
9-5	Reserved	00000
4	System reset detected	0	no reset since last ‘clear status register’ command
		1	reset detected (hard or soft reset command or supply fail) - default
3-2	Reserved	00
1	Command status	0	last command executed successfully
		1	last command not processed. Invalid or failed checksum
0	Write data checksum status	0	checksum of last write correct
		1	checksum of last write transfer failed

Operation

See examples.

Future

Must

keep in sync with SHT85 library.
keep derived SHT31_SW builds green

Should

check TODO in code.
rename MAGIC numbers. e.g. in dataReady()

Could

move code from .h to .cpp

Wont

Support

If you appreciate my libraries, you can support the development and maintenance. Improve the quality of the libraries by providing issues and Pull Requests, or donate through PayPal or GitHub sponsors.

Thank you,

RobTillaart / SHT31