SHT85

Arduino library for the SHT85 temperature and humidity sensor.

Based upon the SHT31 library - https://github.com/RobTillaart/SHT31 however this library will be leading in the future as it implements derived classes for the following sensors: SHT30, SHT31, SHT35 and SHT85.

WARNING to keep self-heating below 0.1°C, the SHT85 sensor should not be used for more than 10% of the time.

Description

Always check datasheet before connecting!

    //  TOPVIEW      SHT85
    //             +-------+
    //  +-----\    | SDA 4 -----
    //  | /-+  ----+ GND 3 -----
    //  | +-+  ----+ +5V 2 -----
    //  +-----/    | SCL 1 -----
    //             +-------+

The SHT85 sensors should work (I2C) up to 1000 KHz. During tests with an Arduino UNO it stopped between 500 - 550 KHz. So to be safe I recommend not to use the sensor above 400 KHz. Also the differences in read time becomes quite small. (max 15% gain).

See indicative output example sketch. SPS (= samples per second) are added later.

I2C speed	read ms	SPS	notes
50 KHz	6.60	123
100 KHz	5.11	140	default
150 KHz	4.79
200 KHz	4.64	140
250 KHz	4.56
300 KHz	4.50	164
350 KHz	4.47
400 KHz	4.45	164
450 KHz	4.43
500 KHz	4.42	163
550 KHz	----		fail

At 10% load the SHT85 can be used to make about 10 - 15 SPS.

Compatibility

The SHT85 is protocol compatible with the SHT3x series. Main difference is the accuracy and the SHT85 only has address 0x44. Compare the data sheets to see all differences.

Accuracy table:

Sensor	Temperature	Humidity	Verified }
SHT30	~0.3°	2.0%	N
SHT31	~0.3°	1.5%	Y
SHT35	~0.2°	1.5%	N
SHT85	~0.2°	1.5%	Y

Note: The SHT40, SHT41 and SHT45 are not protocol compatible with SHT3x and SHT85. The SHT4x series is slightly faster than the SHT3x series.

Multiple SHT85

The SHT3x comes with two I2C address options, 0x44 and 0x45. The SHT85 only has 0x44 as I2C address, so it is not possible to have more than one on a single I2C bus. This means you need to use multiple I2C buses (if your board support this), a software I2C (below) or an I2C multiplexer e.g. https://github.com/RobTillaart/TCA9548

0.6.0 Breaking change

Version 0.6.0 introduced a breaking change. The parameters from begin() moved to the constructor. 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/SHT2x
• https://github.com/RobTillaart/SHT31
• https://github.com/RobTillaart/SHT31_SW = softWire based I2C.
• https://github.com/RobTillaart/tinySHT2x

An elaborated library for the SHT31 sensor can be found here

• https://github.com/hawesg/SHT31D_Particle_Photon_ClosedCube

Dewpoint, heatindex, related functions and conversions.

• https://github.com/RobTillaart/Temperature

I2C multiplexer

• https://github.com/RobTillaart/TCA9548

Interface

#include "SHT85.h"

Constructor

• SHT(uint8_t address, TwoWire *wire = &Wire) constructor of the base class. Note that getType() will return 0.
• SHT30(uint8_t address, TwoWire *wire = &Wire) constructor. Optional select the address and the I2C bus (Wire, Wire1 etc).
• SHT31(uint8_t address, TwoWire *wire = &Wire) constructor. Optional select the address and the I2C bus (Wire, Wire1 etc).
• SHT35(uint8_t address, TwoWire *wire = &Wire) constructor. Optional select the address and the I2C bus (Wire, Wire1 etc).
• SHT85(uint8_t address, TwoWire *wire = &Wire) constructor. Optional select the address and the I2C bus (Wire, Wire1 etc).
• uint8_t getType() returns numeric part of sensor type. Returns 0 for the base class.
• bool begin() Returns false if device address is incorrect or device cannot be reset.
• uint8_t getAddress() returns address set in constructor.

Status

• bool isConnected() checks if address of the sensor is reachable over I2C. Returns false if not connected.
• 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 fails.

Synchronous read

• bool read(bool fast = true) blocks 4 (fast) or 15 (slow) milliseconds + actual read + math. Reads both the temperature and humidity from the device.

Note: the medium level is not supported (yet).

Asynchronous read

See async example for usage.

• bool requestData(bool fast = true) requests a new measurement. Returns false if the request fails. Records the timestamp of the request if successful.
• bool dataReady(bool fast = true) Checks if appropriate time (4 / 15 ms) has past since request to read the data. The parameter fast should be the same as in requestData().
• bool readData(bool fast = true) fast = true skips the CRC check. Returns false if reading the data fails or if CRC check failed.
• uint32_t getLastRequest() returns timestamp of last successful call to requestData. This function is used to check if the request is too long ago.

Temperature and humidity

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().

• float getHumidity() computes the relative humidity in % based on the latest raw reading. This includes the optional offset.
• float getTemperature() computes the temperature in °C based on the latest raw reading. This includes the optional offset.
• float getFahrenheit() computes the temperature in °F based on the latest raw reading, and returns it. Note that the optional offset is set in °Celsius.

The getRawHumidity() and getRawTemperature() can be used to minimize storage or communication as the data type is 50% smaller. Another application is faster comparison with a previous value or threshold. However comparisons are quite fast.

• 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.

The library has no CelsiusToRaw() function although this is relative easy.

rawTemperatureC = (tempC + 45) * (65535 / 175.0); 
rawTemperatureF = (tempF + 49) * (65535 / 315.0); 
rawHumidity     = humidity * 655.35;

Temperature and humidity offset

Default the offset is zero for both temperature and humidity. These functions allows one to adjust them a little. Note there is no limit to the offset so one can use huge values. This allows to use an offset of 273.15 effectively creating °Kelvin instead of Celsius.

Note: the offset is defined in degrees Celsius. To set an offset in degrees Fahrenheit, multiply the Fahrenheit offset with 0.55555556 to get Celsius steps (divide by 1.8 is slower). So an offset of 4 °F becomes 2.2222 °C.

• void setTemperatureOffset(float offset = 0) set the offset in °C, default is zero removing the offset. The library converts this internally
• float getTemperatureOffset() returns the set offset in °C.
• void setHumidityOffset(float offset = 0) set the offset, default is zero removing the offset.
• float getHumidityOffset() returns the set offset.

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	SHT_OK	no error
0x81	SHT_ERR_WRITECMD	I2C write failed
0x82	SHT_ERR_READBYTES	I2C read failed
0x83	SHT_ERR_HEATER_OFF	Could not switch off heater
0x84	SHT_ERR_NOT_CONNECT	Could not connect
0x85	SHT_ERR_CRC_TEMP	CRC error in temperature
0x86	SHT_ERR_CRC_HUM	CRC error in humidity
0x87	SHT_ERR_CRC_STATUS	CRC error in status field
0x88	SHT_ERR_HEATER_COOLDOWN	Heater need to cool down
0x89	SHT_ERR_HEATER_ON	Could not switch on heater
0x8A	SHT_ERR_SERIAL	Could not read serial number

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 explicitly 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 the heat cycle on if not already on. Returns false if this fails, setting error to SHT_ERR_HEATER_COOLDOWN or to SHT_ERR_HEATER_ON.
• bool heatOff() switches the heat cycle off. Returns false if fails, setting error to SHT_ERR_HEATER_OFF.
• bool isHeaterOn() is the sensor still in a heating cycle? Replaces heatUp(). Will switch the heater off if maximum heating time has passed.

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	reserved
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

SHT85 specific

• uint32_t GetSerialNumber() Returns a 32 bit unique serial number. This command seems to be timing sensitive, it uses a delay of 500us which may need "tuning". So function is bit experimental.

Future

Must

• improve documentation.
  • reorder interface

Should

• more testing (including heater)
• verify working with ESP32
• support for medium level read.
  • 3 levels instead of 2.

Could

• remember the fast parameter of requestData() so it can become obsolete for radaReady().
• investigate command ART (auto sampling at 4 Hz)
• investigate command BREAK (stop auto sampling)
• test SHT30/35

Won't

• rename the library? to SHT ? or sensirion.h ?
  • not on short term
• create a SHT85 simulator
  • I2C slave sketch with e.g. a DHT22 sensor/
  • not within this library.
• software I2C experiments
  • see https://github.com/RobTillaart/SHT31_SW
• merge with other SHT sensors if possible?
  • derived classes fixes this enough.
• getKelvin() wrapper? (no => check temperature class) ==> set Offset to 273.15 !

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