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# SHT85
Arduino library for the SHT85 temperature and humidity sensor
Based upon the SHT31 library - https://github.com/RobTillaart/SHT31
**Warning:** to keep self-heating below 0.1<EFBFBD> C, the SHT85 sensor should
not be used for more than 10% of the time.
## Description
```
// TOPVIEW
// +-------+
// +-----\ | SDA 4 -----
// | /-+ ----+ VCC 3 -----
// | +-+ ----+ GND 2 -----
// +-----/ | SCL 1 -----
// +-------+
```
The SHT85 sensors should work up to 1000 KHz, however during tests
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with an Arduino UNO it stopped between 500 - 550 KHz so to be safe I recommend
not to use it above 400 KHz. Also the differences in read time becomes
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quite small. (max 15% gain). See output example sketch.
| I2C speed | read ms | notes |
|:---------:|:-------:|:------|
| 100 KHz | 5.11 | |
| 150 KHz | 4.79 | |
| 200 KHz | 4.64 | |
| 250 KHz | 4.56 | |
| 300 KHz | 4.50 | |
| 350 KHz | 4.47 | |
| 400 KHz | 4.45 | |
| 450 KHz | 4.43 | |
| 500 KHz | 4.42 | |
| 550 KHz | ---- | fail |
### Compatibility
This library should also work for SHT30, SHT31 and SHT35 but
this is not verified yet.
| SENSOR | Temperature accuracy | Humidity accuracy |
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|:------:|:------:|:-----:|
| SHT30 | ~0.3 | 2.0 |
| SHT31 | ~0.3 | 1.5 |
| SHT35 | ~0.2 | 1.5 |
| SHT85 | ~0.2 | 1.5 |
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Need to investigate if the interface is identical?
If so the libraries might be merged.
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An elaborated library for the SHT31 sensor can be found here
https://github.com/hawesg/SHT31D_Particle_Photon_ClosedCube
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## Interface
#### Base interface
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- **SHT85()** constructor.
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- **begin(address, dataPin, clockPin)** begin function for ESP8266 & ESP32; **WARNING: not verified yet**
returns false if device address is incorrect or device cannot be reset.
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- **bool begin(address, TwoWire \*wire = & Wire)** for platforms with multiple I2C busses.
- **bool read(bool fast = true)** blocks 4 (fast) or 15 (slow) milliseconds + actual read + math.
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Does read both the temperature and humidity.
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- **bool isConnected()** check sensor is reachable over I2C. Returns false if not connected.
- **uint16_t readStatus()** details see datasheet and **Status fields** below.
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- **uint32_t lastRead()** in milliSeconds since start of program.
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- **bool reset(bool hard = false)** resets the sensor, soft reset by default. Returns false if fails.
- **float getHumidity()** computes the relative humidity in % based off the latest raw reading, and returns it.
- **float getTemperature()** computes the temperature in <20> 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.
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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.
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#### Error interface
- **getError()** returns last set error flag and clear it.
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Be sure to clear the error flag by calling **getError()** before calling any command as the error flag could be from a previous command.
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| 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 statusfield |
| 0x88 | SHT_ERR_HEATER_COOLDOWN | Heater need to cool down |
| 0x88 | SHT_ERR_HEATER_ON | Could not switch on heater |
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#### Heater interface
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**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.
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**WARNING:** The user is responsible to switch the heater off manually!
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The class does **NOT** do this automatically.
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Switch off the heater by directly calling **heatOff()** or indirectly by calling **isHeaterOn()** .
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- **void setHeatTimeout(uint8_t seconds)** Set the time out of the heat cycle.
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This value is truncated to max 180 seconds.
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- **uint8_t getHeatTimeout
- **bool heatOn()** switches heat cycle on if not already on.
Returns false if fails, setting error to **SHT_ERR_HEATER_COOLDOWN**
or to **SHT_ERR_HEATER_ON** .
- **bool heatOff()** switches heat cycle off.
Returns false if fails, setting error to **SHT_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.
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#### Async interface
See async example for usage
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- **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.
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## Status fields
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| 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 <20> clear status register<65> 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
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## Future
- verify working with ESP32
- merge with other SHT sensors if possible
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- SHT_BASE class ?
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- investigate command ART (auto sampling at 4 Hz)
- investigate command BREAK (stop auto sampling)
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- direct Fahrenheit formula ?
- improve error handling / status. (all code paths)
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## Operation
See examples
