The SHT30 command codes

The SHt30 uses 16 bit command codes with a 3 bit CRC (cyclic redundancy check) calculated over the upper 13 bit added on the lowest significant 3 bits.
Let us have a look at the "one time measurement" commands in order to understand the command layout. Here is the table from the data sheet:

If we analyze the codes we get:

As you can see, knowing the repeatability and the clock stretching parameter we can easily construct the command code: Starting from a command code 0x2400 we set bit 11 if clock stretching is enabled and we "or" 01 into bits 3 and 4 for medium and 10 for low repeatability to get the command code. This code we use as a key into a dictionary from where we can pick up the 3 bit checksum, which we or to bits 0..2 .

sht30.readAlert(high=True,set=True,raw=False): Reads the alert limits. Returns temperature and humidity limits for alerts in °C and % or the raw data read out if raw == True.

sht30.writeAlert(self,tempC,humi,high=True,set=True): Write the alert limits

Here you find the driver itself:

https://afnog.iotworkshop.africa/pub/IoT_Course_English/SHT30NopI2CTemperatureAndHumiditySensor/sht3x.py.txt

and here some test code exercising the driver:

https://afnog.iotworkshop.africa/pub/IoT_Course_English/SHT30NopI2CTemperatureAndHumiditySensor/sht3xTest.py.txt

And here is the result printed out by the test program:

>>> %Run -c $EDITOR_CONTENT
Serial number: 0x317d411
Measure with clock stretching, the delay between writing the cmd and reading back the result
is calculated from the repeatabilty parameter (see data sheet)
Temperature: 24.26994 °C, Humidity: 61.74011 %
Measure without clock stretching
After sending the command the SHT30 is polled for the result every 1ms
Generates a timeout exception if the SHT30 does not respond in time
Temperature: 24.28329 °C, Humidity: 61.75385 %
Read and print the alert settings: High limit to set the alert
The data sheet says that this should be 60°C and 80%
Temperature limit high: 59.93164 °C Humidity limit high: 79.6875 %
Set accelerated response time
Print the current status register contents
--------------------------------------------------
Status register content: 0x2060
--------------------------------------------------
No alert pending
Heater is enabled
Last command succeeded
Checksum of last write command was correct
Clear the status register, switch on the heater and print the values again
Switch on the heater
--------------------------------------------------
Status register content: 0x20a0
--------------------------------------------------
No alert pending
Heater is enabled
Last command succeeded
Checksum of last write command was correct
Start periodic measurement
We pass the function printContinuousValues as callback routine, printing the temperature and humidity values
read out in each measurement cycle
Period measurement will take 20.0 s
Interval: 2.0
Temperature: 24.21387 °C, Humidity: 61.6684 %
Temperature: 24.26994 °C, Humidity: 61.78131 %
Temperature: 24.28329 °C, Humidity: 61.79504 %
Temperature: 24.24057 °C, Humidity: 61.80878 %
Temperature: 24.25392 °C, Humidity: 61.83167 %
Temperature: 24.24057 °C, Humidity: 61.82861 %
Temperature: 24.22722 °C, Humidity: 61.85608 %
Temperature: 24.24057 °C, Humidity: 61.91101 %
Temperature: 24.22722 °C, Humidity: 61.92932 %
Temperature: 24.21387 °C, Humidity: 61.90643 %
--------------------------------------------------
Status register content: 0x2000
--------------------------------------------------
No alert pending
Heater is enabled
Last command succeeded
Checksum of last write command was correct
>>>

The SHT30 I2C Temperature and Humidity Sensor

Methods available in the SHT3X class

Here is a list of the available methods of the SHT3Xclass:

__init__(scl_pin=machine.Pin(22),sda_pin=machine.Pin(21),i2c,address=0x45): creates an instance of the SHT3X object. The default values for scl_pin, sda_pin and i2c_address should be ok for the ESP32 and our version of the SHT30 shield. The method raises an SHT3XError.BUS_ERROR if the sht30 chip is not found on the I2C bus

from sht3x import SHT3X

sht30 = SHT3X()

should therefore work and create the SHT3X object.

sht30.isPresent(): returns True if the sht30 is chip is found on the I2C bus, False otherwise

sht30.serialNumber(): Returns the 32 bit serial number of the sht30 chip

sht30.readStatus(): Returns the status bits (16 bits) from the status register

sht30.printStatus(): Prints the status bits from the sht30 status register in a humanly readable form

sht30.clearStatus(): Clears the status register

sht30.softReset(): Resets the sht30

sht30.enableHeater(): Switches the heater on

sht30.disableHeater(): Switches the heater off

sht30.getTempAndHumi(clockStretching=!SHT3X.CLOCK_STRETCH, repeatability=!SHT3X.REP_S_LOW, raw=False, timeout=100)

get temperature and humidity in the selected mode. If clock stretching as been selected the corresponding command 0x2cxx is issued followed by a wait until the measurement has been completed. The wait time used is the maximum measurement time for that mode found in table 4 chapter 2.2 of the data sheet.

If no clock stretching is selected the method will poll the sht30 every ms for the result data. It within "timeout ms" the sht30 does not return the result an SHT3X.TIMEOUT error is raised.

After the wait, the data are read out and the checksum checked. If the checksum received is wrong an SHT3X.CRC_ERROR is raised.

If raw mode has been selected the 6 byte result (16 bits for temperature, 16 bits humidity and 2*8 bits checksums) is returned. Otherwise the raw values are converted to physical temperature values (in °C) and relative humidity (in %) which are returned to the caller.

sht30.Celsius2Fahrenheit(tempC): takes a temperature value in °C and converts it to °F

sht30.startPeriodicMeas(mps=0.5,repeatability=self.REP_P_HIGH): starts repetitive measurements in the selected mode. In "continuous mode" clock stretching is not done. Here I wait for the period selected before the data are read out.

sht30.stopPeriodicMeas(): Stops "continuous measurement mode"

The SHT30 I2C Temperature an Humidity Sensor

Introduction

The SHT30 is a temperature and humidity sensor for the I2C bus. Typical accuracy for temperature is +- 0.1 °C and +- 1.5% relative humidity. Sensirion, the company behind the SHT30 supplies a series of documents including the data sheet, several application notes and sample code, written in C STM32F10x microcontroller. You can find it at https://www.sensirion.com/en/download-center/humidity-sensors/digital-humidity-sensors-for-various-applications.

There is also a SHT30 driver written for MicroPython on the ESP8266 but unfortunately it did not work on the ESP32 out of the box and it implements only a small subset of the SHT30's functions. I therefore developed a driver called sht3x translating the Sensirion code into MicroPython.

Here is a list of functions, their command codes and if they are implemented in the SHT30 driver:

-- Uli Raich - 2020-05-22

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