The SHT30 I2C Temperature and Humidity Sensor

Introduction

The SHT30 is a temperature and humidity sensor that communicates over 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 for the STM32F10x micro-controller. You can find them at
https://www.sensirion.com/products/catalog/SHT30-DIS-B

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. Making it work for the ESP32 is not too difficult however. This is the subject of an exercise on the SHT30 ( The I2C bus and the SHT30 Temperature and Humidity Sensor ). Since the SHT30 driver only implements a small subset of the SHT30 functionality, I developed a driver called SHT3X translating the Sensirion code into MicroPython and taking over some of the SHT30 code.

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

Function	Repeatability	measurement per s	Clock Stretching	Command Code	Implemented in SHT30 driver
Single shot data acquisition	High		enabled	0x2C06	no
	Medium			0x2C0D	no
	Low			0x2C10	no
	High		disabled	0x2400	yes
	Medium			0x240B	no
	Low			0x2416	no
Continuous data acquisition	High	0.5	disabled	0x2032	no
	Medium			0x2024	no
	Low			0x202F	no
	High	1		0x2130	no
	Medium			0x2120	no
	Low			0x212D	no
	High	2		0x2236	no
	Medium			0x2220	no
	Low			0x222B	no
	High	4		0x2334	no
	Medium			0x2322	no
	Low			0x2329	no
	High	10		0x2737	no
	Medium			0x2722	no
	Low			0x272A	no
Read serial number				0x3780	no
Fetch Data				0xE000	no
Accelerated Response Time				0x2B32	no
Break continuous acquisition				0x3093	no
Soft Reset				0x30A2	yes
Heater enable				0x306D	yes
Heater disable				0x3066	yes
Read out status register				0xF32D	yes
Clear status register				0x3041	yes
Read high alert limit	set			0xE11F	no
	clear			0xE114	no
Read low alert limit	set			0xE109	no
	clear			0xE102	no
Write high alert limit	set			0x611D	no
	clear			0x6116	no
Write low alert limit	set			0x610B	no
	clear			0x6100	no

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:

Repeatability	Clock Stretching	Command code	bit for clock stretching	rest of command code	bits for repeatability	CRC
High	enabled	0010	1	100	0000 0	110
Medium		0010	1	100	0000 1	101
Low		0010	1	100	0001 0	000
High	disabled	0010	0	100	0000 0	000
Medium		0010	0	100	0000 1	011
Low		0010	0	100	0001 0	110

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 .

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)

gets 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. If 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"

sht30.measPeriodic(self,mps=0.5, repeatability=REP_P_HIGH, raw=False, noOfMeas=50, callback=None): Starts continuous measurements using sht30.startPeriodicMeas(), reads out the data after each measurement and stops continuous measurements using stopPeriodicMeas() as soon as "noOfMeas" measurements have been reached. You can specify a callback function which is called with [tempC,humi], the temperature in °C and the humidity value in % as parameter if raw is False or with the raw data [tempRaw,humiRaw,checksum] as parameter if raw is True.

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
>>>

-- Uli Raich - 2020-05-22

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