Si7021 I2C Humidity and Temperature Sensor example in Micropython

In this example we connect a Si7021 I2C Humidity and Temperature Sensor to an ESP32 running Micropython

Parts List

I actually used a GY-21P which is a combined BMP280 and Si7021 sensor in one module. We only deal with the Si7021 module here

Connection

An easy module to connect to an ESP32, SCL is 22 and SDA is 21 on the Wemos board I used, you can see this is the schematic below

Code

You can use any method to upload files or an IDE for development.

The following is based on a github library – . The first part of this is the library which I upload to my ESP32

This is the library called si7021.py

[codesyntax lang=”python”]

'''This module implements a driver for the Si7021 humidity and temperature
sensor.

Datasheet:
https://www.silabs.com/Support%20Documents%2FTechnicalDocs%2FSi7021-A20.pdf

'''

from time import sleep

class CRCError(Exception):
    'Data failed a CRC check.'
    pass


class Si7021(object):
    'Driver for the Si7021 temperature sensor.'
    SI7021_DEFAULT_ADDRESS = 0x40
    SI7021_MEASTEMP_NOHOLD_CMD = bytearray([0xF3])
    SI7021_MEASRH_NOHOLD_CMD = bytearray([0xF5])
    SI7021_RESET_CMD = bytearray([0xFE])
    SI7021_ID1_CMD = bytearray([0xFA, 0x0F])
    SI7021_ID2_CMD = bytearray([0xFC, 0xC9])
    I2C_WAIT_TIME = 0.025


    def __init__(self, i2c, address=SI7021_DEFAULT_ADDRESS):
        'Initialize an Si7021 sensor object.'
        self.i2c = i2c
        self.address = address
        self.serial, self.identifier = self._get_device_info()


    @property
    def temperature(self):
        'Return the temperature in Celcius.'
        temperature = self._get_data(self.SI7021_MEASTEMP_NOHOLD_CMD)
        celcius = temperature * 175.72 / 65536 - 46.85
        return celcius


    @temperature.setter
    def temperature(self, value):
        raise AttributeError('can\'t set attribute')


    @property
    def relative_humidity(self):
        'Return the relative humidity as a percentage. i.e. 35.59927'
        relative_humidity = self._get_data(self.SI7021_MEASRH_NOHOLD_CMD)
        relative_humidity = relative_humidity * 125 / 65536 - 6
        return relative_humidity


    @relative_humidity.setter
    def relative_humidity(self, value):
        raise AttributeError('can\'t set attribute')


    def reset(self):
        'Reset the sensor.'
        self.i2c.writeto(self.address, self.SI7021_RESET_CMD)
        sleep(self.I2C_WAIT_TIME)


    def _get_data(self, command):
        'Retrieve data from the sensor and verify it with a CRC check.'
        data = bytearray(3)
        self.i2c.writeto(self.address, command)
        sleep(self.I2C_WAIT_TIME)

        self.i2c.readfrom_into(self.address, data)
        value = self._convert_to_integer(data[:2])

        verified = self._verify_checksum(data)
        if not verified:
            raise CRCError('Data read off i2c bus failed CRC check.',
                           data[:2],
                           data[-1])
        return value


    def _get_device_info(self):
        '''Get the serial number and the sensor identifier. The identifier is
        part of the bytes returned for the serial number.

        '''
        # Serial 1st half
        self.i2c.writeto(self.address, self.SI7021_ID1_CMD)
        id1 = bytearray(8)
        sleep(self.I2C_WAIT_TIME)
        self.i2c.readfrom_into(self.address, id1)

        # Serial 2nd half
        self.i2c.writeto(self.address, self.SI7021_ID2_CMD)
        id2 = bytearray(6)
        sleep(self.I2C_WAIT_TIME)
        self.i2c.readfrom_into(self.address, id2)

        combined_id = bytearray([id1[0], id1[2], id1[4], id1[6],
                                 id2[0], id2[1], id2[3], id2[4]])

        serial = self._convert_to_integer(combined_id)
        identifier = self._get_device_identifier(id2[0])

        return serial, identifier

    def _convert_to_integer(self, bytes_to_convert):
        'Use bitwise operators to convert the bytes into integers.'
        integer = None
        for chunk in bytes_to_convert:
            if not integer:
                integer = chunk
            else:
                integer = integer << 8
                integer = integer | chunk
        return integer


    def _get_device_identifier(self, identifier_byte):
        '''Convert the identifier byte to a device identifier. Values are based
        on the information from page 24 of the datasheet.

        '''
        if identifier_byte == 0x00 or identifier_byte == 0xFF:
            return 'engineering sample'
        elif identifier_byte == 0x0D:
            return 'Si7013'
        elif identifier_byte == 0x14:
            return 'Si7020'
        elif identifier_byte == 0x15:
            return 'Si7021'
        else:
            return 'unknown'


    def _verify_checksum(self, data):
        ''''Verify the checksum using the polynomial from page 19 of the
        datasheet.

        x8 + x5 + x4 + 1 = 0x131 = 0b100110001

        Valid Example:
        byte1: 0x67 [01100111]
        byte2: 0x8c [10001100]
        byte3: 0xfc [11111100] (CRC byte)

        '''
        crc = 0
        values = data[:2]
        checksum = int(data[-1])
        for value in values:
            crc = crc ^ value
            for _ in range(8, 0, -1):
                if crc & 0x80: #10000000
                    crc <<= 1
                    crc ^= 0x131 #100110001
                else:
                    crc <<= 1
        if crc != checksum:
            return False
        else:
            return True

def convert_celcius_to_fahrenheit(celcius):
    'Convert a Celcius measurement into a Fahrenheit measurement.'
    return celcius * 1.8 + 32

[/codesyntax]

Now you upload this library and then you can create or modify the main.py with this example code

[codesyntax lang=”python”]

import si7021
import machine
i2c = machine.I2C(sda=machine.Pin(21),scl=machine.Pin(22))

temp_sensor = si7021.Si7021(i2c)

print('Serial:              {value}'.format(value=temp_sensor.serial))
print('Identifier:          {value}'.format(value=temp_sensor.identifier))
print('Temperature:         {value}'.format(value=temp_sensor.temperature))
print('Relative Humidity:   {value}'.format(value=temp_sensor.relative_humidity))

[/codesyntax]

Testing

Open up the REPL window. Here is what I saw in uPyCraft

Ready to download this file,please wait!
….
download ok
exec(open(‘./main.py’).read(),globals())
Serial: 9108027469264322559
Identifier: Si7021
Temperature: 23.03474
Relative Humidity: 36.35458
>>>

Links

https://www.silabs.com/documents/public/data-sheets/Si7021-A20.pdf