SI7021 I2C sensor



  • I'm trying to get the SI7021 sensor working, which is connected on the I2C bus. Connectivity basically works and I can also write and read bytes. But I strugle with converting the example Arduino code to MicroPython. I've also found another example using the smbus Python library, but I also couldn't find out how to convert this.

    Here is what I've done so far:

    from machine import I2C
    import time
    
    ADDR = 0x40
    SI7021_MEASTEMP_NOHOLD_CMD = 0xF3
    SI7021_MEASRH_NOHOLD_CMD = 0xF5
    
    # configure the I2C bus
    i2c = I2C(0, I2C.MASTER, baudrate=100000)
    
    # Humidity
    i2c.writeto(ADDR, bytes([SI7021_MEASRH_NOHOLD_CMD]))
    time.sleep(0.3)
    hum0 = i2c.readfrom(ADDR, 1)
    hum1 = i2c.readfrom(ADDR, 1)
    print(hum0)
    print(hum1)
    
    #humidity = ((hum0 * 256 + hum1) * 125 / 65536.0) - 6
    #print(humidity)
    
    # Temperature
    i2c.writeto(ADDR, bytes([SI7021_MEASTEMP_NOHOLD_CMD]))
    time.sleep(0.3)
    temp0 = i2c.readfrom(ADDR, 1)
    temp1 = i2c.readfrom(ADDR, 1)
    print(temp0)
    print(temp1)
    
    #cTemp = ((temp0 * 256 + temp1) * 175.72 / 65536.0) - 46.85
    #fTemp = cTemp * 1.8 + 32
    
    #print(cTemp)
    

    Output:

    b'F'
    b'F'
    b'h'
    b'h'
    

    Currently I don't understand how I can read and convert the data correctly. Sending the commands works and I also get some sort of data back when calling the i2c.readfrom function, but then I'm lost. F.e. I don't know how many bytes I must read and then how to convert it to an actual useable value.

    Any help is much appreciated!

    Cheers,
    Tobias



  • @crankshaft said in SI7021 I2C sensor:

    Try This;

    WOW! You're great, works like a charm... Thank you very very much!



  • Try This;

    from time import sleep_ms
    from machine import Pin, I2C
    
    # Default Address
    SI7021_I2C_DEFAULT_ADDR = 0x40
    
    # Commands
    CMD_MEASURE_RELATIVE_HUMIDITY_HOLD_MASTER_MODE = 0xE5
    CMD_MEASURE_RELATIVE_HUMIDITY = 0xF5
    CMD_MEASURE_TEMPERATURE_HOLD_MASTER_MODE = 0xE3
    CMD_MEASURE_TEMPERATURE = 0xF3
    CMD_READ_TEMPERATURE_VALUE_FROM_PREVIOUS_RH_MEASUREMENT = 0xE0
    CMD_RESET = 0xFE
    CMD_WRITE_RH_T_USER_REGISTER_1 = 0xE6
    CMD_READ_RH_T_USER_REGISTER_1 = 0xE7
    CMD_WRITE_HEATER_CONTROL_REGISTER = 0x51
    CMD_READ_HEATER_CONTROL_REGISTER = 0x11
    
    class SI7021(object):
      def __init__(self, i2c=None):
        self.i2c = i2c
        self.addr = SI7021_I2C_DEFAULT_ADDR
        self.cbuffer = bytearray(2)
        self.cbuffer[1] = 0x00
        
      def write_command(self, command_byte):
        self.cbuffer[0] = command_byte
        self.i2c.writeto(self.addr, self.cbuffer)
    
      def readTemp(self):
        self.write_command(CMD_MEASURE_TEMPERATURE)
        sleep_ms(25)
        temp = self.i2c.readfrom(self.addr,3)
        temp2 = temp[0] << 8
        temp2 = temp2 | temp[1]
        return (175.72 * temp2 / 65536) - 46.85
    
      def readRH(self):
        self.write_command(CMD_MEASURE_RELATIVE_HUMIDITY)
        sleep_ms(25)
        rh = self.i2c.readfrom(self.addr, 3)
        rh2 = rh[0] << 8
        rh2 = rh2 | rh[1]
        return (125 * rh2 / 65536) - 6
    


Looks like your connection to Pycom Forum was lost, please wait while we try to reconnect.