SI7021 I2C sensor

[tobru]

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]

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

[tobru]

@crankshaft said in SI7021 I2C sensor:

Try This;

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