HTU21D Raspberry pi Python Script

Enable I2C with raspi-config and reboot

Check if the sensor is connected:

i2cdetect -y 1

The output should look like this:

root@rpikiosk:~# i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- 23 -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: 40 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- 77

Our HTU21D device is detected as the number 40 in the line 40, so the address is 0X40.

#!/usr/bin/python
import struct, array, time, io, fcntl

I2C_SLAVE=0x0703
HTU21D_ADDR = 0x40
CMD_READ_TEMP_HOLD = "\xE3"
CMD_READ_HUM_HOLD = "\xE5"
CMD_READ_TEMP_NOHOLD = "\xF3"
CMD_READ_HUM_NOHOLD = "\xF5"
CMD_WRITE_USER_REG = "\xE6"
CMD_READ_USER_REG = "\xE7"
CMD_SOFT_RESET= "\xFE"

class i2c(object):
   def __init__(self, device, bus):

      self.fr = io.open("/dev/i2c-"+str(bus), "rb", buffering=0)
      self.fw = io.open("/dev/i2c-"+str(bus), "wb", buffering=0)

      # set device address

      fcntl.ioctl(self.fr, I2C_SLAVE, device)
      fcntl.ioctl(self.fw, I2C_SLAVE, device)

   def write(self, bytes):
      self.fw.write(bytes)

   def read(self, bytes):
      return self.fr.read(bytes)

   def close(self):
      self.fw.close()
      self.fr.close()

class HTU21D(object):
   def __init__(self):
      self.dev = i2c(HTU21D_ADDR, 1) #HTU21D 0x40, bus 1
      self.dev.write(CMD_SOFT_RESET) #soft reset
      time.sleep(.1)

   def ctemp(self, sensorTemp):
      tSensorTemp = sensorTemp / 65536.0
      return -46.85 + (175.72 * tSensorTemp)

   def chumid(self, sensorHumid):
      tSensorHumid = sensorHumid / 65536.0
      return -6.0 + (125.0 * tSensorHumid)

   def crc8check(self, value):
      # Ported from Sparkfun Arduino HTU21D Library: https://github.com/sparkfun/HTU21D_Breakout
      remainder = ( ( value[0] << 8 ) + value[1] ) << 8
      remainder |= value[2]
      
      # POLYNOMIAL = 0x0131 = x^8 + x^5 + x^4 + 1
      # divsor = 0x988000 is the 0x0131 polynomial shifted to farthest left of three bytes
      divsor = 0x988000
      
      for i in range(0, 16):
         if( remainder & 1 << (23 - i) ): remainder ^= divsor divsor = divsor >> 1
      
      if remainder == 0:
         return True
      else:
         return False
   
   def read_tmperature(self):
      self.dev.write(CMD_READ_TEMP_NOHOLD) #measure temp
      time.sleep(.1)

      data = self.dev.read(3)
      buf = array.array('B', data)

      if self.crc8check(buf):
         temp = (buf[0] << 8 | buf [1]) & 0xFFFC
         return self.ctemp(temp)
      else:
         return -255
         
   def read_humidity(self):
      self.dev.write(CMD_READ_HUM_NOHOLD) #measure humidity
      time.sleep(.1)

      data = self.dev.read(3)
      buf = array.array('B', data)
      
      if self.crc8check(buf):
         humid = (buf[0] << 8 | buf [1]) & 0xFFFC
         return self.chumid(humid)
      else:
         return -255
         
if __name__ == "__main__":
   obj = HTU21D()
   print "Temp:", obj.read_tmperature(), "C"
   print "Humid:", obj.read_humidity(), "% rH"

Run the script:

root@rpikiosk:~# ./HTU21DF.py
Temp: 22.9382104492 C
Humid: 45.0101318359 % rH

If you connect the ground wire to the wrong Rpi pin, you’ll get this error:

IOError: [Errno 5] Input/output error

 

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