HTU21DF.py

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# Raspberry Pi Driver for Adafruit HTU21D-F
# Go buy one at https://www.adafruit.com/products/1899
# written by D. Alex Gray dalexgray@mac.com
# Thanks to egutting at the adafruit.com forums
# Thanks to Joan on the raspberrypi.org forums
# This requires the pigpio library
# Get pigpio at http://abyz.co.uk/rpi/pigpio/index.html
# No warranty offered or implied.  God help you if you use this.
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import time
import pigpio
import math

pi = pigpio.pi()

# HTU21D-F Address
addr = 0x40

# i2c bus, if you have a Raspberry Pi Rev A, change this to 0
bus = 1

# HTU21D-F Commands
rdtemp = 0xE3
rdhumi = 0xE5
wtreg = 0xE6
rdreg = 0xE7
reset = 0xFE

def htu_reset():
   handle = pi.i2c_open(bus, addr) # open i2c bus
   pi.i2c_write_byte(handle, reset) # send reset command
   pi.i2c_close(handle) # close i2c bus
   time.sleep(0.2) # reset takes 15ms so let's give it some time

def read_temperature():
   handle = pi.i2c_open(bus, addr) # open i2c bus
   pi.i2c_write_byte(handle, rdtemp) # send read temp command
   time.sleep(0.055) # readings take up to 50ms, lets give it some time
   (count, byteArray) = pi.i2c_read_device(handle, 3) # vacuum up those bytes
   pi.i2c_close(handle) # close the i2c bus
   t1 = byteArray[0] # most significant byte msb
   t2 = byteArray[1] # least significant byte lsb
   temp_reading = (t1 * 256) + t2 # combine both bytes into one big integer
   temp_reading = math.fabs(temp_reading) # I'm an idiot and can't figure out any other way to make it a float 
   temperature = ((temp_reading / 65536) * 175.72 ) - 46.85 # formula from datasheet
   return temperature

def read_humidity():
   handle = pi.i2c_open(bus, addr) # open i2c bus
   pi.i2c_write_byte(handle, rdhumi) # send read humi command
   time.sleep(0.055) # readings take up to 50ms, lets give it some time
   (count, byteArray) = pi.i2c_read_device(handle, 3) # vacuum up those bytes
   pi.i2c_close(handle) # close the i2c bus
   h1 = byteArray[0] # most significant byte msb
   h2 = byteArray[1] # least significant byte lsb
   humi_reading = (h1 * 256) + h2 # combine both bytes into one big integer
   humi_reading = math.fabs(humi_reading) # I'm an idiot and can't figure out any other way to make it a float
   uncomp_humidity = ((humi_reading / 65536) * 125 ) - 6 # formula from datasheet
   # to get the compensated humidity we need to read the temperature
   temperature = read_temperature()
   humidity = ((25 - temperature) * -0.15) + uncomp_humidity
   return humidity