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executable file 92 lines (67 sloc) 2.24 KB
#!/usr/bin/python
import time
from Adafruit_I2C import Adafruit_I2C
# ===========================================================================
# HTU21D Class
#
# Code only test with a Sparkfun HTU21D Sensor module on a Beaglebone Black.
# It has been reported that an I2C address issue was seen on a Pi.
# ===========================================================================
class HTU21D:
i2c = None
# HTU21D Address
address = 0x40
# Commands
TRIGGER_TEMP_MEASURE_HOLD = 0xE3
TRIGGER_HUMD_MEASURE_HOLD = 0xE5
READ_USER_REG = 0xE7
# Constructor
def __init__(self):
self.i2c = Adafruit_I2C(self.address)
def readUserRegister(self):
"Read the user register byte"
return self.i2c.readU8(self.READ_USER_REG)
def readTemperatureData(self):
"Read 3 temperature bytes from the sensor"
# value[0], value[1]: Raw temperature data
# value[2]: CRC
value = self.i2c.readList(self.TRIGGER_TEMP_MEASURE_HOLD, 3)
# CRC Check
if not self.crc8check(value):
return -255
rawTempData = ( value[0] << 8 ) + value[1]
# Clear the status bits
rawTempData = rawTempData & 0xFFFC;
# Calculate the actual temperature
actualTemp = -46.85 + (175.72 * rawTempData / 65536)
return actualTemp
def readHumidityData(self):
"Read 3 humidity bytes from the sensor"
# value[0], value[1]: Raw relative humidity data
# value[2]: CRC
value = self.i2c.readList(self.TRIGGER_HUMD_MEASURE_HOLD, 3)
# CRC Check
if not self.crc8check(value):
return -255
rawRHData = ( value[0] << 8 ) + value[1]
# Clear the status bits
rawRHData = rawRHData & 0xFFFC;
# Calculate the actual RH
actualRH = -6 + (125.0 * rawRHData / 65536)
return actualRH
def crc8check(self, value):
"Calulate the CRC8 for the data received"
# 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