added library for HTU21D I2C relative humidity sensor

bbio/libraries/HTU21D/HTU21D.py

@@ -0,0 +1,121 @@
+"""
+ HTU21D
+ Copyright 2015 - Alexander Hiam <alex@graycat.io>
+
+ A PyBBIO library for controlling HTU21D I2C humidity sensor.
+
+ HTU21D is released as part of PyBBIO under its MIT license.
+ See PyBBIO/LICENSE.txt
+"""
+import bbio, math
+
+class HTU21D(object):
+  I2C_ADDRESS = 0x40
+
+  CMD_TEMP          = 0xf3
+  CMD_RH            = 0xf5
+  CMD_USER_REG_READ = 0xe7 
+  CMD_RESET         = 0xfe
+
+  USER_REGISTER_DEFAULT = 0x02 # Reset value of the user register
+
+  CRC_DIVISOR = 0b100110001
+
+  def __init__(self, i2c):
+    self.i2c = i2c
+    if i2c == -1:
+      # Testing mode, don't try to open an I2C interface
+      return
+    i2c.begin()
+    bbio.delay(15)
+    i2c.write(self.I2C_ADDRESS, [self.CMD_RESET])
+    bbio.delay(15)
+    usr_reg = i2c.readTransaction(self.I2C_ADDRESS, self.CMD_USER_REG_READ, 1)
+    assert usr_reg[0] == self.USER_REGISTER_DEFAULT, \
+      "HTU21D not detected on I2C bus"
+
+  def getHumidity(self):
+    """ Reads and returns the current relative humidity
+
+    Received value is checked against a CRC and an AssertionError is thrown if 
+    it is invalid.
+    """
+    self.i2c.write(self.I2C_ADDRESS, [self.CMD_RH])
+    bbio.delay(50)
+    msb, lsb, crc = self.i2c.read(self.I2C_ADDRESS, 3)
+
+    raw_value = (msb<<8) | lsb
+    assert self.checkCRC(raw_value, crc), "received invalid data"
+    # Should that really throw an error?
+
+    # Conversion formula from datasheet:
+    return -6.0 + 125.0 * (raw_value/65536.0)
+
+  def getTemp(self):
+    """ Reads and returns the current ambient temperature in Celsius
+
+    Received value is checked against a CRC and an AssertionError is thrown if 
+    it is invalid.
+    """
+    self.i2c.write(self.I2C_ADDRESS, [self.CMD_TEMP])
+    bbio.delay(50)
+    msb, lsb, crc = self.i2c.read(self.I2C_ADDRESS, 3)
+
+    raw_value = (msb<<8) | lsb
+    assert self.checkCRC(raw_value, crc), "received invalid data"
+    # Should that really throw an error?
+
+    # Conversion formula from datasheet:
+    return -46.85 + 175.72 * (raw_value/65536.0)
+
+  def getTempF(self):
+    """ Reads and returns the current ambient temperature in fahrenheit
+
+    Received value is checked against a CRC and an AssertionError is thrown if 
+    it is invalid.
+    """
+    tempC = self.readTemp()
+    return tempC * 9./5 + 32
+
+  def calculateDewPoint(self, rh, temp):
+    """ Calculates and returns the dew point for the given RH and temp C
+
+    >>> round(HTU21D(-1).calculateDewPoint(50.0, 25.0))
+    14.0
+    >>> round(HTU21D(-1).calculateDewPoint(65.0, -10.0))
+    -15.0
+    """
+    # A, B and C are constants in the dew point calculations
+    A = 8.1332
+    B = 1762.39
+    C = 235.66
+    pp = 10**(A - B / (temp + C)) # Partial pressure
+    return -(C + B / (math.log(rh * (pp/100.), 10) - A))
+
+  def checkCRC(self, value, crc):
+    """ Checks the given 2-byte value against the given CRC
+        
+    Uses the HTU21D's divisor polynomial of x^8 + x^5 + x^4 + 1 given in 
+    the datasheet.
+    See http://en.wikipedia.org/wiki/Computation_of_cyclic_redundancy_checks
+
+    >>> HTU21D(-1).checkCRC(0xDC, 0x79)
+    True
+    >>> HTU21D(-1).checkCRC(0x683A, 0x7C)
+    True
+    >>> HTU21D(-1).checkCRC(0x683A, 0x01)
+    False
+    """
+    value <<= 8
+    divisor = self.CRC_DIVISOR << 15
+    for i in range(16):
+      if value & 1<<(23-i):
+        # There's a 1 above the x^8 bit of the divisor polynomial
+        value ^= divisor
+      divisor >>= 1
+    if (value & 0xff) == crc: return True
+    return False
+
+if __name__ == "__main__":
+    import doctest
+    doctest.testmod()

bbio/libraries/HTU21D/__init__.py

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+# __init__.py file for PyBBIO's HTU21D library
+from HTU21D import HTU21D

examples/HTU21D_test.py

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+"""
+ HTU21D_test.py 
+ Alexander Hiam <alex@graycat.io> 
+ 
+ An example to demonstrate the use of the HTU21D library included 
+ with PyBBIO. Interfaces with the HTU21D I2C relative humidity sensor.
+ 
+ This example program is in the public domain.
+"""
+
+from bbio import *
+# Import the HTU21D class from the HTU21D library:
+from bbio.libraries.HTU21D import HTU21D
+
+# Create a new instance of the HTU21D class using the Wire2 I2C bus
+htu = HTU21D(Wire2)
+
+def setup():
+  # The HTU21D is initialized when the HTU21D class is instantiated,
+  # so there's nothing to do here
+  pass
+
+def loop():
+  # Read the current relative humidity:
+  rh = htu.getHumidity()
+  # Read the current ambient temperature:
+  temp = htu.getTemp()
+  # Calculate the current dew point:
+  dew_point = htu.calculateDewPoint(rh, temp)
+
+  print "\nrelative humidity : %0.2f %%" % rh
+  print "temperature       : %0.2f C" % temp
+  print "dew point         : %0.2f C" % dew_point
+
+  delay(2000)
+
+run(setup, loop)