sht21.py

#!/usr/bin/python
import fcntl
import time
import unittest

class SHT21:
    """Class to read temperature and humidity from SHT21, much of class was 
    derived from: #http://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/Humidity/Sensirion_Humidity_SHT21_Datasheet_V3.pdf
    and Martin Steppuhn's code from http://www.emsystech.de/raspi-sht21"""

    #control constants
    _SOFTRESET = 0xFE
    _I2C_ADDRESS = 0x40
    _TRIGGER_TEMPERATURE_NO_HOLD = 0xF3
    _TRIGGER_HUMIDITY_NO_HOLD = 0xF5

    #From: /linux/i2c-dev.h
    I2C_SLAVE = 0x0703
    I2C_SLAVE_FORCE = 0x0706

    def __init__(self, device_number=0):
	"""Opens the i2c device (assuming that the kernel modules have been
	loaded).  Note that this has only been tested on first revision 
	raspberry pi where the device_number = 0, but it should work 
	where device_number=1"""
        self.i2c = open('/dev/i2c-%s'%(device_number),'r+',0)
        fcntl.ioctl(self.i2c, self.I2C_SLAVE,0x40)
        self.i2c.write(chr(self._SOFTRESET))
        time.sleep(0.050)


    def read_temperature(self):    
        """Reads the temperature from the sensor.  Not that this call blocks
	for 250ms to allow the sensor to return the data"""
        self.i2c.write(chr(self._TRIGGER_TEMPERATURE_NO_HOLD))
        time.sleep(0.250)
        data = self.i2c.read(3)
        if _calculate_checksum(data,2) == ord(data[2]):
            return _get_temperature_from_buffer(data)
        

    def read_humidity(self):    
        """Reads the humidity from the sensor.  Not that this call blocks 
	for 250ms to allow the sensor to return the data"""
        self.i2c.write(chr(self._TRIGGER_HUMIDITY_NO_HOLD))
        time.sleep(0.250)
        data = self.i2c.read(3)
        if _calculate_checksum(data,2) == ord(data[2]):
            return _get_humidity_from_buffer(data)    
        

    def close(self):
        """Closes the i2c connection"""
        self.i2c.close()


    def __enter__(self):
        """used to enable python's with statement support"""
        return self
        

    def __exit__(self, type, value, traceback):
        """with support"""
        self.close()

        
def _calculate_checksum(data, nbrOfBytes):
    """5.7 CRC Checksum using teh polynomial given in the datasheet"""
    # CRC
    POLYNOMIAL = 0x131 # //P(x)=x^8+x^5+x^4+1 = 100110001
    crc = 0
    #calculates 8-Bit checksum with given polynomial
    for byteCtr in range(nbrOfBytes):
        crc ^= (ord(data[byteCtr]))
        for bit in range(8,0,-1):
            if (crc & 0x80):
                crc = (crc << 1) ^ POLYNOMIAL
            else:
                crc = (crc << 1)
    return crc


def _get_temperature_from_buffer(data):
    """This function reads the first two bytes of data and 
    returns the temperature in C by using the following function:
    T = =46.82 + (172.72 * (ST/2^16))
    where ST is the value from the sensor
    """
    unadjusted = (ord(data[0]) << 8) + ord(data[1])
    unadjusted *= 175.72
    unadjusted /= 1 << 16 # divide by 2^16
    unadjusted -= 46.85
    return unadjusted


def _get_humidity_from_buffer(data):
    """This function reads the first two bytes of data and returns 
    the relative humidity in percent by using the following function:
    RH = -6 + (125 * (SRH / 2 ^16))
    where SRH is the value read from the sensor
    """
    unadjusted = (ord(data[0]) << 8) + ord(data[1])
    unadjusted *= 125
    unadjusted /= 1 << 16 # divide by 2^16
    unadjusted -= 6
    return unadjusted
    

class SHT21Test(unittest.TestCase):
    """simple sanity test.  Run from the command line with 
    python -m unittest sht21 to check they are still good"""

    def test_temperature(self):
        """Unit test to check the checksum method"""
        calc_temp = _get_temperature_from_buffer([chr(99),chr(172)])
        #floating point comparison, not pretty.
        self.failUnless(abs(calc_temp - 21.5653979492) < 0.1)

    def test_checksum(self):
        """Unit test to check the checksum method.  Uses values read"""
        self.failUnless(_calculate_checksum([chr(99),chr(172)] , 2) == 249)
        self.failUnless(_calculate_checksum([chr(99),chr(160)] , 2) == 132)

if __name__ == "__main__":
    try:
        with SHT21(0) as sht21:
            print "Temperature: %s"%sht21.read_temperature()
            print "Humidity: %s"%sht21.read_humidity()
    except IOError, e:
        print e
        print 'Error creating connection to i2c.  This must be run as root'