#!/usr/bin/python
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#-*- coding: utf-8 -*-
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import fcntl
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import time
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import unittest
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class SHT21:
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"""Class to read temperature and humidity from SHT21, much of class was
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derived from:
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http://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/Humidity/Sensirion_Humidity_SHT21_Datasheet_V3.pdf
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and Martin Steppuhn's code from http://www.emsystech.de/raspi-sht21"""
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# control constants
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_SOFTRESET = 0xFE
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_I2C_ADDRESS = 0x40
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_TRIGGER_TEMPERATURE_NO_HOLD = 0xF3
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_TRIGGER_HUMIDITY_NO_HOLD = 0xF5
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_STATUS_BITS_MASK = 0xFFFC
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# From: /linux/i2c-dev.h
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I2C_SLAVE = 0x0703
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I2C_SLAVE_FORCE = 0x0706
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# datasheet (v4), page 9, table 7, thanks to Martin Milata
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# for suggesting the use of these better values
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# code copied from https://github.com/mmilata/growd
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_TEMPERATURE_WAIT_TIME = 0.086 # (datasheet: typ=66, max=85)
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_HUMIDITY_WAIT_TIME = 0.030 # (datasheet: typ=22, max=29)
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def __init__(self, device_number=0):
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"""Opens the i2c device (assuming that the kernel modules have been
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loaded). Note that this has only been tested on first revision
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raspberry pi where the device_number = 0, but it should work
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where device_number=1"""
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device_number = 1
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self.i2c = open('/dev/i2c-%s' % device_number, 'r+', 0)
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fcntl.ioctl(self.i2c, self.I2C_SLAVE, 0x40)
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self.i2c.write(chr(self._SOFTRESET))
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time.sleep(0.050)
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def read_temperature(self):
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"""Reads the temperature from the sensor. Not that this call blocks
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for ~86ms to allow the sensor to return the data"""
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self.i2c.write(chr(self._TRIGGER_TEMPERATURE_NO_HOLD))
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time.sleep(self._TEMPERATURE_WAIT_TIME)
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data = self.i2c.read(3)
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if self._calculate_checksum(data, 2) == ord(data[2]):
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return self._get_temperature_from_buffer(data)
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def read_humidity(self):
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"""Reads the humidity from the sensor. Not that this call blocks
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for ~30ms to allow the sensor to return the data"""
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self.i2c.write(chr(self._TRIGGER_HUMIDITY_NO_HOLD))
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time.sleep(self._HUMIDITY_WAIT_TIME)
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data = self.i2c.read(3)
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if self._calculate_checksum(data, 2) == ord(data[2]):
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return self._get_humidity_from_buffer(data)
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def close(self):
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"""Closes the i2c connection"""
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self.i2c.close()
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def __enter__(self):
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"""used to enable python's with statement support"""
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return self
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def __exit__(self, type, value, traceback):
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"""with support"""
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self.close()
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@staticmethod
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def _calculate_checksum(data, number_of_bytes):
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"""5.7 CRC Checksum using the polynomial given in the datasheet"""
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# CRC
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POLYNOMIAL = 0x131 # //P(x)=x^8+x^5+x^4+1 = 100110001
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crc = 0
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# calculates 8-Bit checksum with given polynomial
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for byteCtr in range(number_of_bytes):
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crc ^= (ord(data[byteCtr]))
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for bit in range(8, 0, -1):
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if crc & 0x80:
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crc = (crc << 1) ^ POLYNOMIAL
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else:
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crc = (crc << 1)
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return crc
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@staticmethod
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def _get_temperature_from_buffer(data):
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"""This function reads the first two bytes of data and
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returns the temperature in C by using the following function:
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T = -46.85 + (175.72 * (ST/2^16))
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where ST is the value from the sensor
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"""
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unadjusted = (ord(data[0]) << 8) + ord(data[1])
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unadjusted &= SHT21._STATUS_BITS_MASK # zero the status bits
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unadjusted *= 175.72
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unadjusted /= 1 << 16 # divide by 2^16
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unadjusted -= 46.85
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return unadjusted
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@staticmethod
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def _get_humidity_from_buffer(data):
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"""This function reads the first two bytes of data and returns
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the relative humidity in percent by using the following function:
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RH = -6 + (125 * (SRH / 2 ^16))
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where SRH is the value read from the sensor
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"""
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unadjusted = (ord(data[0]) << 8) + ord(data[1])
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unadjusted &= SHT21._STATUS_BITS_MASK # zero the status bits
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unadjusted *= 125.0
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unadjusted /= 1 << 16 # divide by 2^16
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unadjusted -= 6
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return unadjusted
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class SHT21Test(unittest.TestCase):
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"""simple sanity test. Run from the command line with
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python -m unittest sht21 to check they are still good"""
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def test_temperature(self):
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"""Unit test to check the checksum method"""
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calc_temp = SHT21._get_temperature_from_buffer([chr(99), chr(172)])
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self.failUnless(abs(calc_temp - 21.5653979492) < 0.1)
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def test_humidity(self):
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"""Unit test to check the humidity computation using example
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from the v4 datasheet"""
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calc_temp = SHT21._get_humidity_from_buffer([chr(99), chr(82)])
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self.failUnless(abs(calc_temp - 42.4924) < 0.001)
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def test_checksum(self):
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"""Unit test to check the checksum method. Uses values read"""
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self.failUnless(SHT21._calculate_checksum([chr(99), chr(172)], 2) == 249)
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self.failUnless(SHT21._calculate_checksum([chr(99), chr(160)], 2) == 132)
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if __name__ == "__main__":
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try:
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with SHT21(0) as sht21:
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print "Temperature: %s" % sht21.read_temperature()
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print "Humidity: %s" % sht21.read_humidity()
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except IOError, e:
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print e
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print "Error creating connection to i2c. This must be run as root"
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