#!/usr/bin/python """ from https://raw.githubusercontent.com/kif/sht21_python/refs/heads/python3/sht21.py from https://github.com/kif/sht21_python/tree/python3 forked from jaques/sht21_python with The MIT License (MIT) Copyright (c) 2013 Richard Jaques Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. """ 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 _STATUS_BITS_MASK = 0xFFFC # From: /linux/i2c-dev.h I2C_SLAVE = 0x0703 I2C_SLAVE_FORCE = 0x0706 # datasheet (v4), page 9, table 7, thanks to Martin Milata # for suggesting the use of these better values # code copied from https://github.com/mmilata/growd _TEMPERATURE_WAIT_TIME = 0.086 # (datasheet: typ=66, max=85) _HUMIDITY_WAIT_TIME = 0.030 # (datasheet: typ=22, max=29) 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, 'rb+', 0) fcntl.ioctl(self.i2c, self.I2C_SLAVE, 0x40) self.i2c.write(bytes([self._SOFTRESET])) time.sleep(0.050) def read_temperature(self): """Reads the temperature from the sensor. Not that this call blocks for ~86ms to allow the sensor to return the data""" self.i2c.write(bytes([self._TRIGGER_TEMPERATURE_NO_HOLD])) time.sleep(self._TEMPERATURE_WAIT_TIME) data = self.i2c.read(3) if self._calculate_checksum(data, 2) == data[2]: return self._get_temperature_from_buffer(data) def read_humidity(self): """Reads the humidity from the sensor. Not that this call blocks for ~30ms to allow the sensor to return the data""" self.i2c.write(bytes([self._TRIGGER_HUMIDITY_NO_HOLD])) time.sleep(self._HUMIDITY_WAIT_TIME) data = self.i2c.read(3) if self._calculate_checksum(data, 2) == data[2]: return self._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() @staticmethod def _calculate_checksum(data, number_of_bytes): """5.7 CRC Checksum using the 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(number_of_bytes): crc ^= data[byteCtr] for bit in range(8, 0, -1): if crc & 0x80: crc = (crc << 1) ^ POLYNOMIAL else: crc = (crc << 1) return crc @staticmethod 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 = -46.85 + (175.72 * (ST/2^16)) where ST is the value from the sensor """ unadjusted = (data[0] << 8) + data[1] unadjusted &= SHT21._STATUS_BITS_MASK # zero the status bits unadjusted *= 175.72 unadjusted /= 1 << 16 # divide by 2^16 unadjusted -= 46.85 return unadjusted @staticmethod 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 = (data[0] << 8) + data[1] unadjusted &= SHT21._STATUS_BITS_MASK # zero the status bits unadjusted *= 125.0 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 = SHT21._get_temperature_from_buffer([chr(99), chr(172)]) self.failUnless(abs(calc_temp - 21.5653979492) < 0.1) def test_humidity(self): """Unit test to check the humidity computation using example from the v4 datasheet""" calc_temp = SHT21._get_humidity_from_buffer([chr(99), chr(82)]) self.failUnless(abs(calc_temp - 42.4924) < 0.001) def test_checksum(self): """Unit test to check the checksum method. Uses values read""" self.failUnless(SHT21._calculate_checksum([chr(99), chr(172)], 2) == 249) self.failUnless(SHT21._calculate_checksum([chr(99), chr(160)], 2) == 132) if __name__ == "__main__": try: with SHT21(1) as sht21: print("Temperature: %s" % sht21.read_temperature()) print("Humidity: %s" % sht21.read_humidity()) except IOError as e: print(type(e), e) print("Error creating connection to i2c. This must be run as root")