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sht21.py
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sht21.py
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#!/usr/bin/env python3
# -*- coding: UTF-8 -*-
# Copyright (c) 2015 Martin Steppuhn, www.emsystech.de. All rights reserved.
#
# Redistribution and use in source and binary, must retain the above copyright notice, and the following disclaimer.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
# INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
# DISCLAIMED. IN NO EVENT SHALL BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
# OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
# THE POSSIBILITY OF SUCH DAMAGE.
#
# History:
# 24.06.2015 Martin Steppuhn Initial version
import time
import rpi_i2c
class SHT21:
"""Interface Class for SHT21 Temperture and Humidity Sensor from www.sensirion.com
Hardware: Breakoutboard with SHT21 Sensor for Raspberry Pi by www.emsystech.de
The Class can be used in two ways.
1. With the build in I2C Port from Raspberry Pi using I2C Driver
I2C Driver must be enabled: https://learn.adafruit.com/adafruits-raspberry-pi-lesson-4-gpio-setup/configuring-i2c
2. rpi_i2c can emulate an I2C Bus on any GPIO Pin. This is more flexible and more than one sensor can be used.
But it needs Pullups für SCL and SDA und root for GPIO access. (sudo)
The method "measure" does a complete cycle and returns a tupple with the values:
(temperature, humidity) = sht21.measure(1) # I2C-1 Port
If you want it more flexible e.g. multiple measurements, only humidity or temperature use:
sht21.open(1)
...
temperature = sht21.read_temperature()
humidity = sht21.read_humidity()
...
sht21.close()
"""
i2c = rpi_i2c.I2C() # I2C Wrapper Class
def measure(self, dev=1, scl=3, sda=2):
"""Complete cycle including open, measurement und close, return tuple of temperature and humidity"""
self.open(dev, scl, sda)
t = self.read_temperature()
rh = self.read_humidity()
self.i2c.close()
return (t, rh)
def open(self, dev=1, scl=3, sda=2):
"""Hardware I2C Port, B,B+,Pi 2 = 1 the first Pi = 0"""
self.i2c.open(0x40,dev, scl, sda)
self.i2c.write([0xFE]) # execute Softreset Command (default T=14Bit RH=12)
time.sleep(0.050)
def read_temperature(self):
""" Temperature measurement (no hold master), blocking for ~ 88ms !!! """
self.i2c.write([0xF3])
time.sleep(0.086) # wait, typ=66ms, max=85ms @ 14Bit resolution
data = self.i2c.read(3)
if (self._check_crc(data, 2)):
t = ((data[0] << 8) + data[1]) & 0xFFFC # set status bits to zero
t = -46.82 + ((t * 175.72) / 65536) # T = 46.82 + (175.72 * ST/2^16 )
return round(t, 1)
else:
return None
def read_humidity(self):
""" RH measurement (no hold master), blocking for ~ 32ms !!! """
self.i2c.write([0xF5]) # Trigger RH measurement (no hold master)
time.sleep(0.03) # wait, typ=22ms, max=29ms @ 12Bit resolution
data = self.i2c.read(3)
if (self._check_crc(data, 2)):
rh = ((data[0] << 8) + data[1]) & 0xFFFC # zero the status bits
rh = -6 + ((125 * rh) / 65536)
if (rh > 100): rh = 100
return round(rh, 1)
else:
return None
def close(self):
"""Closes the i2c connection"""
self.i2c.close()
def _check_crc(self, data, length):
"""Calculates checksum for n bytes of data and compares it with expected"""
crc = 0
for i in range(length):
crc ^= (ord(chr(data[i])))
for bit in range(8, 0, -1):
if crc & 0x80:
crc = (crc << 1) ^ 0x131 # CRC POLYNOMIAL
else:
crc = (crc << 1)
return True if (crc == data[length]) else False
if __name__ == "__main__":
sht21 = SHT21()
print("SHT21 Demo by www.emsystech.de")
while True:
try:
############################################################################################################
# Example 1 Using the I2C Driver
############################################################################################################
(temperature, humidity) = sht21.measure(1) # I2C-1 Port
print("Temperature: %s °C Humidity: %s %%" % (temperature, humidity))
############################################################################################################
# Example 2 Using GPIOs on I2C Pins (without Driver), must be executed with sudo
############################################################################################################
#(temperature, humidity) = sht21.measure(None) # No I2C-Port/Driver --> GPIO2, GPIO3
#print("Temperature: %s °C Humidity: %s %%" % (temperature, humidity))
############################################################################################################
# Example 3 Using multiple Sensors (without Driver), must be executed with sudo, Pullups required
############################################################################################################
#(t0, rh0) = sht21.measure(None,25,8) # Use GPIOs SCL=3, SDA=2
#(t1, rh1) = sht21.measure(None,7,11) # Use GPIOs SCL=3, SDA=2
#print("%s°C\t%s%%\t%s°C\t%s%%" % (t0,rh0,t1,rh1))
############################################################################################################
except:
print("SHT21 I/O Error")
time.sleep(2)