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<![CDATA[import time
import RPi.GPIO as GPIO
class DHT11Result:
'DHT11 sensor result returned by DHT11.read() method'
ERR_NO_ERROR = 0
ERR_MISSING_DATA = 1
ERR_CRC = 2
error_code = ERR_NO_ERROR
temperature = -1
humidity = -1
def __init__(self, error_code, temperature, humidity):
self.error_code = error_code
self.temperature = temperature
self.humidity = humidity
def is_valid(self):
return self.error_code == DHT11Result.ERR_NO_ERROR
class DHT11:
'DHT11 sensor reader class for Raspberry'
__pin = 0
def __init__(self, pin):
self.__pin = pin
def read(self):
GPIO.setup(self.__pin, GPIO.OUT)
# send initial high
self.__send_and_sleep(GPIO.HIGH, 0.05)
# pull down to low
self.__send_and_sleep(GPIO.LOW, 0.02)
# change to input using pull up
GPIO.setup(self.__pin, GPIO.IN, GPIO.PUD_UP)
# collect data into an array
data = self.__collect_input()
# parse lengths of all data pull up periods
pull_up_lengths = self.__parse_data_pull_up_lengths(data)
# if bit count mismatch, return error (4 byte data + 1 byte checksum)
if len(pull_up_lengths) != 40:
return DHT11Result(DHT11Result.ERR_MISSING_DATA, 0, 0)
# calculate bits from lengths of the pull up periods
bits = self.__calculate_bits(pull_up_lengths)
# we have the bits, calculate bytes
the_bytes = self.__bits_to_bytes(bits)
# calculate checksum and check
checksum = self.__calculate_checksum(the_bytes)
if the_bytes[4] != checksum:
return DHT11Result(DHT11Result.ERR_CRC, 0, 0)
# ok, we have valid data, return it
return DHT11Result(DHT11Result.ERR_NO_ERROR, the_bytes[2], the_bytes[0])
def __send_and_sleep(self, output, sleep):
GPIO.output(self.__pin, output)
time.sleep(sleep)
def __collect_input(self):
# collect the data while unchanged found
unchanged_count = 0
# this is used to determine where is the end of the data
max_unchanged_count = 100
last = -1
data = []
while True:
current = GPIO.input(self.__pin)
data.append(current)
if last != current:
unchanged_count = 0
last = current
else:
unchanged_count += 1
if unchanged_count > max_unchanged_count:
break
return data
def __parse_data_pull_up_lengths(self, data):
STATE_INIT_PULL_DOWN = 1
STATE_INIT_PULL_UP = 2
STATE_DATA_FIRST_PULL_DOWN = 3
STATE_DATA_PULL_UP = 4
STATE_DATA_PULL_DOWN = 5
state = STATE_INIT_PULL_DOWN
lengths = [] # will contain the lengths of data pull up periods
current_length = 0 # will contain the length of the previous period
for i in range(len(data)):
current = data[i]
current_length += 1
if state == STATE_INIT_PULL_DOWN:
if current == GPIO.LOW:
# ok, we got the initial pull down
state = STATE_INIT_PULL_UP
continue
else:
continue
if state == STATE_INIT_PULL_UP:
if current == GPIO.HIGH:
# ok, we got the initial pull up
state = STATE_DATA_FIRST_PULL_DOWN
continue
else:
continue
if state == STATE_DATA_FIRST_PULL_DOWN:
if current == GPIO.LOW:
# we have the initial pull down, the next will be the data pull up
state = STATE_DATA_PULL_UP
continue
else:
continue
if state == STATE_DATA_PULL_UP:
if current == GPIO.HIGH:
# data pulled up, the length of this pull up will determine whether it is 0 or 1
current_length = 0
state = STATE_DATA_PULL_DOWN
continue
else:
continue
if state == STATE_DATA_PULL_DOWN:
if current == GPIO.LOW:
# pulled down, we store the length of the previous pull up period
lengths.append(current_length)
state = STATE_DATA_PULL_UP
continue
else:
continue
return lengths
def __calculate_bits(self, pull_up_lengths):
# find shortest and longest period
shortest_pull_up = 1000
longest_pull_up = 0
for i in range(0, len(pull_up_lengths)):
length = pull_up_lengths[i]
if length < shortest_pull_up:
shortest_pull_up = length
if length > longest_pull_up:
longest_pull_up = length
# use the halfway to determine whether the period it is long or short
halfway = shortest_pull_up + (longest_pull_up - shortest_pull_up) / 2
bits = []
for i in range(0, len(pull_up_lengths)):
bit = False
if pull_up_lengths[i] > halfway:
bit = True
bits.append(bit)
return bits
def __bits_to_bytes(self, bits):
the_bytes = []
byte = 0
for i in range(0, len(bits)):
byte = byte << 1
if (bits[i]):
byte = byte | 1
else:
byte = byte | 0
if ((i + 1) % 8 == 0):
the_bytes.append(byte)
byte = 0
return the_bytes
def __calculate_checksum(self, the_bytes):
return the_bytes[0] + the_bytes[1] + the_bytes[2] + the_bytes[3] & 255]]>
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