dht11.py

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