dht.py

import pyb
from pyb import Pin
from pyb import ExtInt

# We need to use global properties here as any allocation of a memory (aka declaration of a variable)
# during the read cycle causes non-acceptable delay and we are loosing data than
data = None
timer = None
micros = None
dhttype = 0 #0=DHT11 1=DHT21/DHT22

FALL_EDGES = 42 # we have 42 falling edges during data receive

times = list(range(FALL_EDGES))
index = 0

# The interrupt handler
def edge(line):
	global index
	global times
	global micros
	times[index] = micros.counter()
	if index < (FALL_EDGES - 1): # Avoid overflow of the buffer in case of any noise on the line
		index += 1

def init(timer_id = 2, data_pin = 'Y9', the_dhttype='DHT11'):
	global data
	global micros
	global timer
	global dhttype
   
	if(the_dhttype=='DHT11'):
		dhttype = 0
	else:
		dhttype = 1
	
	# Configure the pid for data communication
	data = Pin(data_pin)
	# Save the ID of the timer we are going to use
	timer = timer_id
	# setup the 1uS timer
	micros = pyb.Timer(timer, prescaler=83, period=0x3fffffff) # 1MHz ~ 1uS
	# Prepare interrupt handler
	ExtInt(data, ExtInt.IRQ_FALLING, Pin.PULL_UP, None)
	ExtInt(data, ExtInt.IRQ_FALLING, Pin.PULL_UP, edge)

# Start signal
def do_measurement():
	global data
	global micros
	global index
	# Send the START signal
	data.init(Pin.OUT_PP)
	data.low()
	micros.counter(0)
	while micros.counter() < 25000:
		pass
	data.high()
	micros.counter(0)
	while micros.counter() < 20:
		pass
	# Activate reading on the data pin
	index = 0
	data.init(Pin.IN, Pin.PULL_UP)
	# Till 5mS the measurement must be over
	pyb.delay(5)

# Parse the data read from the sensor
def process_data():
	global dhttype
	global times
	i = 2 # We ignore the first two falling edges as it is a respomse on the start signal
	result_i = 0
	result = list([0, 0, 0, 0, 0])
	while i < FALL_EDGES:
		result[result_i] <<= 1
		if times[i] - times[i - 1] > 100:
			result[result_i] += 1
		if (i % 8) == 1:
			result_i += 1
		i += 1
	[int_rh, dec_rh, int_t, dec_t, csum] = result
	
	if dhttype==0:		#dht11
		humidity = int_rh 		#dht11 20% ~ 90%
		temperature = int_t 	#dht11 0..50°C
	else:			#dht21,dht22
		humidity = ((int_rh * 256) + dec_rh)/10 
		temperature = (((int_t & 0x7F) * 256) + dec_t)/10
		if (int_t & 0x80) > 0:
			temperature *= -1
	
	comp_sum = int_rh + dec_rh + int_t + dec_t
	if (comp_sum & 0xFF) != csum:
		raise ValueError('Checksum does not match')
	return (humidity, temperature)#,int_rh,dec_rh,int_t,dec_t)

def measure():
	do_measurement()
	if index != (FALL_EDGES -1):
		raise ValueError('Data transfer failed: %s falling edges only' % str(index))
	return process_data()

def hum():
	return measure()[0]

def temp():
	return measure()[1]


#using:
#import dht
#dht.init()
#dht.measure()