Time & Interrupts

[MaSePreetz]

Dear Developers,

i am currently trying to develop a driver for the DHT11/DHT22 by using
interrupt and callback handlers.
The sensor sends its data by toggling the data pin from high to low.
These flanks are already detected by my code, i can count them and all are
detected.

But i need to measure the time difference between the flanks, a 1
has the length of approx. 130 us and a 0 of approx. 78 us.
When executing the code, the recorded time stamps are all above 130 us,
so they are unusable. Is there any possibility to speed things up here?
I assume that the queuing of interrupts produces those long times.

Here is the result of a measurement:

[120.575, 256.6, 401.2, 547.225, 682.625, 829.225, 973.1, 1109.5, 1255.1, 1402.6, 1548.6, 1681.4, 1826.45, 1979.675, 2122.75, 2265.425, 2389.325, 2535.075, 2668.4, 2802.9, 2957.15, 3092.35, 3227.575, 3360.9, 3505.95, 3640.4, 3786.925, 3925.85, 4060.325, 4184.05, 4307.825, 4431.625, 4555.325, 4679.05, 4802.825, 4934.1, 5057.8, 5181.525, 5305.3, 5429.1, 5552.8, 5676.525]

Here is the code of the handler:

def handler(self, arg):
	self._samples[self._samplesIndex] = self._chronometer.read_us()
	# Increase the index if we are still within the bounds of the array	
	if(self._samplesIndex < self._MAXSAMPLES - 1):
		self._samplesIndex = self._samplesIndex + 1

and here the code to register the handler:

self._dataPin.callback(Pin.IRQ_FALLING, self._irqHandler.handler)

Any ideas how to improve the situation?

Many thanks in advance,
Marc

[robert-hh]

@Emmanuel-Goudot Yes, that's the piece of code below, which works for my DHT22 on a Lopy. It is buried far below in a thread, where two people fight to get it running(https://forum.pycom.io/topic/1172/pure-python-library-for-reading-dht-sensor-on-pycom-boards). No clue what their problem is, since I consider it solved, even if not bullet proof.

import time
from machine import enable_irq, disable_irq,  Pin


class DTHResult:
    'DHT sensor result returned by DHT.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 == DTHResult.ERR_NO_ERROR


class DTH:
    'DHT sensor (dht11, dht21,dht22) reader class for Pycom'

    __pin = Pin('P3', mode=Pin.OPEN_DRAIN)
    __dhttype = 0

    def __init__(self, pin, sensor=0):
	self.__pin = pin
	self.__pin(1)
	self.__dhttype = sensor

    def read(self):
	time.sleep(1)

	# send initial high
	#self.__send_and_sleep(1, 25000)

	# pull down to low
	#self.__send_and_sleep(0, 40000)

	# 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)
	print(pull_up_lengths)
	if len(pull_up_lengths) != 40:
	    return DTHResult(DTHResult.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)
	print(the_bytes)
	# calculate checksum and check
	checksum = self.__calculate_checksum(the_bytes)
	if the_bytes[4] != checksum:
	    return DTHResult(DTHResult.ERR_CRC, 0, 0)

	# ok, we have valid data, return it
	[int_rh, dec_rh, int_t, dec_t, csum] = the_bytes
	if self.__dhttype==0:		#dht11
		rh = int_rh		#dht11 20% ~ 90%
		t = int_t	#dht11 0..50°C
	else:			#dht21,dht22
		rh = ((int_rh * 256) + dec_rh)/10
		t = (((int_t & 0x7F) * 256) + dec_t)/10
		if (int_t & 0x80) > 0:
			t *= -1
	return DTHResult(DTHResult.ERR_NO_ERROR, t, rh)

    def __send_and_sleep(self, output, mysleep):
	self.__pin(output)
	time.sleep_us(mysleep)

    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 = []
	m = [1]*500	   # needs long sample size to grab all the bits from the DHT
	self.__pin(0)
	time.sleep_us(20000)
	self.__pin(1)
	irqf = disable_irq()
	for i in range(len(m)):
	    m[i] = self.__pin()      ## sample input and store value
	enable_irq(irqf)
	for i in range(len(m)):
	    current = m[i]
	    data.append(current)
	    if last != current:
		unchanged_count = 0
		last = current
	    else:
		unchanged_count += 1
		if unchanged_count > max_unchanged_count:
		    break
	#print(data)
	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_UP

	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 == 0:
		    # ok, we got the initial pull down
		    state = STATE_INIT_PULL_UP
		    continue
		else:
		    continue
	    if state == STATE_INIT_PULL_UP:
		if current == 1:
		    # 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 == 0:
		    # 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 == 1:
		    # 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 == 0:
		    # 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
	#print(the_bytes)
	return the_bytes

    def __calculate_checksum(self, the_bytes):
	return the_bytes[0] + the_bytes[1] + the_bytes[2] + the_bytes[3] & 255

[Emmanuel Goudot]

Hello @robert-hh ,
I guess you have succeeded working with DHT11/DHT22 ?
Could you share python library (that work) ?
Thanks a lot,
Emmanuel.

[robert-hh]

@MaSePreetz There was a discussion about pin interrupt latency here: https://forum.pycom.io/topic/936/pin-interrupt-latency
Unless newer version of the firmware improve things drastically, there is no chance to get data from the DHT11/DHT22 with an interrupt using driver. The only Python method that works for me is a busy polling method.

[Emmanuel Goudot]

Hello @livius ,
with such interrupt handler:

tested with

I get such results:

empty loop ~25µs, loop with hadler: ~85µs (handler alone: 65µs ?)
deltas are differences of time.ticks_us() and minimum value I get is ~84µs

Would it be possible to add value of timer inside the object passed to interrupt handler, or as second parameter ???

BR,
Emmanuel.

[Erik]

Hi Marc,

I've also tried to measure a DHT22's pulse length using an interrupt callbacks. Even with the smallest (and hopefully fastest) interrupt routine I got the same results as you did; I never measured a pulse length below 100 us.

For what I did and the results see here: https://github.com/erikdelange/WiPy-2.0-DHT22

In the end the only thing which worked for me was to use polling, and even then I had to temporarily disable interrupts to get a usable result.

Regards
Erik

[livius]

@MaSePreetz

When executing the code, the recorded time stamps are all above 130 us,

You register for IRQ_FALLING, then how you detect how long is 0 and how long is 1?

For IRQ_FALLING you get info that it is changing from 1 to 0. and e.g. it stay in 0 for 50us and it raising to 1 and stay for 100us and then back you get IRQ_FALLING
you count this as 150us but do not know how long is 0 and how long is 1.

EDIT
Ah i forgot that DHT send low 50us and variable length of high like 28 for 0 and 70 for 1
then your concept is good - you should got 78us for 0 or 120us for 1

But for dht better is to Pull pin
https://github.com/adafruit/DHT-sensor-library/blob/master/DHT.cpp
and i do not know how long your operation in handler take time?

Try to measure it in e.g. 1000 loop - what you got?