小实验目录

树莓派科学小实验
001 点亮第一盏LED灯
002 点亮LED灯组
003_开关控制LED灯
004_获取温湿度

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前言

在这个实验中将接触到时钟信号，高低电平的接收。字节转换

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一、实验部件

1 实验元器件

2 连接的GPIO针脚

使用的针脚：4,17，27,19

二、 DHT11 和 单片机或树莓派通信的过程

由于python无法控制us的时钟的精度，所以在实际收取时会出现无法获取或获取信号不完整的现象。但是基本原理如图下。

三、代码部分

在这里引用了github上的开源项目：https://github.com/szazo/DHT11_Python

1 DHT11的实际工作代码

# -*- coding: utf-8 -*-
"""
#这个是对DHT11的类，将完成传感器的初始化，获取值，以及将值转换为10进制并返回，文章参考了：https://github.com/szazo/DHT11_Pythonversion = '0.0.1'
make day=2022-01-26
"""
__docformat__ = "restructuredtext en"__all__ = []__license__ = "MIT license"import timeimport RPi.GPIO as GPIO
import RPi# 定义显示类用来显示和输出
class DHT11Result:'DHT11 返回从DHT11.read（）中读取的值'ERR_NO_ERROR = 0ERR_MISSING_DATA = 1ERR_CRC = 2error_code = ERR_NO_ERRORtemperature = -1humidity = -1def __init__(self, error_code, temperature, humidity):self.error_code = error_codeself.temperature = temperatureself.humidity = humiditydef is_valid(self):return self.error_code == DHT11Result.ERR_NO_ERROR# 定义探头初始化和获取的类
class DHT11:__pin = 0 #定义Pin的值def __init__(self, pin):self.__pin = pindef __send_and_sleep(self, output, sleep):'''设定发送的电平值和休眠的时间:param output: 电平值:param sleep: 休眠时间:return:'''GPIO.output(self.__pin, output)time.sleep(sleep)def __collect_input(self):'''这部分完成了对输入数据的采集'''unchanged_count = 0max_unchanged_count = 100last=-1data=[]while True:curent=GPIO.input(self.__pin)data.append(curent)if last != curent:unchanged_count=0last=curentelse:unchanged_count+=1if unchanged_count>max_unchanged_count:breakreturn datadef __parse_data_pull_up_lengths(self, data):'''这部分完成了对数据的初步处理''' STATE_INIT_PULL_DOWN = 1STATE_INIT_PULL_UP = 2STATE_DATA_FIRST_PULL_DOWN = 3STATE_DATA_PULL_UP = 4STATE_DATA_PULL_DOWN = 5state = STATE_INIT_PULL_DOWNlengths = []  # will contain the lengths of data pull up periodscurrent_length = 0  # will contain the length of the previous periodfor i in range(len(data)):current = data[i]current_length += 1print(f"id:{i},value:{current},state:{state}")if state == STATE_INIT_PULL_DOWN:if current == RPi.GPIO.LOW:# ok, we got the initial pull downstate = STATE_INIT_PULL_UPcontinueelse:continueif state == STATE_INIT_PULL_UP:if current == RPi.GPIO.HIGH:# ok, we got the initial pull upstate = STATE_DATA_FIRST_PULL_DOWNcontinueelse:continueif state == STATE_DATA_FIRST_PULL_DOWN:if current == RPi.GPIO.LOW:# we have the initial pull down, the next will be the data pull upstate = STATE_DATA_PULL_UPcontinueelse:continueif state == STATE_DATA_PULL_UP:if current == RPi.GPIO.HIGH:# data pulled up, the length of this pull up will determine whether it is 0 or 1current_length = 0state = STATE_DATA_PULL_DOWNcontinueelse:continueif state == STATE_DATA_PULL_DOWN:if current == RPi.GPIO.LOW:# pulled down, we store the length of the previous pull up periodlengths.append(current_length)state = STATE_DATA_PULL_UPcontinueelse:continuereturn lengthsdef __calculate_bits(self, pull_up_lengths):'''这部分完成了对数据的整形''' # find shortest and longest periodshortest_pull_up = 1000longest_pull_up = 0for i in range(0, len(pull_up_lengths)):length = pull_up_lengths[i]if length < shortest_pull_up:shortest_pull_up = lengthif length > longest_pull_up:longest_pull_up = length# use the halfway to determine whether the period it is long or shorthalfway = shortest_pull_up + (longest_pull_up - shortest_pull_up) / 2bits = []for i in range(0, len(pull_up_lengths)):bit = Falseif pull_up_lengths[i] > halfway:bit = Truebits.append(bit)return bitsdef __bits_to_bytes(self, bits):'''bit转bytes''' the_bytes = []byte = 0for i in range(0, len(bits)):print(f"left before:{byte}")byte = byte << 1print(f"left after:{byte}")if (bits[i]):byte = byte | 1else:byte = byte | 0if ((i + 1) % 8 == 0):the_bytes.append(byte)byte = 0print(f"the_bytes:{the_bytes}")return the_bytesdef __calculate_checksum(self, the_bytes):return the_bytes[0] + the_bytes[1] + the_bytes[2] + the_bytes[3] & 255def read(self):'''读取数据''' RPi.GPIO.setup(self.__pin, RPi.GPIO.OUT)# send initial highself.__send_and_sleep(RPi.GPIO.HIGH, 0.05)# pull down to lowself.__send_and_sleep(RPi.GPIO.LOW, 0.02)# change to input using pull upRPi.GPIO.setup(self.__pin, RPi.GPIO.IN, RPi.GPIO.PUD_UP)# collect data into an arraydata = self.__collect_input()# parse lengths of all data pull up periodspull_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 periodsbits = self.__calculate_bits(pull_up_lengths)# we have the bits, calculate bytesthe_bytes = self.__bits_to_bytes(bits)# calculate checksum and checkchecksum = self.__calculate_checksum(the_bytes)if the_bytes[4] != checksum:return DHT11Result(DHT11Result.ERR_CRC, 0, 0)# ok, we have valid data# The meaning of the return sensor values# the_bytes[0]: humidity int# the_bytes[1]: humidity decimal# the_bytes[2]: temperature int# the_bytes[3]: temperature decimaltemperature = the_bytes[2] + float(the_bytes[3]) / 10humidity = the_bytes[0] + float(the_bytes[1]) / 10return DHT11Result(DHT11Result.ERR_NO_ERROR, temperature, humidity)

2 外层调用代码

# -*- coding: utf-8 -*-
"""
#这个实验完成了使用DHT11来读取温度和相对湿度
author = "Derek Tian"
version = '0.0.1'
make day=2022-01-25
"""
__docformat__ = "restructuredtext en"__all__ = []__license__ = "MIT license"import timeimport RPi.GPIO as GPIO
import DHT11 as dht11
import time
import datetime# initialize GPIO
GPIO.setwarnings(True)
GPIO.setmode(GPIO.BCM)
# read data using pin 14
instance = dht11.DHT11(pin=4)try:while True:result = instance.read()if result.is_valid():print("Last valid input: " + str(datetime.datetime.now()))print("Temperature: %-3.1f C" % result.temperature)print("Humidity: %-3.1f %%" % result.humidity)time.sleep(6)except KeyboardInterrupt:print("Cleanup")GPIO.cleanup()

四、 代码分析

1） 在主函数中：

在这里使用的RPi.GPIO的库，来直接操作GPIO

import RPi.GPIO as GPIO

设定使用BCM方式操作GPIO，设定DHT11 连接的针脚为4并初始化DHT11的工作类

GPIO.setwarnings(True)
GPIO.setmode(GPIO.BCM)
# read data using pin 14
instance = dht11.DHT11(pin=4)

使用循环来获取值：

 while True:result = instance.read()if result.is_valid():print("Last valid input: " + str(datetime.datetime.now()))print("Temperature: %-3.1f C" % result.temperature)print("Humidity: %-3.1f %%" % result.humidity)

2） 对DTH11.py 的分析

(1) 定义发送信号的函数，在这里同时定义了休眠时间（这个是关键，因为这是使用软件在模拟PWM的数字方式），因为需要多次调用，所以抽象为独立函数

 def __send_and_sleep(self, output, sleep):GPIO.output(self.__pin, output)time.sleep(sleep)

（2）__collect_input() 函数，接收采集到的数值

 def __collect_input(self):'''这部分完成了对输入数据的采集'''unchanged_count = 0max_unchanged_count = 100last=-1data=[]while True:curent=GPIO.input(self.__pin)data.append(curent)if last != curent:unchanged_count=0last=curentelse:unchanged_count+=1if unchanged_count>max_unchanged_count:breakreturn data

通过循环来接收信号采集到数字值，并设定读取失败的最大次数为100次。采集到的是0 or 1的数字信号
可以通过添加print打印并显示出来
（3）__parse_data_pull_up_lengths函数完成了对原始数据采集的类型整理
程序的关键是定义了5个状态

        STATE_INIT_PULL_DOWN = 1 #发送前的前置低电平STATE_INIT_PULL_UP = 2  #发送前的前置高电平，这里将告诉主机准备接收STATE_DATA_FIRST_PULL_DOWN = 3  # 发送低电平标志数据准备发送STATE_DATA_PULL_UP = 4  #发送数据STATE_DATA_PULL_DOWN = 5  #数据发送结束

通过循环处理接收到的数据包，挑出其中有效数据帧存储，下面是对于每一个bit值得处理输出：

当数据状态转换为“STATE_DATA_PULL_UP”并且新的bit位为高电平时，将初始化

 if state == STATE_DATA_PULL_UP:if current == RPi.GPIO.HIGH:# data pulled up, the length of this pull up will determine whether it is 0 or 1current_length = 0state = STATE_DATA_PULL_DOWNcontinueelse:continue

当数据状态为“STATE_DATA_PULL_DOWN”并且新的bit位为低电平值时，将count的值写入list中

            if state == STATE_DATA_PULL_DOWN:if current == RPi.GPIO.LOW:# pulled down, we store the length of the previous pull up periodlengths.append(current_length)state = STATE_DATA_PULL_UPcontinueelse:continue

最终返回的值：
（4）__calculate_bits函数完成了将数值从count值准换为二进制的工作，在这里作者采用了中间值来判断的方式。
设定初始的上下边界，0,1000.由于最大接收的字节有效bit不可能超过1000，所以这里设定为1000（个人因为比较浪费）

        shortest_pull_up = 1000longest_pull_up = 0

通过找出上下边界，计算中间值

 for i in range(0, len(pull_up_lengths)):length = pull_up_lengths[i]print(f"length:{length}|shortest_pull_up:{shortest_pull_up}|longest_pull_up:{longest_pull_up}")if length < shortest_pull_up:shortest_pull_up = lengthif length > longest_pull_up:longest_pull_up = length# use the halfway to determine whether the period it is long or shorthalfway = shortest_pull_up + (longest_pull_up - shortest_pull_up) / 2

根据中间值判断来生成二进制list

 for i in range(0, len(pull_up_lengths)):bit = Falseif pull_up_lengths[i] > halfway:bit = Truebits.append(bit)

（5）__bits_to_bytes 通过按位计算来获取采集到的值：
在这里，作者通过对bit位的左移操作来完成了2进制对10进制的转换。对bit位的连续左移到第8个bit位时，数值将是10进制的值。

 for i in range(0, len(bits)):print(f"left before:{byte}")byte = byte << 1print(f"left after:{byte}")if (bits[i]):byte = byte | 1else:byte = byte | 0if ((i + 1) % 8 == 0):the_bytes.append(byte)

（6）__calculate_checksum 将生成的4位数值写入一个数组，这里就不多说了。

red（）的读取过程
在函数初始化中，第一步是设定PIN针脚的工作方向：（输出、输入），这里设定为输出，用于激活DHT11模块

 RPi.GPIO.setup(self.__pin, RPi.GPIO.OUT)

调用 ”__send_and_sleep”函数来对针脚进行控制
输出一个高电平，并休眠0.05秒，激活DHT11模块

  self.__send_and_sleep(RPi.GPIO.HIGH, 0.05)

输出一个低电平，并休眠0.02秒，准备接收DHT11模块的输出值

 self.__send_and_sleep(RPi.GPIO.LOW, 0.02)

在休眠0.02秒后，将PIN针脚转为输入模式，并设定高电平为信号采集位

RPi.GPIO.setup(self.__pin, RPi.GPIO.IN, RPi.GPIO.PUD_UP)

接收PIN的输入

data = self.__collect_input() 

调用函数对接收到的数据帧进行状态转换

 pull_up_lengths = self.__parse_data_pull_up_lengths(data)

生成4个8字节的byte的数据和1个8byte的效验位数据，

 if len(pull_up_lengths) != 40:return DHT11Result(DHT11Result.ERR_MISSING_DATA, 0, 0)

 实际上这个效验位数据就是前4个byte数据的总和。

最后的63就是效验位

# calculate bits from lengths of the pull up periods
bits = self.__calculate_bits(pull_up_lengths)

    # we have the bits, calculate bytesthe_bytes = self.__bits_to_bytes(bits)# calculate checksum and checkchecksum = self.__calculate_checksum(the_bytes)if the_bytes[4] != checksum:return DHT11Result(DHT11Result.ERR_CRC, 0, 0)# ok, we have valid data# The meaning of the return sensor values# the_bytes[0]: humidity int# the_bytes[1]: humidity decimal# the_bytes[2]: temperature int# the_bytes[3]: temperature decimaltemperature = the_bytes[2] + float(the_bytes[3]) / 10humidity = the_bytes[0] + float(the_bytes[1]) / 10return DHT11Result(DHT11Result.ERR_NO_ERROR, temperature, humidity)修改代码，在最外层调用函数中添加按键处理和LED灯显示（按下时点亮，有数据时点亮）

# -*- coding: utf-8 -*-
"""
#这个实验完成了使用DHT11来读取温度和相对湿度
author = "Derek Tian"
version = '0.0.1'
make day=2022-01-25
"""
__docformat__ = "restructuredtext en"__all__ = []__license__ = "MIT license"import time
from signal import pauseimport RPi.GPIO as GPIO
import DHT11 as dht11
import time
import datetime
from  gpiozero import LED,Button# initialize GPIO
GPIO.setwarnings(True)
GPIO.setmode(GPIO.BCM)
# read data using pin 14
instance = dht11.DHT11(pin=4)button=Button(19)
red=LED(17)
yellow=LED(27)
bl=LED(5)
bl.off()
def getValue():red.on()result = instance.read()if result.is_valid():print("Last valid input: " + str(datetime.datetime.now()))print("Temperature: %-3.1f C" % result.temperature)print("Humidity: %-3.1f %%" % result.humidity)yellow.on()else:yellow.off()print("not get value !!!!")def fGetValue():red.off()yellow.off()if __name__=='__main__':try:button.when_pressed = getValuebutton.when_released = fGetValuepause()# while True:#     result = instance.read()#     if result.is_valid():#         print("Last valid input: " + str(datetime.datetime.now()))##         print("Temperature: %-3.1f C" % result.temperature)#         print("Humidity: %-3.1f %%" % result.humidity)##     time.sleep(6)except KeyboardInterrupt:print("Cleanup")GPIO.cleanup()

最终效果：

按下按键后红灯点亮，通过成功获取数值（黄色灯点亮）：


按下按键后红色灯点亮，获取数值失败，黄色灯没有点亮：

总结：

对于DHT11来说，获取信号的时钟信息时非常关键的，如果无法正确获取到时钟是无法获取值或激活传感器的。而python对于这种需要us来控制的代码不太擅长，如果使用c或golang比较好。