Lecture8:DHT11TemperatureAndHumiditySensor < Embedded

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---+ DHT11 Temperature and Humidity Sensor
<h2 align="center">Lecture 8</h2> <h2 align="center">Uli Raich</h2> <h2 align="center">UCC semester 2017/2018</h2>

---++ 
---+ A single GPIO pin

We have seen how we can drive an LED from a single GPIO pin

programmed as output pin or how we can read its state

through another GPIO pin, programmed as input pin.<br /><br />Can one do more with a single pin?

---++ 

---+ DHT11 pinout
| The DHT11 has only<br />3 pins:<br /><br /> <ul> <li>Vcc</li> </ul><ul> <li>Ground</li> </ul><ul> <li>one data pin</li> </ul><p>Be careful:<br /><b>The pinout of the DHT11 in our sensor kit</b><br /><b>is different from the one shown on the photo!</b></p> <p>The PCB layout of the PCB board in my<br />sensor kit is different!</p> | <img alt="dht11.png" height="236" src="%ATTACHURL%/dht11.png" title="dht11.png" width="144" /> |
---++ 
---+ Serial Protocol

How can we, with a single pin<br /><br />
   * initiate a measurement
   * read humidity and temperature data
   * make sure the data are correct?
The answer is: <b>a serial protocol</b><br />and the DHT11 implements its own serial protocol

---++ 
---+ Reading and understanding the data sheet

Let’s have a look at the [[https://www.mouser.com/datasheet/2/758/DHT11-Technical-Data-Sheet-Translated-Version-1143054.pdf][ DHT11 data sheet]]:

Most of the following information is just a copy from the data sheet.

<img alt="dht11Intro.png" height="245" src="%ATTACHURL%/dht11Intro.png" title="dht11Intro.png" width="765" />

---++ 
---+ How does a resistive humidity measurement work?

<img alt="resistiveHumiMeas.png" height="191" src="%ATTACHURL%/resistiveHumiMeas.png" title="resistiveHumiMeas.png" width="319" />

---++ 
---+ Resistive Humidity Measurement(2)
   * Variations in relative humidity produce changes variations <br />of resistivity,which is measured.
   * The resistance of certain hygroscopic materials such as<br />hygroscopic salts have such a property
   * These materials are mounted as thin films on <br />the substrate with metal contacts
   * The devices can be very small
---++ 
---+ The NTC Thermistor

<img alt="ntcThermistor.png" height="398" src="%ATTACHURL%/ntcThermistor.png" title="ntcThermistor.png" width="728" />

---++ 
---+ A processor on chip

In order to convert these measurements into numeric values

and send them to the end user through a serial protocol,

a preprogrammed micro-controller must be implemented on the chip.

In the case of the DHT11 this is an 8 bit micro-controller,

which does the conversion into binary and which creates the serial protocol

---++ 
---+ Text from the data sheet

<img alt="humiCalib.png" height="186" src="%ATTACHURL%/humiCalib.png" title="humiCalib.png" width="746" />

---++ 
---+ Measurement Precision

<img alt="dht11Specs.png" height="133" src="%ATTACHURL%/dht11Specs.png" title="dht11Specs.png" width="774" />

---++ 
---+ How to connect the device

<img alt="connections.png" height="368" src="%ATTACHURL%/connections.png" title="connections.png" width="650" />

---++ 
---+ Powering the device

As we can see from the specs below, the DHT11 power line

can be directly connected to the cobbler 3.3V (or the 5V) line

<img alt="power.png" height="150" src="%ATTACHURL%/power.png" title="power.png" width="711" />

---++ 
---+ Single Wire two way interface

<img alt="twoWay.png" height="258" src="%ATTACHURL%/twoWay.png" title="twoWay.png" width="744" />

---++ 
---+ Overall Communication Process

<img alt="overall.png" height="493" src="%ATTACHURL%/overall.png" title="overall.png" width="804" />

---++ 
---+ How to initiate a Measurement?

<img alt="initiate.png" height="420" src="%ATTACHURL%/initiate.png" title="initiate.png" width="769" />

---++ 
---+ What does this mean for our program?

We must:
   * Program the GPIO pin onto which we connected our DHT11 as output
   * We must pull this line done (send a login level zero) and <br />keep it low for a minimum of 18 ms
   * We must pull it high again and wait for another 20-40 &mu;s
   * Finally we must re-program the pin as input and wait for the DHT11 to respond.
---++ 
---+ Response from the DHT11

<img alt="response.png" height="224" src="%ATTACHURL%/response.png" title="response.png" width="734" />

---++ 
---+ A zero bit

<img alt="zero.png" height="484" src="%ATTACHURL%/zero.png" title="zero.png" width="732" />

---++ 
---+ A one bit

<img alt="one.png" height="438" src="%ATTACHURL%/one.png" title="one.png" width="770" />

---++ 
---+ Complicated?

Wow, this looks complicated. How can we write a program to do all this?

Let's start slowly:
   * Initiating a measurement seems do-able
   * Why not just read the data from the device every 5 µs <br />and print out the information?
The zeros and ones do not look too convincing. <br /><br />
---+ What the user of the device wants

The user of the device would like to have a library which hides

all these details. He wants functions to<br /><br />
   * Initialize the device
   * Start a measurement
   * Make sure the checksum is ok
   * Read temperature and humidity values
   * Maybe know when the measurement was made
---++ 
---+ The library

How do we write a library?<br /><br />A library consist of min. 2 files:<br /><br />
   * an include file (dht11.h)
   * and implementation file (dht11.c)
Very often you have a single include but several implementation files.

---++ 
---+ The include file of the dht11 library

<img alt="dht11H-1.png" height="343" src="%ATTACHURL%/dht11H-1.png" title="dht11H-1.png" width="370" />

---++ 
---+ dht11.h (2)

<img alt="dht11H-2.png" height="300" src="%ATTACHURL%/dht11H-2.png" title="dht11H-2.png" width="385" />

---++ 
---+ The library code
   * dht11Init simply calls wiringPiSetup()
   * dht11Measurement
   * Programs the gpio pin as output and sends the start sequence
   * Switches back the gpio pin to input and reads the data coming from the dht11 every 5 µs and saves the data into an array
   * Analyses the data and extracts temperature, humidity and the checksum
   * Calculates the checksum and compares it with the one coming from the device
   * If checksums are ok, sends the data to “validData”
   * Keeps the time stamp of the measurement

---++ 
---+ Reading the final data
   * dht11GetValidTemperature()
   * dht11GetValidHumidity()
   * dht11getValidMeasTime()
   * dht11getChecksum()
   * dht11getDeviceChecksum()
---++ 
---+ Starting the measurement

The data pin has been programmed as output earlier

<img alt="measurement.png" height="217" src="%ATTACHURL%/measurement.png" title="measurement.png" width="609" />

You can print these data and have a look at them with gnuplot. Try analyzing be hand.

---++ 
---+ Analyzing the data

This is the tricky bit and you are invited to give it a try. I have written a

routine reading a single bit, which is called 40 times for all the 40 bits in the data.

Please note one nasty problem:<br />Linux is a multi-tasking system and permanently receives interrupts.

While these interrupts are treated the DHT11 data are continuing to flow

but are not taking into account, leading to corrupt data.

Since the duration of the protocol takes ~ 4 ms the probability is non negligible

and the checksum test in obligatory to make sure the data are consistent.

In case of a checksum error you must repeat the measurement

---++ 
---+ Creating a shared library

The Makefile says it all

<img alt="shareLib.png" height="402" src="%ATTACHURL%/shareLib.png" title="shareLib.png" width="545" />

---++ 
---+ Where is the include file, where the binary of the shared lib?

I installed the include file in /opt/ucc/include

and the library in /opt/ucc/lib.

These are non-standard directories and must be declared in the Makefile

-I /opt/ucc/include for the include file in CFLAGS

-L /opt/ucc/lib for the library path

Since the shared library is loaded separately when running the

main program, it must be found by the system:

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/opt/ucc/lib
---++ 
---+ Documentation

When writing a library that is supposed to be used by other people,

documentation is of utmost importance

Of course the function prototypes in the include file give

some indication but this normally not enough.

I use the _doxygen_ in code documentation system which allows to

generate documentation from the source layout and

comments in the code with special tags.

It creates html of latex doc

<br />%SLIDESHOWEND%

-- %USERSIG{UliRaich - 2017-10-20}%

---++ Comments

<br />%COMMENT%

Attachments

Topic attachments
I	Attachment	History	Action	Size	Date	Who
png	connections.png	r1	manage	42.6 K	2017-10-20 - 16:27	UnknownUser
png	dht11.png	r1	manage	597.9 K	2017-10-20 - 16:34	UnknownUser
png	dht11H-1.png	r1	manage	24.8 K	2017-10-23 - 13:47	UnknownUser
png	dht11H-2.png	r1	manage	22.3 K	2017-11-01 - 08:05	UnknownUser
png	dht11H-2v2.png	r1	manage	22.3 K	2017-11-01 - 08:05	UnknownUser
png	dht11Intro.png	r1	manage	26.5 K	2017-10-20 - 16:27	UnknownUser
png	dht11Specs.png	r1	manage	11.5 K	2017-10-20 - 16:27	UnknownUser
png	humiCalib.png	r1	manage	24.3 K	2017-10-20 - 16:27	UnknownUser
png	initiate.png	r1	manage	153.8 K	2017-10-20 - 16:27	UnknownUser
odp	lecture_8.odp	r1	manage	1715.1 K	2017-11-01 - 09:25	UnknownUser
png	measurement.png	r1	manage	29.9 K	2017-10-23 - 13:55	UnknownUser
png	ntcThermistor.png	r1	manage	80.6 K	2017-10-20 - 16:27	UnknownUser
png	one.png	r1	manage	149.6 K	2017-10-20 - 16:27	UnknownUser
png	overall.png	r1	manage	180.3 K	2017-10-20 - 16:27	UnknownUser
png	power.png	r1	manage	14.5 K	2017-10-20 - 16:27	UnknownUser
png	resistiveHumiMeas.png	r1	manage	54.9 K	2017-10-20 - 16:27	UnknownUser
png	response.png	r1	manage	28.8 K	2017-10-20 - 16:28	UnknownUser
png	shareLib.png	r1	manage	27.9 K	2017-10-23 - 14:25	UnknownUser
png	twoWay.png	r1	manage	32.0 K	2017-10-20 - 16:28	UnknownUser
png	zero.png	r1	manage	181.6 K	2017-10-20 - 16:28	UnknownUser

Topic revision: r7 - 2023-11-19 - UliRaich