(r2) 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>

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---+ 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?

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---+ 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> | <img alt="dht11.png" height="236" src="%ATTACHURL%/dht11.png" title="dht11.png" width="144" /> |
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---+ 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

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---+ Reading and understanding the data sheet

Let’s have a look at the [[http://www.micropik.com/PDF/dht11.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" />

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

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

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---+ 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
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---+ The NTC Thermistor

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

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---+ 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

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---+ Text from the data sheet

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

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---+ Measurement Precision

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

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---+ How to connect the device

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

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---+ 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" />

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---+ Single Wire two way interface

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

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---+ Overall Communication Process

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

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---+ How to initiate a Measurement?

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

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---+ 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.
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---+ Response from DHT11<img alt="response.png" height="224" src="%ATTACHURL%/response.png" title="response.png" width="734" />

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---+ A zero bit

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

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---+ A one bit

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

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---+ 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 />Let’s save those onto a file and look at it with gnuplot<br /><br />

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

---++ Comments

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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	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
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	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: r2 - 2017-10-20 - uli