Lecture 8

Uli Raich

UCC semester 2017/2018

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.

Can one do more with a single pin?

DHT11 pinout

The DHT11 has only 3 pins: Vcc Ground one data pin

Serial Protocol

How can we, with a single pin

• initiate a measurement
• read humidity and temperature data
• make sure the data are correct?

Reading and understanding the data sheet

Let’s have a look at the DHT11 data sheet:

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

How does a resistive humidity measurement work?

Resistive Humidity Measurement(2)

• Variations in relative humidity produce changes variations
  of resistivity,which is measured.
• The resistance of certain hygroscopic materials such as
  hygroscopic salts have such a property
• These materials are mounted as thin films on
  the substrate with metal contacts
• The devices can be very small

The NTC Thermistor

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

Measurement Precision

How to connect the device

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

Single Wire two way interface

Overall Communication Process

How to initiate a Measurement?

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
  keep it low for a minimum of 18 ms
• We must pull it high again and wait for another 20-40 μs
• Finally we must re-program the pin as input and wait for the DHT11 to respond.

Response from DHT11

A zero bit

A one bit

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
  and print out the information?

Slide 1: DHT11 Temperature and Humidity Sensor

-- Uli Raich - 2017-10-20

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