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?

Serial Protocol

How can we, with a single pin

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

The answer is: a serial protocol
and the DHT11 implements its own serial protocol

Reading and understanding the data sheet

Let\x92s 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

This sensor includes a resistive-type humidity measurement component and an NTC temperature measurement component,

and connects to a high-performance 8-bit microcontroller, offering excellent quality, fast response, anti-interference ability

and cost-effectiveness. Each DHT11 element is strictly calibrated in the laboratory that is extremely accurate on humidity calibration.

The calibration coefficients are stored as programmes in the OTP memory, which are used by the sensor\x92s internal signal detecting process.

The single-wire serial interface makes system integration quick and easy. Its small size, low power consumption and

up-to-20 meter signal transmission making it the best choice for various applications, including those most demanding ones.

The component is 4-pin single row pin package.

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

Communication Format can be seperated into three stages

1) Request
2) Response
3) Data Reading

Request Reading

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

Data Reading

Logic Analyzer

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

• Send request
• Read response
• Read each data segment and save it to a buffer
• Sum the segments and check if the result is the same as CheckSum

• If the CheckSum is correct, the values are correct so we can use them. If CheckSum is wrong we discard the packet.

  To read the data bits can use a counter and start count uSeconds of High level. For counts > 24uS we replace with bit '1'. For counts <=24 we replace with bit'0'

Reading the final data

The dht11 library uses a custom 1-wire protocol to get the measurements from the sensor.

The payload consists of a humidity value, a temperature value and a checksum.

Then measure and read their values with:

• measure()

• temperature()

• humidity() 

Starting the measurement

The data pin has to be programmed as output

d = dht.DHT11(machine.Pin(16))

And read data as

d.measure()
d.temperature() # eg. 23.6 (\xB0C)
d.humidity()    # eg. 41.3 (% RH)