Exploring Edison - Bit Banging the DHT11/DHT22
Monday, 25 January 2016
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Exploring Edison - Bit Banging the DHT11/DHT22
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In this chapter we make use of all of the ideas introduced in earlier chapters to create a raw interface with the low cost DHT11/22 temperature and humidity sensor. It is an exercise in interfacing two logic families and implementing a protocol directly in C. 

This is a chapter from our ebook on the Intel Edison. The full contents can be seen below. 

As this is a work in progress you can use the comments or email harry.fairhead@i-programmer.info with your queries or suggestions.




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Chapter List

  1. Meet Edison
    In this chapter we consider the Edison's pros and cons and get an overview of its structure and the ways in which you can make use of it. If you have ever wondered if you need an Edison or an Arduino or even a Raspberry Pi then this is the place to start. 

  2. First Contact
    When you are prototyping with the Edison you are going to need to use one of the two main breakout boards - the Arduino or the mini. This chapter explains how to set up the Edison for both configurations. 

  3. In C
    You can program the Edison in Python, JavaScript or C/C+ but there are big advantages in choosing C. It is fast, almost as easy as the other languages and gives you direct access to everything. It is worth the effort and in this chapter we show you how to set up the IDE and get coding. 

  4. Mraa GPIO
    Using the mraa library is the direct way to work with the GPIO lines and you have to master it. Output is easy but you do need to be aware of how long everything takes. Input is also easy but using it can be more difficult. You can use polling or the Edison interrupt system which might not work exactly as you would expect.

  5. Fast Memory Mapped I/O
    There is a faster way to work with GPIO lines - memory mapped I/O. Using this it is possible to generate pulses as short at 0.25 microsecond and read pulse widths of 5 microseconds. However getting things right can be tricky. We look at how to generate fast accurate pulses of a given width and how to measure pulse widths.

  6. Near Realtime Linux 
    You need to be aware how running your programs under a non-realtime operating system like Yocto Linux effects timings and how accurately you can create pulse trains and react to the outside world. In this chapter we look the realtime facilities in every version of Linux. 

  7. Sophisticated GPIO - Pulse Width Modulation 
    Using the PWM mode of the GPIO lines is often the best way of solving control problems. PWM means you can dim an LED or position a servo and all using mraa. 

  8. Sophisticated GPIO -  I2C 
    I2C is a simple communications bus that allows you to connect any of a very large range of sensors. 

  9. I2C - Measuring Temperature  
    After looking at the theory of using I2C here is a complete case study using the SparkFun HTU21D hardware and software. 
  10. Life At 1.8V
    How to convert a 1.8V input or output to work with 5V or 3.3V including how to deal with bidirectional pull-up buses.

  11. Using the DHT11/22 Temperature Humidity Sensor at 1.8V 
    In this chapter we make use of all of the ideas introduced in earlier chapters to create a raw interface with the low cost DHT11/22 temperature and humidity sensor. It is an exercise in interfacing two logic families and implementing a protocol directly in C. 

  12. The DS18B20 1-Wire Temperature 
    The Edison doesn't have built in support for the Maxim 1-Wire bus and this means you can't use the very popular DS18B20 temperature sensor. However with a little careful planning you can and you can do it from user rather than kernel space. 

  13. Using the SPI Bus 
    The SPI bus can be something of a problem because it doesn't have a well defined standard that every device conforms to. Even so, if you only want to work with one specific device it is usually easy to find a configuration that works - as long as you understand what the possibilities are. 

  14. SPI in Practice The MCP3008 AtoD 
    The SPI bus can be difficult to make work at first, but once you know what to look for about how the slave claims to work it gets easier. To demonstrate how its done let's add eight channels of 12-bit AtoD using the MCP3008.

  15. Beyond mraa - Controlling the features mraa doesn't. 
    There is a Linux-based approach to working with GPIO lines and serial buses that is worth knowing about because it provides an alternative to using the mraa library. Sometimes you need this because you are working in a language for which mraa isn't available. It also lets you access features that mraa doesn't make available. 


The Device

The DHT22 is a more accurate version of the DHT11 and it is used in this project but the hardware and software will work with both version and with the AM2302 which is similar to the DHT22. 

Model AM2302/DHT22

Power supply 3.3-5.5V DC
Output signal digital signal via 1-wire bus 
Sensing element Polymer humidity capacitor
Operating range
  humidity 0-100%RH;
  temperature -40~80Celsius
 humidity +-2%RH(Max +-5%RH);
 temperature +-0.5Celsius
Resolution or sensitivity
 humidity 0.1%RH;
 temperature 0.1Celsius
 humidity +-1%RH;
 temperature +-0.2Celsius



So the device will work at 3.3V and it makes use of a 1-wire open collector style bus which will need to be converted to a 1.8V bus to work with the Edison. Unfortunately the one wire bus isn't standard and is only used by this family of devices so we have little choice but to implement the protocol in C. 


 The pin outs are:

  1. VDD
  2. SDA serial data
  3. not used
  4. GND

and the standard way of connecting the device is:


The serial protocol is also fairly simple:

  • The host pulls the line low for between 0.8 and 29 ms,
    usually 1ms

  • It then releases the bus which is pulled high 

  • After between 20 and 200 microseconds, usually 30 microseconds, the device starts to send data by pulling the line down for around 80 microseconds and then lets float high for another 80 microseconds. 

  • Next 80 bits of data are sent using a 70 microsecond high for a 1 and a 26 microsecond high for a zero the high pluses are separated by arond 50 micorosecond low period. 




So what we have to do is pull the line low for 1ms or so to start the device sending data and this is very easy. Then we have to wait for the device to pull the line down and let it pull up again - about 160 microsecond and then read the time that the line is high 80 times.

A one corresponds to 70 microseconds and a zero corresponds to 26 microseconds. This is within the range of pulse measurement that can be achieved using fast memory mapped I/O. There is also a 50 microsecond period between each data bit an this can be used to do some limited processing. Notice that we are only interested in the time that the line is held high. 





Last Updated ( Friday, 03 June 2016 )