DHT22 lib working on 1Mhz?

[TimO]

I'm working with the library that is provided with the mysensors download.
Works with an internal clock set to 1 MHz for me.

[tomkxy]

Actually, I am using the same lib.
I found a thread in the Arduino forum which explains that it can not work in principle: http://forum.arduino.cc/index.php?topic=291695.0

Hmmm, quite interesting that it seems to work at least in certain cases.

[Stric]

how about 2 or 4MHz if 8 is too high?

[tomkxy]

@Stric said:

how about 2 or 4MHz if 8 is too high?

Will try and report back.

[tomkxy]

I tried it. Even on 4Mhz only the first 4 transmits show readings. Then it only returns errors.
So it only work on 8Mhz for me.

[tomkxy]

So after 2 weeks fiddling around with the different libraries at different clock frequencies, I have some conclusions I would like to share with you.

I tested with a ProMini and two DHT22 sensors, one from Adafruits and one from China.
As I already reported, I had problems with clock frequencies below 8MHz with all different libraries. After switching to 8MHz, I realized another problem. Temperature readings did not change. They stayed the same value, only when I reset the ProMini I had a correct reading.
I had this behaviour with the Adafruit and the library included in the MySensors project.

Since yesterday evening I am testing the library from Rob Tillaart (http://arduino.cc/playground/Main/DHTLib) which is working correctly giving me correct readings.
I am presuming that this is related to sleeping the ProMini and may be the low power mode the DHT22 is going into, I don't know...(at least in Rob's lib there is a wakeup delay which seems to address that point)

Anyway, it seems I have at least now a working solution. I will re-try to lower the clock frequency with that library.

Apart from that I also realized that the DHT library included in MySensor will perform a sensor read for a call to readTemperature and readHumidity which means that you would need to put a delay of 2s between a readTemperature and readHumidity. Since the sensor delivers the values in a "single" read this is rather unfortunate for battery powered sensor.

[hek]

If it verifies ok you could perhaps create a pull request with the new library and update the DHT example sketch accordingly?

[tomkxy]

Ok. I'll do that. Might however take a couple of days.

[tomkxy]

I retried again with Rob Tillaart's DHT22 library whether I can reduce clock frequency.
The results:

• 1 MHz -> wrong readings, like -100 C
• 2 MHz -> "
• 4 MHz -> from time to time wrong reading

Overall, I would state that you need to run the DHT22 with the libraries I tested on 8 Mhz.

[hek]

Ok, thanks for the update.

[tbowmo]

@tomkxy

Are you compiling for the different clock frequencies in arduino? Or is it compiled for 8/16Mhz target, and then you switch frequency in the sketch?

[tomkxy]

I used the Sensebender sketch which switches in the sketch.

[tlustoch]

Sorry for a stupid question, but how do you run at 1 Mhz?
See http://www.mysensors.org/hardware/micro#comment-2196823229
Are you able to switch to 1 Mhz at runtime?

[tomkxy]

Have a look at the sensebender sketch http://www.mysensors.org/hardware/micro#example-sketch

 
  if ((measureCount == 5) && highfreq) 
  {
    clock_prescale_set(clock_div_8); // Switch to 1Mhz for the reminder of the sketch, save power.
    highfreq = false;
  } 

The clock frequency is scaled down by software.