Adafruit_DHT_Driver incorrect readings on long wires (10m) using DHT11 + possible code fix

[adafruit]

Adafruit_DHT_Driver incorrect readings on long wires (10m) using DHT11 + possible code fix

[gunkl]

@tdicola Fail, the new code in 1.3.1 also doesn't work for DHT11 over a long cable, in my case a rasp pi 1.

I made these changes and fixed it:
// Set pin at input.
pi_mmio_set_input(pin);
// Need a very short delay before reading pins or else value is sometimes still low.
//for (volatile int i = 0; i < 50; ++i) {
for (volatile int i = 0; i < 500; ++i) {
}

[bayshorebeachboy]

First thanks for this thread it seems to address the issue I am having with the dht-22, however; a question.

I made the changes to pi_2_dht_read.c and assume that this should be compiled but am unclear what (executable) to compile it to.

Or if it is compiled at runtime e.g. I am not getting the desired result i.e. I still get the same bad data on a long cable

[talkleasy]

Great piece of detective work! Solved the problem I was having on a 20 metre cable to the sensor.

Thanks very much for posting the solution.

[tdicola]

Thanks for raising the issue and apologies that it couldn't be reviewed earlier. We've actually deprecated this Adafruit_DHT_Driver code and refactored the DHT sensor reading C code into the Adafruit_Python_DHT library here: https://github.com/adafruit/Adafruit_Python_DHT The timing functions in the new library are slightly refactored to use nanosleep for longer delays and busy waiting for shorter delays. I'm curious if this helps with stability over long wires. I'll close this issue for now, but it's great to have as a reference to point folks towards if they are also using very long wires. Thanks again for raising the issue.

[etatus]

Thank you very much @gunkl!!!. Your fix worked like a charm for DHT22 over long cable in my RaspPi 1. (Adafruit_Python_DHT version 1.3.1)

[travc]

@tdicola, This should be reopened and @gunkl's fix (or something based on it) adopted.

[gunkl]

created pull request adafruit/Adafruit_Python_DHT#98

[etatus]

@gunkl approach stopped working for me and I find a new solution here: #161