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winlinvip / SimpleDHT

Simple, Stable and Fast Arduino Temp & Humidity Sensors for DHT11 and DHT22. http://learn.adafruit.com/dht
MIT License
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Getting error `SimpleDHTErrStartLow` unnecessarily #35

Open timcharper opened 5 years ago

timcharper commented 5 years ago

In my DHT sensor, I'm consistently seeing a low time of 23-24us measured here:

https://github.com/winlinvip/SimpleDHT/blob/1.0.12/SimpleDHT.cpp#L220

I'm not sure if it's because the microcontroller I'm using has timing issues, or it's an odd DHT sensor, but simply reducing the threshold from 30 to 20 is the difference between the sensor failing continuously, and consistently reading successfully.

Would it make sense to allow this threshold to be customizable? Will it break things to allow slightly faster values?

timcharper commented 5 years ago

@winlinvip I experimented with this and found the issue; pinMode can take a variable amount of microseconds to complete. Further, per the spec, you're supposed to leave the pin HIGH for 25 microseconds, and then read (and expect a high value from the sensor within 80 microseconds).

I rewrote the timing code to use a stop-watch style approach and it made things much more stable; allowing the pinMode change to be included in the 80 microseconds for which we're waiting for the high signal from the sensor has completely erased the non-determinism I was seeing and I'm getting significantly more stable readings.

https://github.com/timcharper/UniversalDHT/blob/master/UniversalDHT.cpp#L132

winlinvip commented 5 years ago

👍