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Removed "End the start signal by setting data line high for 40 micros… #51

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Removed "End the start signal by setting data line high for 40 micros… #51

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10 changes: 7 additions & 3 deletions DHT.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -152,9 +152,13 @@ boolean DHT::read(bool force) {
InterruptLock lock;

// End the start signal by setting data line high for 40 microseconds.
digitalWrite(_pin, HIGH);
delayMicroseconds(40);

// digitalWrite(_pin, HIGH);
// delayMicroseconds(40);
// WEB: This is an error. DHT tries to pull the line low as soon as it came back to high.
// therefore the low-high transition after the start signal "low" needs to be realised by
// the pullup resistor only, uC pin must already be in INPUT mode.
// ref: https://github.com/adafruit/DHT-sensor-library/issues/48#issue-119801608

// Now start reading the data line to get the value from the DHT sensor.
pinMode(_pin, INPUT_PULLUP);
delayMicroseconds(10); // Delay a bit to let sensor pull data line low.
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@vlad-sh vlad-sh Sep 20, 2016 •
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I found out there are must be some "smart" delay instead delayMicroseconds(10):

pinMode(_pin, INPUT_PULLUP); delayMicroseconds(10); // Delay a bit to let sensor pull data line low.

The delay may be like:

if (expectPulse(HIGH) == 0) { DEBUG_PRINTLN(F("Timeout waiting for start signal high pulse.")); _lastresult = false; return _lastresult; }

If no delay, next expectPulse(LOW) function often returns 0, because _pin is still HIGH (sensor takes from 20 to 200 microseconds to pull-down line to LOW, according datasheet).

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