RobTillaart/DHTNew

DTH11 gets too short start impulse

arnowa opened this issue · 16 comments

arnowa commented

I just tried this library with a DHT11. Did not work. A test with an oscilloscope showed a LOW pulse of just 3.3ms, while the DHT11 needs 18ms.

The problem seems to be calling delayMicroseconds() with a value larger than 16383: https://www.arduino.cc/reference/en/language/functions/time/delaymicroseconds/

A very simple fix is (line 329 in dhtnew.cpp):

delayMicroseconds(_wakeupDelay * 1100UL);
->
delayMicroseconds(_wakeupDelay * 550UL);
delayMicroseconds(_wakeupDelay * 550UL);

RobTillaart commented

Thank you for reporting plus providing a simple solution.

RobTillaart commented

Which processor do you use?
Which IDE version/ develop platform?
Thanks

RobTillaart commented

There is a related issue with the delayMicroseconds(18000).
Some applications using RTOS need to call yield() for context switching, e.g. to handle a Wifi incoming buffer etc.
So the solution will be something more "complicated" than just splitting the call.

RobTillaart commented

@arnowa
Created a Pull Request #73 with a rewrite of the wake-up delay.
Please verify if this fix works for you.

Replaced the delayMicroseconds(_wakeupDelay * 1100UL); with a loop that calls yield() every iteration.
For a DHT22 alike it will be called only once, for the DHT11 alike it will be called 18 times.

  // WAKE UP - add 10% extra for timing inaccuracies in sensor.
  uint32_t startWakeup = micros();
  do
  {
    // HANDLE PENDING IRQ
    yield();
    delayMicroseconds(1100UL);
  }
  while((micros() - startWakeup) < (_wakeupDelay * 1100UL));

Non-RTOS environments (e.g. UNO) implement yield() as an empty function which will be optimized away.

arnowa commented
arnowa commented
arnowa commented
RobTillaart commented

Good point, could make it 550.

RobTillaart commented

@arnowa

Which board do you use?
Which IDE?

arnowa commented

plain cheapduino Uno R3 clone (i.e. ATMega 328) and latest Arduino IDE 1.8.19 on 64 bit Linux.

RobTillaart commented

Thanks.
Did some math and every 360 us would match 18 x 1100 = 19.800 exactly. Given the loop has some overhead it would at most 'overflow' 359 us.
3x better than 1100.

arnowa commented
RobTillaart commented

plain cheapduino Uno R3 clone (i.e. ATMega 328) and latest Arduino IDE 1.8.19 on 64 bit Linux.

Ok that allows me to recreate/test a bit

RobTillaart commented

Did some more math/testruns and came to 180.

For DHT11 ==> 19808 us
For DHT22 ==> 1104 us

So quite a perfect (including loop overhead).

  // WAKE UP - add 10% extra for timing inaccuracies in sensor.
  uint32_t startWakeup = micros();
  do
  {
    // HANDLE PENDING IRQ
    yield();
    delayMicroseconds(180UL);
  }
  while((micros() - startWakeup) < (_wakeupDelay * 1100UL));
RobTillaart commented

The problem seems to be calling delayMicroseconds() with a value larger than 16383: https://www.arduino.cc/reference/en/language/functions/time/delaymicroseconds/

The parameter of delayMicros() is unsigned int that implies one could go up to 65535
(it is hand-tuned code, so I stop digging)

parameter us is multiplied by 4 here causing overflow...

	// the following loop takes 1/4 of a microsecond (4 cycles)
	// per iteration, so execute it four times for each microsecond of
	// delay requested.
	us <<= 2; // x4 us, = 4 cycles
RobTillaart commented

Solution merged in master