[DHT22] modified interpretation of temperature values in byte 2 and 3, such that negative temperature values are handled correctly

[bkormann80]

This pull request solves the issue described in #221.

Negative temperature values from a DHT22 sensor can be interpreted as a 16 bit signed integer (short). There is no additional differentiation needed.

float DHT::readTemperature(bool S, bool force) {
  float f = NAN;
  short s = 0;
...
    case DHT22:
      s = ((short) data[2]) << 8 | data[3];
      f = s * 0.1;
      if (S) {
        f = convertCtoF(f);
      }
      break;
... 

The patch was successfully tested using five different DHT22 sensors on an Arduino UNO and an ESP8266 at positive and negative temperatures.

[bkormann80]

Is any information missing?

[caternuson]

Negative temperature values from a DHT22 sensor can be interpreted as a 16 bit signed integer (short).

Are you sure about this? This doesn't look like twos complement, looks like sign/magnitude:

ex:

#include <stdio.h>
#include <stdint.h>

uint8_t  b2 = 0b10000000;
uint8_t  b3 = 0b01100101;

short t;

int main() {
  t  = ((short) b2) << 8 | b3;
  printf("t = %d\n", t);
  return 0;
}

yields:

t = -32667

Can you dump the actual bytes you are getting from whatever sensor you are using when there is a negative temperature?

[bkormann80]

I am sure. In issue #221 I described this mismatch between documentation and all of my sensors. Documentation doesn't describe two complement, but sensor returns data this way.
If you want to keep both variants, simply add this flavor as another sensor type like DHT22b.