Problem: DHT22 does not send data in sign-magnitude format

[argltuc]

• Arduino board 1: ESP8266 / NodeMCU 1.0 (ESP12-E Module)
• Arduino board 2: ESP-WROOM-32 / ESP32 DEVKIT V1
• Arduino IDE version (found in Arduino -> About Arduino menu): 1.8.13
• installed DHT-sensor-library version: 1.4.1 / installed by Arduino IDE Lib manager

This problem is directly connected to #177

After some more research on negative temperatures with DHT22 and ESP8266 / ESP32, i figure out, that my DHT22 sensor does not send data as expected.

This library assumed, sensor temperature data is in sign-magnitude format. This is also documented here: https://cdn-shop.adafruit.com/datasheets/Digital+humidity+and+temperature+sensor+AM2302.pdf

But this DHT22 which I have available for testing, seems to send data in two's complement format. With one exception: During transition from positive to negative values, there is one point with sign-magnitude. Going from 0.1°C (0x0001) -> 0°C (0x0000) -> -0°C (0x8000) -> -0.1°C (0xFFFF)

I have also a log of this, where I add a hex print of data[2]/data[3] at end of readTemperature():

22:07:01.473 -> 0003    0.3
22:07:01.473 -> Temperature: 0.30C
22:07:03.485 -> 8000    -3276.8
22:07:03.485 -> Temperature: -3276.80C
22:07:05.500 -> FFFD    -0.3
22:07:05.500 -> Temperature: -0.30C

[dl2sba]

Also my finding - I have fixed this code in DHT.cpp.

   case DHT22:
   case DHT21:
     int16_t d = ((int16_t)data[2]) << 8 | data[3];
	  //	msb set == negativ
	  if (d & 0x8000) {
		  d &= 0x7fff;
		  //	0x8000 == 0x0000 == 0)
		  if ( d == 0 ) d = 0x7fff;
		  d = 0x7fff - d;
		  d *= -1;
	  }
	  f = d * 0.1f;

     if (S) {
       f = convertCtoF(f);
     }
     break;

[JavanXD]

Having the same problem on Raspberry Pi using the latest Adafruit_DHT package.

[RamitArko]

I am also facing the same problem using Raspberry Pi 4