Incorrect temperature value below 0 C with DHT22 sensor

[djsomi]

Device

Sonoff POW R2

Version

ESPURNA 1.19.0 built 2025-02-18 20:05:14

Bug description

I get invalid data from DHT22 if temperature under 0 C.

404.6°C shows when temperature is -4.6°C.

What can be the problem, and how to fix it?

Steps to reproduce

No response

Build tools used

PlatformIO, VScode

Any relevant log output (when available)

Decoded stack trace (when available)

[mcspr]

Looks like the problem is with the number representation (encoding) in memory. Current code expects one thing, while the device returns something completely different when number is negative.

Some references
adafruit/DHT-sensor-library#182
arendst/Tasmota#14165
arendst/Tasmota#14173
https://en.wikipedia.org/wiki/Signed_number_representations#Sign%E2%80%93magnitude

Sensor code has to
a) prefer s16 in initial conversion
b) check hardware temperature limits for the sake of sanity
c) detect sign-magnitude and swap bits for dht22s like yours in this block

espurna/code/espurna/sensors/DHTSensor.h

Lines 283 to 289 in 1366713

	} else {
	_temperature = (dhtData[2] & 0x7F) * 256 + dhtData[3];
	_temperature /= 10;
	if (dhtData[2] & 0x80) {
	_temperature *= -1;
	}
	}