dht: fix humidity and temperature extraction for DHT22 (#358)

dht/device.go

@@ -0,0 +1,44 @@
+package dht
+
+import (
+	"encoding/binary"
+)
+
+// DeviceType is the enum type for device type
+type DeviceType uint8
+
+const (
+	DHT11 DeviceType = iota
+	DHT22
+)
+
+// extractData parses information received from the sensor.
+// The 2 first buffers are for the humidity and
+// the 2 following corresponds to the temperature.
+func (d DeviceType) extractData(buf []byte) (temp int16, hum uint16) {
+	switch d {
+	case DHT11:
+		hum = 10*uint16(buf[0]) + uint16(buf[1])
+		temp = int16(buf[2])
+		if buf[3]&0x80 > 0 {
+			temp = -1 - temp
+		}
+		temp *= 10
+		temp += int16(buf[3] & 0x0f)
+	case DHT22:
+		hum = binary.BigEndian.Uint16(buf[0:2])
+		temp = int16(buf[2]&0x7f)<<8 + int16(buf[3])
+		// the first bit corresponds to the sign bit
+		if buf[2]&0x80 > 0 {
+			temp = -temp
+		}
+	default:
+		// keeping this for retro-compatibility but not tested
+		hum = binary.LittleEndian.Uint16(buf[0:2])
+		temp = int16(buf[3])<<8 + int16(buf[2]&0x7f)
+		if buf[2]&0x80 > 0 {
+			temp = -temp
+		}
+	}
+	return
+}