Added DHT22 example

examples/dht22/dht22.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"log"
+	"time"
+
+	"github.com/stianeikeland/go-rpio/v4"
+)
+
+func main() {
+	err := rpio.Open()
+	if err != nil {
+		log.Panicf("Could not open GPIO: %s", err)
+	}
+	// change the pin number to the BCM pin where your DHT22 is attached
+	log.Println(readDHT22(rpio.Pin(24)))
+}
+
+func readDHT22(pin rpio.Pin) (float32, float32, error) {
+	// let data line be pulled high by pull-up
+	pin.Input()
+	time.Sleep(1000 * time.Microsecond)
+	// pull data line low for 1100 micros to signal ready to read
+	pin.Output()
+	pin.Low()
+	time.Sleep(1100 * time.Microsecond)
+	// leave data line floating again, sensor writes data now
+	pin.Input()
+	pos := 0
+	var now int64
+	level := rpio.Low
+	lastChange := time.Now().UnixMicro()
+	syncCycles, dataCycles := make([]uint16, 50), make([]uint16, 50)
+	// wait for incoming pulses from sensor until buffer is full or timeout is reached
+	for {
+		now = time.Now().UnixMicro()
+		if pin.Read() != level {
+			if level == rpio.Low {
+				level = rpio.High
+				// level changed to HIGH, sync cycle
+				syncCycles[pos] = uint16(now - lastChange)
+
+			} else {
+				level = rpio.Low
+				// level changed to LOW, data cycle
+				dataCycles[pos] = uint16(now - lastChange)
+				// increment position
+				pos++
+				if pos >= 50 {
+					// buffer is full, stop reading
+					break
+				}
+			}
+			lastChange = now
+		} else if now-lastChange >= 8000 {
+			// pin does not change anymore, stop reading
+			break
+		}
+	}
+	// we need at least 40 pulses for a valid data packet
+	if pos < 40 {
+		return 0, 0, fmt.Errorf("timeout: %d packets received", pos)
+	}
+	// calculate average sync pulse duration
+	offset := pos - 40
+	var syncAverage float32 = 0
+	for i := offset; i < 40; i++ {
+		syncAverage += float32(syncCycles[i])
+	}
+	syncAverage /= 40
+	// extract data bits
+	data := make([]uint8, 5)
+	for i := 0; i < 40; i++ {
+		if dataCycles[i+offset] > uint16(syncAverage) {
+			data[i/8] |= 1 << (7 - i%8)
+		}
+	}
+	// verify checksum
+	if data[4] != ((data[0] + data[1] + data[2] + data[3]) & 0xFF) {
+		return 0, 0, errors.New("checksum error")
+	}
+	// calculate temperature and humidity
+	var tmp, hum float32
+	tmp = float32(uint16(data[2]&0x7F)<<8|uint16(data[3])) * 0.1
+	hum = float32(uint16(data[0])<<8|uint16(data[1])) * 0.1
+	if tmp < -40 || tmp > 80 || hum < 0 || hum > 100 {
+		return 0, 0, errors.New("out of range")
+	}
+	return tmp, hum, nil
+}