Added HC-SR04 ultrasonic distance sensor. (#143)

* Added HC-SR04 ultrasonic distance sensor.

hcsr04/hcsr04.go

@@ -0,0 +1,81 @@
+// Package hcsr04 provides a driver for the HC-SR04 ultrasonic distance sensor
+//
+// Datasheet:
+// https://cdn.sparkfun.com/datasheets/Sensors/Proximity/HCSR04.pdf
+package hcsr04
+
+import (
+	"machine"
+	"time"
+)
+
+const TIMEOUT = 23324 // max sensing distance (4m)
+
+// Device holds the pins
+type Device struct {
+	trigger machine.Pin
+	echo    machine.Pin
+}
+
+// New returns a new ultrasonic driver given 2 pins
+func New(trigger, echo machine.Pin) Device {
+	return Device{
+		trigger: trigger,
+		echo:    echo,
+	}
+}
+
+// Configure configures the pins of the Device
+func (d *Device) Configure() {
+	d.trigger.Configure(machine.PinConfig{Mode: machine.PinOutput})
+	d.echo.Configure(machine.PinConfig{Mode: machine.PinInput})
+}
+
+// ReadDistance returns the distance of the object in mm
+func (d *Device) ReadDistance() int32 {
+	pulse := d.ReadPulse()
+
+	// sound speed is 343000 mm/s
+	// pulse is roundtrip measured in microseconds
+	// distance = velocity * time
+	// 2 * distance = 343000 * (pulse/1000000)
+	return (pulse * 1715) / 10000 //mm
+}
+
+// ReadPulse returns the time of the pulse (roundtrip) in microseconds
+func (d *Device) ReadPulse() int32 {
+	t := time.Now()
+	d.trigger.Low()
+	time.Sleep(2 * time.Microsecond)
+	d.trigger.High()
+	time.Sleep(10 * time.Microsecond)
+	d.trigger.Low()
+	i := uint8(0)
+	for {
+		if d.echo.Get() {
+			t = time.Now()
+			break
+		}
+		i++
+		if i > 10 {
+			if time.Since(t).Microseconds() > TIMEOUT {
+				return 0
+			}
+			i = 0
+		}
+	}
+	i = 0
+	for {
+		if !d.echo.Get() {
+			return int32(time.Since(t).Microseconds())
+		}
+		i++
+		if i > 10 {
+			if time.Since(t).Microseconds() > TIMEOUT {
+				return 0
+			}
+			i = 0
+		}
+	}
+	return 0
+}