Added driver for BH1750 digital ambient light sensor (#15)

* Added driver for BH1750 digital ambient light sensor

bh1750/bh1750.go

@@ -0,0 +1,70 @@
+// Package bh1750 provides a driver for the BH1750 digital Ambient Light
+//
+// Datasheet:
+// https://www.mouser.com/ds/2/348/bh1750fvi-e-186247.pdf
+//
+package bh1750
+
+import (
+	"time"
+
+	"machine"
+)
+
+// SamplingMode is the sampling's resolution of the measurement
+type SamplingMode byte
+
+// Device wraps an I2C connection to a bh1750 device.
+type Device struct {
+	bus  machine.I2C
+	mode SamplingMode
+}
+
+// New creates a new bh1750 connection. The I2C bus must already be
+// configured.
+//
+// This function only creates the Device object, it does not touch the device.
+func New(bus machine.I2C) Device {
+	return Device{
+		bus:  bus,
+		mode: CONTINUOUS_HIGH_RES_MODE,
+	}
+}
+
+// Configure sets up the device for communication
+func (d *Device) Configure() {
+	d.bus.Tx(Address, []byte{POWER_ON}, nil)
+	d.SetMode(d.mode)
+}
+
+// RawSensorData returns the raw value from the bh1750
+func (d *Device) RawSensorData() uint16 {
+
+	buf := []byte{1, 0}
+	d.bus.Tx(Address, nil, buf)
+	return (uint16(buf[0]) << 8) | uint16(buf[1])
+}
+
+// Illuminance returns the adjusted value in mlx (milliLux)
+func (d *Device) Illuminance() int32 {
+
+	lux := uint32(d.RawSensorData())
+	var coef uint32
+	if d.mode == CONTINUOUS_HIGH_RES_MODE || d.mode == ONE_TIME_HIGH_RES_MODE {
+		coef = HIGH_RES
+	} else if d.mode == CONTINUOUS_HIGH_RES_MODE_2 || d.mode == ONE_TIME_HIGH_RES_MODE_2 {
+		coef = HIGH_RES2
+	} else {
+		coef = LOW_RES
+	}
+	// 100 * coef * lux * (5/6)
+	// 5/6 = measurement accuracy as per the datasheet
+	return int32(250 * coef * lux / 3)
+}
+
+// SetMode changes the reading mode for the sensor
+func (d *Device) SetMode(mode SamplingMode) {
+	d.mode = mode
+	d.bus.Tx(Address, []byte{byte(d.mode)}, nil)
+	time.Sleep(10 * time.Millisecond)
+}