Implementation of the BME280 Sensor (#38)

* Driver for bme280

Makefile

@@ -41,5 +41,6 @@ smoke-test:
 	tinygo build -size short -o ./build/test.elf -target=microbit ./examples/waveshare-epd/epd2in13/main.go
 	tinygo build -size short -o ./build/test.elf -target=microbit ./examples/waveshare-epd/epd2in13x/main.go
 	tinygo build -size short -o ./build/test.elf -target=circuitplay-express ./examples/ws2812/main.go
+	tinygo build -size short -o ./build/test.elf -target=trinket-m0 ./examples/bme280/main.go
 
 test: clean fmt-check smoke-test

bme280/bme280.go

@@ -0,0 +1,251 @@
+package bme280
+
+import (
+	"machine"
+	"math"
+)
+
+// calibrationCoefficients reads at startup and stores the calibration coefficients
+type calibrationCoefficients struct {
+	t1 uint16
+	t2 int16
+	t3 int16
+	p1 uint16
+	p2 int16
+	p3 int16
+	p4 int16
+	p5 int16
+	p6 int16
+	p7 int16
+	p8 int16
+	p9 int16
+	h1 uint8
+	h2 int16
+	h3 uint8
+	h4 int16
+	h5 int16
+	h6 int8
+}
+
+// Device wraps an I2C connection to a BME280 device.
+type Device struct {
+	bus                     machine.I2C
+	Address                 uint16
+	calibrationCoefficients calibrationCoefficients
+}
+
+// New creates a new BME280 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,
+		Address: Address,
+	}
+}
+
+// Configure sets up the device for communication and
+// read the calibration coefficientes.
+func (d *Device) Configure() {
+
+	var data [24]byte
+	err := d.bus.ReadRegister(uint8(d.Address), REG_CALIBRATION, data[:])
+	if err != nil {
+		return
+	}
+
+	var h1 [1]byte
+	err = d.bus.ReadRegister(uint8(d.Address), REG_CALIBRATION_H1, h1[:])
+	if err != nil {
+		return
+	}
+
+	var h2lsb [7]byte
+	err = d.bus.ReadRegister(uint8(d.Address), REG_CALIBRATION_H2LSB, h2lsb[:])
+	if err != nil {
+		return
+	}
+
+	d.calibrationCoefficients.t1 = readUintLE(data[0], data[1])
+	d.calibrationCoefficients.t2 = readIntLE(data[2], data[3])
+	d.calibrationCoefficients.t3 = readIntLE(data[4], data[5])
+	d.calibrationCoefficients.p1 = readUintLE(data[6], data[7])
+	d.calibrationCoefficients.p2 = readIntLE(data[8], data[9])
+	d.calibrationCoefficients.p3 = readIntLE(data[10], data[11])
+	d.calibrationCoefficients.p4 = readIntLE(data[12], data[13])
+	d.calibrationCoefficients.p5 = readIntLE(data[14], data[15])
+	d.calibrationCoefficients.p6 = readIntLE(data[16], data[17])
+	d.calibrationCoefficients.p7 = readIntLE(data[18], data[19])
+	d.calibrationCoefficients.p8 = readIntLE(data[20], data[21])
+	d.calibrationCoefficients.p9 = readIntLE(data[22], data[23])
+
+	d.calibrationCoefficients.h1 = h1[0]
+	d.calibrationCoefficients.h2 = readIntLE(h2lsb[0], h2lsb[1])
+	d.calibrationCoefficients.h3 = h2lsb[2]
+	d.calibrationCoefficients.h6 = int8(h2lsb[6])
+	d.calibrationCoefficients.h4 = 0 + (int16(h2lsb[3]) << 4) | (int16(h2lsb[4] & 0x0F))
+	d.calibrationCoefficients.h5 = 0 + (int16(h2lsb[5]) << 4) | (int16(h2lsb[4]) >> 4)
+
+	d.bus.WriteRegister(uint8(d.Address), CTRL_HUMIDITY_ADDR, []byte{0x3f})
+	d.bus.WriteRegister(uint8(d.Address), CTRL_MEAS_ADDR, []byte{0xB7})
+	d.bus.WriteRegister(uint8(d.Address), CTRL_CONFIG, []byte{0x00})
+
+}
+
+// Connected returns whether a BME280 has been found.
+// It does a "who am I" request and checks the response.
+func (d *Device) Connected() bool {
+	data := []byte{0}
+	d.bus.ReadRegister(uint8(d.Address), WHO_AM_I, data)
+	return data[0] == CHIP_ID
+}
+
+// Reset the device
+func (d *Device) Reset() {
+	d.bus.WriteRegister(uint8(d.Address), CMD_RESET, []byte{0xB6})
+}
+
+// ReadTemperature returns the temperature in celsius milli degrees (ºC/1000)
+func (d *Device) ReadTemperature() (int32, error) {
+	data, err := d.readData()
+	if err != nil {
+		return 0, err
+	}
+
+	temp, _ := d.calculateTemp(data)
+	return temp, nil
+}
+
+// ReadPressure returns the pressure in milli pascals mPa
+func (d *Device) ReadPressure() (int32, error) {
+	data, err := d.readData()
+	if err != nil {
+		return 0, err
+	}
+	_, tFine := d.calculateTemp(data)
+	pressure := d.calculatePressure(data, tFine)
+	return pressure, nil
+}
+
+// ReadHumidity returns the relative humidity in hundredths of a percent
+func (d *Device) ReadHumidity() (int32, error) {
+	data, err := d.readData()
+	if err != nil {
+		return 0, err
+	}
+	_, tFine := d.calculateTemp(data)
+	humidity := d.calculateHumidity(data, tFine)
+	return humidity, nil
+}
+
+// ReadAltitude returns the current altitude in meters based on the
+// current barometric pressure and estimated pressure at sea level.
+// Calculation is based on code from Adafruit BME280 library
+// 	https://github.com/adafruit/Adafruit_BME280_Library
+func (d *Device) ReadAltitude() (alt int32, err error) {
+	mPa, _ := d.ReadPressure()
+	atmP := float32(mPa) / 100000
+	alt = int32(44330.0 * (1.0 - math.Pow(float64(atmP/SEALEVEL_PRESSURE), 0.1903)))
+	return
+}
+
+// convert2Bytes converts two bytes to int32
+func convert2Bytes(msb byte, lsb byte) int32 {
+	return int32(readUint(msb, lsb))
+}
+
+// convert3Bytes converts three bytes to int32
+func convert3Bytes(msb byte, b1 byte, lsb byte) int32 {
+	return int32(((((uint32(msb) << 8) | uint32(b1)) << 8) | uint32(lsb)) >> 4)
+}
+
+// readUint converts two bytes to uint16
+func readUint(msb byte, lsb byte) uint16 {
+	return (uint16(msb) << 8) | uint16(lsb)
+}
+
+// readUintLE converts two little endian bytes to uint16
+func readUintLE(msb byte, lsb byte) uint16 {
+	temp := readUint(msb, lsb)
+	return (temp >> 8) | (temp << 8)
+}
+
+// readIntLE converts two little endian bytes to int16
+func readIntLE(msb byte, lsb byte) int16 {
+	return int16(readUintLE(msb, lsb))
+}
+
+// readData does a burst read from 0xF7 to 0xF0 according to the datasheet
+// resulting in an slice with 8 bytes 0-2 = pressure / 3-5 = temperature / 6-7 = humidity
+func (d *Device) readData() (data [8]byte, err error) {
+	err = d.bus.ReadRegister(uint8(d.Address), REG_PRESSURE, data[:])
+	if err != nil {
+		println(err)
+		return
+	}
+	return
+}
+
+// calculateTemp uses the data slice and applies calibrations values on it to convert the value to milli degrees
+// it also calculates the variable tFine which is used by the pressure and humidity calculation
+func (d *Device) calculateTemp(data [8]byte) (int32, int32) {
+
+	rawTemp := convert3Bytes(data[3], data[4], data[5])
+
+	var1 := (((rawTemp >> 3) - (int32(d.calibrationCoefficients.t1) << 1)) * int32(d.calibrationCoefficients.t2)) >> 11
+	var2 := (((((rawTemp >> 4) - int32(d.calibrationCoefficients.t1)) * ((rawTemp >> 4) - int32(d.calibrationCoefficients.t1))) >> 12) * int32(d.calibrationCoefficients.t3)) >> 14
+
+	tFine := var1 + var2
+	T := (tFine*5 + 128) >> 8
+	return (10 * T), tFine
+}
+
+// calculatePressure uses the data slice and applies calibrations values on it to convert the value to milli pascals mPa
+func (d *Device) calculatePressure(data [8]byte, tFine int32) int32 {
+
+	rawPressure := convert3Bytes(data[0], data[1], data[2])
+
+	var1 := int64(tFine) - 128000
+	var2 := var1 * var1 * int64(d.calibrationCoefficients.p6)
+	var2 = var2 + ((var1 * int64(d.calibrationCoefficients.p5)) << 17)
+	var2 = var2 + (int64(d.calibrationCoefficients.p4) << 35)
+	var1 = ((var1 * var1 * int64(d.calibrationCoefficients.p3)) >> 8) + ((var1 * int64(d.calibrationCoefficients.p2)) << 12)
+	var1 = ((int64(1) << 47) + var1) * int64(d.calibrationCoefficients.p1) >> 33
+
+	if var1 == 0 {
+		return 0 // avoid exception caused by division by zero
+	}
+	p := int64(1048576 - rawPressure)
+	p = (((p << 31) - var2) * 3125) / var1
+	var1 = (int64(d.calibrationCoefficients.p9) * (p >> 13) * (p >> 13)) >> 25
+	var2 = (int64(d.calibrationCoefficients.p8) * p) >> 19
+
+	p = ((p + var1 + var2) >> 8) + (int64(d.calibrationCoefficients.p7) << 4)
+	p = (p / 256)
+	return int32(1000 * p)
+}
+
+// calculateHumidity uses the data slice and applies calibrations values on it to convert the value to relative humidity in hundredths of a percent
+func (d *Device) calculateHumidity(data [8]byte, tFine int32) int32 {
+
+	rawHumidity := convert2Bytes(data[6], data[7])
+
+	h := float32(tFine) - 76800
+
+	if h == 0 {
+		println("invalid value")
+	}
+
+	var1 := float32(rawHumidity) - (float32(d.calibrationCoefficients.h4)*64.0 +
+		(float32(d.calibrationCoefficients.h5) / 16384.0 * h))
+
+	var2 := float32(d.calibrationCoefficients.h2) / 65536.0 *
+		(1.0 + float32(d.calibrationCoefficients.h6)/67108864.0*h*
+			(1.0+float32(d.calibrationCoefficients.h3)/67108864.0*h))
+
+	h = var1 * var2
+	h = h * (1 - float32(d.calibrationCoefficients.h1)*h/524288)
+	return int32(100 * h)
+
+}

bme280/registers.go

@@ -0,0 +1,25 @@
+package bme280
+
+// Constants/addresses used for I2C.
+
+// The I2C address which this device listens to.
+const Address = 0x76
+
+// Registers. Names, addresses and comments copied from the datasheet.
+const (
+	CTRL_MEAS_ADDR        = 0xF4
+	CTRL_HUMIDITY_ADDR    = 0xF2
+	CTRL_CONFIG           = 0xF5
+	REG_PRESSURE          = 0xF7
+	REG_CALIBRATION       = 0x88
+	REG_CALIBRATION_H1    = 0xA1
+	REG_CALIBRATION_H2LSB = 0xE1
+	CMD_RESET             = 0xE0
+
+	WHO_AM_I = 0xD0
+	CHIP_ID  = 0x60
+)
+
+const (
+	SEALEVEL_PRESSURE float32 = 1013.25 // in hPa
+)

examples/bme280/main.go

@@ -0,0 +1,35 @@
+package main
+
+import (
+	"machine"
+	"strconv"
+	"time"
+
+	"tinygo.org/x/drivers/bme280"
+)
+
+func main() {
+
+	machine.I2C0.Configure(machine.I2CConfig{})
+	sensor := bme280.New(machine.I2C0)
+	sensor.Configure()
+
+	connected := sensor.Connected()
+	if !connected {
+		println("BME280 not detected")
+	}
+	println("BME280 detected")
+
+	for {
+		temp, _ := sensor.ReadTemperature()
+		println("Temperature:", strconv.FormatFloat(float64(temp)/1000, 'f', 2, 64), "ºC")
+		press, _ := sensor.ReadPressure()
+		println("Pressure:", strconv.FormatFloat(float64(press)/100000, 'f', 2, 64), "hPa")
+		hum, _ := sensor.ReadHumidity()
+		println("Humidity:", strconv.FormatFloat(float64(hum)/100, 'f', 2, 64), "%")
+		alt, _ := sensor.ReadAltitude()
+		println("Altitude:", alt, "m")
+
+		time.Sleep(2 * time.Second)
+	}
+}