bme280_driver.go

package i2c

import (
	"bytes"
	"encoding/binary"
	"errors"
)

const bme280RegisterControlHumidity = 0xF2
const bme280RegisterHumidityMSB = 0xFD
const bme280RegisterCalibDigH1 = 0xa1
const bme280RegisterCalibDigH2LSB = 0xe1

type bmeHumidityCalibrationCoefficients struct {
	h1 uint8
	h2 int16
	h3 uint8
	h4 int16
	h5 int16
	h6 int8
}

// BME280Driver is a driver for the BME280 temperature/humidity sensor.
// It implements all of the same functions as the BMP280Driver, but also
// adds the Humidity() function by reading the BME280's humidity sensor.
// For details on the BMP280Driver please see:
// 	https://godoc.org/gobot.io/x/gobot/drivers/i2c#BMP280Driver
//
type BME280Driver struct {
	*BMP280Driver
	hc *bmeHumidityCalibrationCoefficients
}

// NewBME280Driver creates a new driver with specified i2c interface.
// Params:
//		conn Connector - the Adaptor to use with this Driver
//
// Optional params:
//		i2c.WithBus(int):	bus to use with this driver
//		i2c.WithAddress(int):	address to use with this driver
//
func NewBME280Driver(c Connector, options ...func(Config)) *BME280Driver {
	b := &BME280Driver{
		BMP280Driver: NewBMP280Driver(c),
		hc:           &bmeHumidityCalibrationCoefficients{},
	}

	for _, option := range options {
		option(b)
	}

	// TODO: expose commands to API
	return b
}

// Start initializes the BME280 and loads the calibration coefficients.
func (d *BME280Driver) Start() (err error) {
	bus := d.GetBusOrDefault(d.connector.GetDefaultBus())
	address := d.GetAddressOrDefault(bmp180Address)

	if d.connection, err = d.connector.GetConnection(address, bus); err != nil {
		return err
	}

	if err := d.initialization(); err != nil {
		return err
	}

	if err := d.initHumidity(); err != nil {
		return err
	}

	return nil
}

// Humidity returns the current humidity in percentage of relative humidity
func (d *BME280Driver) Humidity() (humidity float32, err error) {
	var rawH uint32
	if rawH, err = d.rawHumidity(); err != nil {
		return 0.0, err
	}
	humidity = d.calculateHumidity(rawH)
	return
}

// read the humidity calibration coefficients.
func (d *BME280Driver) initHumidity() (err error) {
	var coefficients []byte
	if coefficients, err = d.read(bme280RegisterCalibDigH1, 1); err != nil {
		return err
	}
	buf := bytes.NewBuffer(coefficients)
	binary.Read(buf, binary.BigEndian, &d.hc.h1)

	if coefficients, err = d.read(bme280RegisterCalibDigH2LSB, 7); err != nil {
		return err
	}
	buf = bytes.NewBuffer(coefficients)

	// H4 and H5 laid out strangely on the bme280
	var addrE4 byte
	var addrE5 byte
	var addrE6 byte

	binary.Read(buf, binary.LittleEndian, &d.hc.h2) // E1 ...
	binary.Read(buf, binary.BigEndian, &d.hc.h3)    // E3
	binary.Read(buf, binary.BigEndian, &addrE4)     // E4
	binary.Read(buf, binary.BigEndian, &addrE5)     // E5
	binary.Read(buf, binary.BigEndian, &addrE6)     // E6
	binary.Read(buf, binary.BigEndian, &d.hc.h6)    // ... E7

	d.hc.h4 = 0 + (int16(addrE4) << 4) | (int16(addrE5 & 0x0F))
	d.hc.h5 = 0 + (int16(addrE6) << 4) | (int16(addrE5) >> 4)

	d.connection.WriteByteData(bme280RegisterControlHumidity, 0x3F)

	return nil
}

func (d *BME280Driver) rawHumidity() (uint32, error) {
	ret, err := d.read(bme280RegisterHumidityMSB, 2)
	if err != nil {
		return 0, err
	}
	if ret[0] == 0x80 && ret[1] == 0x00 {
		return 0, errors.New("Humidity disabled")
	}
	buf := bytes.NewBuffer(ret)
	var rawH uint16
	binary.Read(buf, binary.BigEndian, &rawH)
	return uint32(rawH), nil
}

// Adapted from https://github.com/BoschSensortec/BME280_driver/blob/master/bme280.c
// function bme280_compensate_humidity_double(s32 v_uncom_humidity_s32)
func (d *BME280Driver) calculateHumidity(rawH uint32) float32 {
	var rawT int32
	var err error
	var h float32

	rawT, err = d.rawTemp()
	if err != nil {
		return 0
	}

	_, tFine := d.calculateTemp(rawT)
	h = float32(tFine) - 76800

	if h == 0 {
		return 0 // TODO err is 'invalid data' from Bosch - include errors or not?
	}

	x := float32(rawH) - (float32(d.hc.h4)*64.0 +
		(float32(d.hc.h5) / 16384.0 * h))

	y := float32(d.hc.h2) / 65536.0 *
		(1.0 + float32(d.hc.h6)/67108864.0*h*
			(1.0+float32(d.hc.h3)/67108864.0*h))

	h = x * y
	h = h * (1 - float32(d.hc.h1)*h/524288)
	return h
}