bme280.go

// Package bme280 provides a driver for the BME280 digital combined
// humidity and pressure sensor by Bosch.
//
// Datasheet:
// https://cdn-shop.adafruit.com/datasheets/BST-BME280_DS001-10.pdf
//
package bme280

import (
	"math"

	"tinygo.org/x/drivers"
)

// 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                     drivers.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 drivers.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 (°C/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)

}