i2c: adds Altitude() function to BMP280/BME280

Signed-off-by: deadprogram <ron@hybridgroup.com>

drivers/i2c/bme280_driver.go

@@ -20,7 +20,12 @@ type bmeHumidityCalibrationCoefficients struct {
 	h6 int8
 }
 
-// BME280Driver is a driver for the BME280 temperature/humidity sensor
+// 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

drivers/i2c/bmp280_driver.go

@@ -3,6 +3,7 @@ package i2c
 import (
 	"bytes"
 	"encoding/binary"
+	"math"
 
 	"gobot.io/x/gobot"
 )
@@ -12,10 +13,10 @@ const (
 	bmp280RegisterConfig       = 0xf5
 	bmp280RegisterPressureData = 0xf7
 	bmp280RegisterTempData     = 0xfa
+	bmp280RegisterCalib00      = 0x88
+	bmp280SeaLevelPressure     = 1013.25
 )
 
-const bmp280RegisterCalib00 = 0x88
-
 type bmp280CalibrationCoefficients struct {
 	t1 uint16
 	t2 int16
@@ -125,6 +126,18 @@ func (d *BMP280Driver) Pressure() (press float32, err error) {
 	return d.calculatePress(rawP, tFine), nil
 }
 
+// Altitude 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 *BMP280Driver) Altitude() (alt float32, err error) {
+	atmP, _ := d.Pressure()
+	atmP /= 100.0
+	alt = float32(44330.0 * (1.0 - math.Pow(float64(atmP/bmp280SeaLevelPressure), 0.1903)))
+
+	return
+}
+
 // initialization reads the calibration coefficients.
 func (d *BMP280Driver) initialization() (err error) {
 	// TODO: set sleep mode here...

examples/firmata_bme280.go

@@ -26,6 +26,9 @@ func main() {
 			p, _ := bme280.Pressure()
 			fmt.Println("Pressure", p)
 
+			a, _ := bme280.Altitude()
+			fmt.Println("Altitude", a)
+
 			h, _ := bme280.Humidity()
 			fmt.Println("Humidity", h)
 		})

examples/firmata_bmp180.go

@@ -20,7 +20,6 @@ func main() {
 
 	work := func() {
 		gobot.Every(1*time.Second, func() {
-			//fmt.Println("Pressure", mpl115a2.Pressure())
 			t, _ := bmp180.Temperature()
 			fmt.Println("Temperature", t)
 		})

examples/firmata_bmp280.go

@@ -0,0 +1,41 @@
+// +build example
+//
+// Do not build by default.
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"gobot.io/x/gobot"
+	"gobot.io/x/gobot/drivers/i2c"
+	"gobot.io/x/gobot/platforms/firmata"
+)
+
+func main() {
+	firmataAdaptor := firmata.NewAdaptor(os.Args[1])
+	bmp280 := i2c.NewBMP280Driver(firmataAdaptor)
+
+	work := func() {
+		gobot.Every(1*time.Second, func() {
+			t, _ := bmp280.Temperature()
+			fmt.Println("Temperature", t)
+
+			p, _ := bmp280.Pressure()
+			fmt.Println("Pressure", p)
+
+			a, _ := bmp280.Altitude()
+			fmt.Println("Altitude", a)
+		})
+	}
+
+	robot := gobot.NewRobot("bmp280bot",
+		[]gobot.Connection{firmataAdaptor},
+		[]gobot.Device{bmp280},
+		work,
+	)
+
+	robot.Start()
+}