bmp180: add ReadAltitude() function copied from BMP280 driver

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

bmp180/bmp180.go

@@ -6,6 +6,7 @@
 package bmp180 // import "tinygo.org/x/drivers/bmp180"
 
 import (
+	"math"
 	"time"
 
 	"tinygo.org/x/drivers"
@@ -123,6 +124,21 @@ func (d *Device) ReadPressure() (pressure int32, err error) {
 	return 1000 * (p + ((x1 + x2 + 3791) >> 4)), 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() (int32, error) {
+	mPa, err := d.ReadPressure()
+	if err != nil {
+		return 0, err
+	}
+	atmP := float32(mPa) / 100000
+
+	return int32(44330.0 * (1.0 - math.Pow(float64(atmP/SEALEVEL_PRESSURE), 0.1903))), nil
+}
+
 // rawTemp returns the sensor's raw values of the temperature
 func (d *Device) rawTemp() (int32, error) {
 	d.bus.WriteRegister(uint8(d.Address), REG_CTRL, []byte{CMD_TEMP})

bmp180/registers.go

@@ -28,3 +28,7 @@ const (
 	// ULTRAHIGHRESOLUTION is the highest oversampling mode of the pressure measurement.
 	ULTRAHIGHRESOLUTION
 )
+
+const (
+	SEALEVEL_PRESSURE float32 = 1013.25 // in hPa
+)

examples/bmp180/main.go

@@ -27,6 +27,9 @@ func main() {
 		pressure, _ := sensor.ReadPressure()
 		println("Pressure", float32(pressure)/100000, "hPa")
 
+		altitude, _ := sensor.ReadAltitude()
+		println("Altitude", altitude, "meters")
+
 		time.Sleep(2 * time.Second)
 	}
 }