private void CalcBarometricPressure(DateTimeOffset timestamp, byte[] data)
{
// this code implements the algorithm in T5400 data sheet at
// http://processors.wiki.ti.com/index.php/SensorTag_User_Guide#Barometric_Pressure_Sensor
if (this.c != null)
{
BarometerMeasurement measurement = new BarometerMeasurement();
//more info about the calculation:
//http://www.epcos.com/web/generator/Web/Sections/ProductCatalog/Sensors/PressureSensors/T5400-ApplicationNote,property=Data__en.pdf;/T5400_ApplicationNote.pdf
int tr = BitConverter.ToInt16(data, 0); // Temperature raw value
int pr = BitConverter.ToUInt16(data, 2); // Pressure raw value from sensor
// Temperature actual value in unit centi degrees celsius
double t_a = (100 * (c[0] * tr / Math.Pow(2, 8) + c[1] * Math.Pow(2, 6))) / Math.Pow(2, 16);
double sensitivity = c[2] + c[3] * tr / Math.Pow(2, 17) + ((c[4] * tr / Math.Pow(2, 15)) * tr) / Math.Pow(2, 19);
double offset = c[5] * Math.Pow(2, 14) + c[6] * tr / Math.Pow(2, 3) + ((c[7] * tr / Math.Pow(2, 15)) * tr) / Math.Pow(2, 4);
measurement.Pascals = (sensitivity * pr + offset) / Math.Pow(2, 14);
OnBarometerMeasurementValueChanged(new BarometerMeasurementEventArgs(measurement, timestamp));
}
}
protected override void OnCharacteristicValueChanged(GattCharacteristic sender, GattValueChangedEventArgs eventArgs)
{
if (sender.Uuid == BarometerCharacteristicUuid)
{
if (_barometerValueChanged != null)
{
uint dataLength = eventArgs.CharacteristicValue.Length;
using (DataReader reader = DataReader.FromBuffer(eventArgs.CharacteristicValue))
{
if (dataLength == 4)
{
// version 1
var data = new byte[dataLength];
reader.ReadBytes(data);
CalcBarometricPressure(eventArgs.Timestamp, data);
}
else if (dataLength == 6)
{
// version 2
uint temp = ReadBigEndianU24bit(reader);
uint pressure = ReadBigEndianU24bit(reader);
BarometerMeasurement measurement = new BarometerMeasurement();
measurement.Temperature = (double)temp / 100.0;
measurement.HectoPascals = (double)pressure / 100.0;
OnBarometerMeasurementValueChanged(new BarometerMeasurementEventArgs(measurement, eventArgs.Timestamp));
}
}
}
}
else if (sender.Uuid == BarometerCharacteristicCalibrationUuid)
{
UpdateCalibrationData(eventArgs);
}
}
public BarometerMeasurementEventArgs(BarometerMeasurement measurement, DateTimeOffset timestamp)
{
Measurement = measurement;
Timestamp = timestamp;
}
