Windows 10 IoT Core用に、BME280センサーのライブラリを作ってみました。BME280は3.3V動作の温度・湿度・気圧センサーで、秋月さんやSwitch Scienceさんから購入できます。
ESP-WROOM-02 ArduinoでBME280を使った際は、Embedded Adventuresさんのライブラリを使っていました。Windows 10 IoT用のライブラリに関しては、類似品であるBMP280用のライブラリならNuGetで取得することができるのですが、BME280用のライブラリは見つかりませんでした。そのため、Adafruitのforumで紹介されていたBMP280用のライブラリをベースに自作（改造）してみました。
BMP280とBME280はレジスターの構成や設定はほんとんど共通のため、湿度読み取り機能を追加し、読み取り値の補正関数をBME280用に修正しています。
またオリジナルのAdafruit版は、I2Cのからのセンサーデーター読み取りなどで非同期処理（async/await）を多用していたのですが、データ読み取りの時間は瞬時で非同期処理を入れるオーバーヘッド（非同期用の別タスクを起こす処理）の方が大きそうな気がしたため、デバイスの初期化部分以外では非同期処理を使わないように変更しました。
作成したライブラリのコードを以下に示します。GitHubでもプロジェエクトファイルを公開しました：
using System;
using System.Threading.Tasks;
using System.Diagnostics;
using Windows.Devices.Enumeration;
using Windows.Devices.I2c;
namespace Sensor.BME280
{
public class BME280_CalibrationData
{
//BME280 Registers
public ushort dig_T1 { get; set; }
public short dig_T2 { get; set; }
public short dig_T3 { get; set; }
public ushort dig_P1 { get; set; }
public short dig_P2 { get; set; }
public short dig_P3 { get; set; }
public short dig_P4 { get; set; }
public short dig_P5 { get; set; }
public short dig_P6 { get; set; }
public short dig_P7 { get; set; }
public short dig_P8 { get; set; }
public short dig_P9 { get; set; }
public byte dig_H1 { get; set; }
public short dig_H2 { get; set; }
public byte dig_H3 { get; set; }
public short dig_H4 { get; set; }
public short dig_H5 { get; set; }
public sbyte dig_H6 { get; set; }
}
public class BME280
{
const byte BME280_Address = 0x76;
const byte BME280_Signature = 0x60;
enum eRegisters : byte
{
BME280_REGISTER_DIG_T1 = 0x88,
BME280_REGISTER_DIG_T2 = 0x8A,
BME280_REGISTER_DIG_T3 = 0x8C,
BME280_REGISTER_DIG_P1 = 0x8E,
BME280_REGISTER_DIG_P2 = 0x90,
BME280_REGISTER_DIG_P3 = 0x92,
BME280_REGISTER_DIG_P4 = 0x94,
BME280_REGISTER_DIG_P5 = 0x96,
BME280_REGISTER_DIG_P6 = 0x98,
BME280_REGISTER_DIG_P7 = 0x9A,
BME280_REGISTER_DIG_P8 = 0x9C,
BME280_REGISTER_DIG_P9 = 0x9E,
BME280_REGISTER_DIG_H1 = 0xA1,
BME280_REGISTER_DIG_H2 = 0xE1,
BME280_REGISTER_DIG_H3 = 0xE3,
BME280_REGISTER_DIG_H4_L = 0xE4,
BME280_REGISTER_DIG_H4_H = 0xE5,
BME280_REGISTER_DIG_H5_L = 0xE5,
BME280_REGISTER_DIG_H5_H = 0xE6,
BME280_REGISTER_DIG_H6 = 0xE7,
BME280_REGISTER_CHIPID = 0xD0,
BME280_REGISTER_SOFTRESET = 0xE0,
BME280_REGISTER_CONTROLHUMID = 0xF2,
BME280_REGISTER_STATUS = 0xF3,
BME280_REGISTER_CONTROL = 0xF4,
BME280_REGISTER_CONFIG = 0xF5,
BME280_REGISTER_PRESSUREDATA_MSB = 0xF7,
BME280_REGISTER_PRESSUREDATA_LSB = 0xF8,
BME280_REGISTER_PRESSUREDATA_XLSB = 0xF9, // bits <7:4>
BME280_REGISTER_TEMPDATA_MSB = 0xFA,
BME280_REGISTER_TEMPDATA_LSB = 0xFB,
BME280_REGISTER_TEMPDATA_XLSB = 0xFC, // bits <7:4>
BME280_REGISTER_HUMIDDATA_MSB = 0xFD,
BME280_REGISTER_HUMIDDATA_LSB = 0xFE,
};
// Enables 2-wire I2C interface when set to ‘0’
public enum interface_mode_e : byte
{
i2c = 0,
spi = 1
};
// t_sb standby options - effectively the gap between automatic measurements
// when in "normal" mode
public enum standbySettings_e : byte
{
tsb_0p5ms = 0,
tsb_62p5ms = 1,
tsb_125ms = 2,
tsb_250ms = 3,
tsb_500ms = 4,
tsb_1000ms = 5,
tsb_10ms = 6,
tsb_20ms = 7
};
// sensor modes, it starts off in sleep mode on power on
// forced is to take a single measurement now
// normal takes measurements reqularly automatically
public enum mode_e :byte
{
smSleep = 0,
smForced = 1,
smNormal = 3
};
// Filter coefficients
// higher numbers slow down changes, such as slamming doors
public enum filterCoefficient_e : byte
{
fc_off = 0,
fc_2 = 1,
fc_4 = 2,
fc_8 = 3,
fc_16 = 4
};
// Oversampling options for humidity
// Oversampling reduces the noise from the sensor
public enum oversampling_e : byte
{
osSkipped = 0,
os1x = 1,
os2x = 2,
os4x = 3,
os8x = 4,
os16x = 5
};
//String for the friendly name of the I2C bus
private const string I2CControllerName = "I2C1";
//Create an I2C device
private I2cDevice bme280 = null;
//Create new calibration data for the sensor
private BME280_CalibrationData CalibrationData;
// Value hold sensor operation parameters
private byte int_mode = (byte)interface_mode_e.i2c;
private byte t_sb;
private byte mode;
private byte filter;
private byte osrs_p;
private byte osrs_t;
private byte osrs_h;
public BME280(standbySettings_e t_sb = standbySettings_e.tsb_0p5ms,
mode_e mode = mode_e.smNormal,
filterCoefficient_e filter = filterCoefficient_e.fc_16,
oversampling_e osrs_p = oversampling_e.os16x,
oversampling_e osrs_t = oversampling_e.os2x,
oversampling_e osrs_h = oversampling_e.os1x)
{
this.t_sb = (byte)t_sb;
this.mode = (byte)mode;
this.filter = (byte)filter;
this.osrs_p = (byte)osrs_p;
this.osrs_t = (byte)osrs_t;
this.osrs_h = (byte)osrs_h;
}
//Method to initialize the BME280 sensor
public async Task Initialize()
{
Debug.WriteLine("BME280::Initialize");
try
{
//Instantiate the I2CConnectionSettings using the device address of the BME280
I2cConnectionSettings settings = new I2cConnectionSettings(BME280_Address);
//Set the I2C bus speed of connection to fast mode
settings.BusSpeed = I2cBusSpeed.FastMode;
//Use the I2CBus device selector to create an advanced query syntax string
string aqs = I2cDevice.GetDeviceSelector(I2CControllerName);
//Use the Windows.Devices.Enumeration.DeviceInformation class to create a collection using the advanced query syntax string
DeviceInformationCollection dis = await DeviceInformation.FindAllAsync(aqs);
//Instantiate the the BME280 I2C device using the device id of the I2CBus and the I2CConnectionSettings
bme280 = await I2cDevice.FromIdAsync(dis[0].Id, settings);
//Check if device was found
if (bme280 == null)
{
Debug.WriteLine("Device not found");
}
}
catch (Exception e)
{
Debug.WriteLine("Exception: " + e.Message + "\n" + e.StackTrace);
throw;
}
byte[] readChipID = new byte[] { (byte)eRegisters.BME280_REGISTER_CHIPID };
byte[] ReadBuffer = new byte[] { 0xFF };
//Read the device signature
bme280.WriteRead(readChipID, ReadBuffer);
Debug.WriteLine("BME280 Signature: " + ReadBuffer[0].ToString());
//Verify the device signature
if (ReadBuffer[0] != BME280_Signature)
{
Debug.WriteLine("BME280::Begin Signature Mismatch.");
return;
}
//Read the coefficients table
CalibrationData = ReadCoefficeints();
//Set configuration registers
WriteConfigRegister();
WriteControlMeasurementRegister();
WriteControlRegisterHumidity();
//Set configuration registers again to ensure configuration of humidity
WriteConfigRegister();
WriteControlMeasurementRegister();
WriteControlRegisterHumidity();
//Dummy read temp to setup t_fine
ReadTemperature();
}
//Method to write the config register (default 16)
//000 100 00
// ↑ ↑ ↑I2C mode
// ↑ ↑Filter coefficient = 16
// ↑t_sb = 0.5ms
private void WriteConfigRegister()
{
byte value = (byte)(int_mode + (filter << 2) + (t_sb << 5));
byte[] WriteBuffer = new byte[] { (byte)eRegisters.BME280_REGISTER_CONFIG, value };
bme280.Write(WriteBuffer);
return;
}
//Method to write the control measurment register (default 87)
//010 101 11
// ↑ ↑ ↑ mode
// ↑ ↑ Pressure oversampling
// ↑ Temperature oversampling
private void WriteControlMeasurementRegister()
{
byte value = (byte)(mode + (osrs_p << 2) + (osrs_t << 5));
byte[] WriteBuffer = new byte[] { (byte)eRegisters.BME280_REGISTER_CONTROL, value };
bme280.Write(WriteBuffer);
return;
}
//Method to write the humidity control register (default 01)
private void WriteControlRegisterHumidity()
{
byte value = osrs_h;
byte[] WriteBuffer = new byte[] { (byte)eRegisters.BME280_REGISTER_CONTROLHUMID, value };
bme280.Write(WriteBuffer);
return;
}
//Method to read a 16-bit value from a register and return it in little endian format
private ushort ReadUInt16_LittleEndian(byte register)
{
ushort value = 0;
byte[] writeBuffer = new byte[] { 0x00 };
byte[] readBuffer = new byte[] { 0x00, 0x00 };
writeBuffer[0] = register;
bme280.WriteRead(writeBuffer, readBuffer);
int h = readBuffer[1] << 8;
int l = readBuffer[0];
value = (ushort)(h + l);
return value;
}
//Method to read an 8-bit value from a register
private byte ReadByte(byte register)
{
byte value = 0;
byte[] writeBuffer = new byte[] { 0x00 };
byte[] readBuffer = new byte[] { 0x00 };
writeBuffer[0] = register;
bme280.WriteRead(writeBuffer, readBuffer);
value = readBuffer[0];
return value;
}
//Method to read the caliberation data from the registers
private BME280_CalibrationData ReadCoefficeints()
{
// 16 bit calibration data is stored as Little Endian, the helper method will do the byte swap.
CalibrationData = new BME280_CalibrationData();
// Read temperature calibration data
CalibrationData.dig_T1 = ReadUInt16_LittleEndian((byte)eRegisters.BME280_REGISTER_DIG_T1);
CalibrationData.dig_T2 = (short)ReadUInt16_LittleEndian((byte)eRegisters.BME280_REGISTER_DIG_T2);
CalibrationData.dig_T3 = (short)ReadUInt16_LittleEndian((byte)eRegisters.BME280_REGISTER_DIG_T3);
// Read presure calibration data
CalibrationData.dig_P1 = ReadUInt16_LittleEndian((byte)eRegisters.BME280_REGISTER_DIG_P1);
CalibrationData.dig_P2 = (short)ReadUInt16_LittleEndian((byte)eRegisters.BME280_REGISTER_DIG_P2);
CalibrationData.dig_P3 = (short)ReadUInt16_LittleEndian((byte)eRegisters.BME280_REGISTER_DIG_P3);
CalibrationData.dig_P4 = (short)ReadUInt16_LittleEndian((byte)eRegisters.BME280_REGISTER_DIG_P4);
CalibrationData.dig_P5 = (short)ReadUInt16_LittleEndian((byte)eRegisters.BME280_REGISTER_DIG_P5);
CalibrationData.dig_P6 = (short)ReadUInt16_LittleEndian((byte)eRegisters.BME280_REGISTER_DIG_P6);
CalibrationData.dig_P7 = (short)ReadUInt16_LittleEndian((byte)eRegisters.BME280_REGISTER_DIG_P7);
CalibrationData.dig_P8 = (short)ReadUInt16_LittleEndian((byte)eRegisters.BME280_REGISTER_DIG_P8);
CalibrationData.dig_P9 = (short)ReadUInt16_LittleEndian((byte)eRegisters.BME280_REGISTER_DIG_P9);
// Read humidity calibration data
CalibrationData.dig_H1 = ReadByte((byte)eRegisters.BME280_REGISTER_DIG_H1);
CalibrationData.dig_H2 = (short)ReadUInt16_LittleEndian((byte)eRegisters.BME280_REGISTER_DIG_H2);
CalibrationData.dig_H3 = ReadByte((byte)eRegisters.BME280_REGISTER_DIG_H3);
short e4 = ReadByte((byte)eRegisters.BME280_REGISTER_DIG_H4_L); // Read 0xE4
short e5 = ReadByte((byte)eRegisters.BME280_REGISTER_DIG_H4_H); // Read 0xE5
CalibrationData.dig_H4 = (short)((e4 << 4) + (e5 & 0x0F));
short e6 = ReadByte((byte)eRegisters.BME280_REGISTER_DIG_H5_H); // Read 0xE6
CalibrationData.dig_H5 = (short)((e5 >> 4) + (e6 << 4));
CalibrationData.dig_H6 = (sbyte)ReadByte((byte)eRegisters.BME280_REGISTER_DIG_H6);
return CalibrationData;
}
//t_fine carries fine temperature as global value
int t_fine;
//Method to return the temperature in DegC. Resolution is 0.01 DegC. Output value of “51.23” equals 51.23 DegC.
private double BME280_compensate_T_double(int adc_T)
{
double var1, var2, T;
//The temperature is calculated using the compensation formula in the BME280 datasheet
var1 = (adc_T / 16384.0 - CalibrationData.dig_T1 / 1024.0) * CalibrationData.dig_T2;
var2 = ((adc_T / 131072.0 - CalibrationData.dig_T1 / 8192.0) *
(adc_T / 131072.0 - CalibrationData.dig_T1 / 8192.0)) * CalibrationData.dig_T3;
t_fine = (int)(var1 + var2);
T = (var1 + var2) / 5120.0;
return T;
}
//Method to returns the pressure in Pa, in Q24.8 format (24 integer bits and 8 fractional bits).
//Output value of “24674867” represents 24674867/256 = 96386.2 Pa = 963.862 hPa
private long BME280_compensate_P_Int64(int adc_P)
{
long var1, var2, p;
//The pressure is calculated using the compensation formula in the BME280 datasheet
var1 = (long)t_fine - 128000;
var2 = var1 * var1 * CalibrationData.dig_P6;
var2 = var2 + ((var1 * CalibrationData.dig_P5) << 17);
var2 = var2 + ((long)CalibrationData.dig_P4 << 35);
var1 = ((var1 * var1 * CalibrationData.dig_P3) >> 8) + ((var1 * CalibrationData.dig_P2) << 12);
var1 = (((long)1 << 47) + var1) * CalibrationData.dig_P1 >> 33;
if (var1 == 0)
{
Debug.WriteLine("BME280_compensate_P_Int64 Jump out to avoid / 0");
return 0; //Avoid exception caused by division by zero
}
//Perform calibration operations as per datasheet:
p = 1048576 - adc_P;
p = (((p << 31) - var2) * 3125) / var1;
var1 = ((long)CalibrationData.dig_P9 * (p >> 13) * (p >> 13)) >> 25;
var2 = ((long)CalibrationData.dig_P8 * p) >> 19;
p = ((p + var1 + var2) >> 8) + ((long)CalibrationData.dig_P7 << 4);
return p;
}
// Returns humidity in %rH as as double. Output value of “46.332” represents 46.332 %rH
private double BME280_compensate_H_double(int adc_H)
{
double var_H;
var_H = t_fine - 76800.0;
var_H = (adc_H - (CalibrationData.dig_H4 * 64.0 + CalibrationData.dig_H5 / 16384.0 * var_H)) *
CalibrationData.dig_H2 / 65536.0 * (1.0 + CalibrationData.dig_H6 / 67108864.0 * var_H *
(1.0 + CalibrationData.dig_H3 / 67108864.0 * var_H));
var_H = var_H * (1.0 - CalibrationData.dig_H1 * var_H / 524288.0);
if (var_H > 100.0)
{
Debug.WriteLine("BME280_compensate_H_double Jump out to 100%");
var_H = 100.0;
} else if (var_H < 0.0)
{
Debug.WriteLine("BME280_compensate_H_double Jump under 0%");
var_H = 0.0;
}
return var_H;
}
public float ReadTemperature()
{
//Read the MSB, LSB and bits 7:4 (XLSB) of the temperature from the BME280 registers
byte tmsb = ReadByte((byte)eRegisters.BME280_REGISTER_TEMPDATA_MSB);
byte tlsb = ReadByte((byte)eRegisters.BME280_REGISTER_TEMPDATA_LSB);
byte txlsb = ReadByte((byte)eRegisters.BME280_REGISTER_TEMPDATA_XLSB); // bits 7:4
//Combine the values into a 32-bit integer
int t = (tmsb << 12) + (tlsb << 4) + (txlsb >> 4);
//Convert the raw value to the temperature in degC
double temp = BME280_compensate_T_double(t);
//Return the temperature as a float value
return (float)temp;
}
public float ReadPreasure()
{
//Read the MSB, LSB and bits 7:4 (XLSB) of the pressure from the BME280 registers
byte pmsb = ReadByte((byte)eRegisters.BME280_REGISTER_PRESSUREDATA_MSB);
byte plsb = ReadByte((byte)eRegisters.BME280_REGISTER_PRESSUREDATA_LSB);
byte pxlsb = ReadByte((byte)eRegisters.BME280_REGISTER_PRESSUREDATA_XLSB); // bits 7:4
//Combine the values into a 32-bit integer
int p = (pmsb << 12) + (plsb << 4) + (pxlsb >> 4);
//Convert the raw value to the pressure in Pa
long pres = BME280_compensate_P_Int64(p);
//Return the pressure as a float value
return ((float)pres) / 256;
}
public float ReadHumidity()
{
//Read the MSB and LSB of the humidity from the BME280 registers
byte hmsb = ReadByte((byte)eRegisters.BME280_REGISTER_HUMIDDATA_MSB);
byte hlsb = ReadByte((byte)eRegisters.BME280_REGISTER_HUMIDDATA_LSB);
//Combine the values into a 32-bit integer
int h = (hmsb << 8) + hlsb;
//Convert the raw value to the humidity in %
double humidity = BME280_compensate_H_double(h);
//Return the humidity as a float value
return (float)humidity;
}
//Method to take the sea level pressure in Hectopascals(hPa) as a parameter and calculate the altitude using current pressure.
public float ReadAltitude(float seaLevel)
{
//Read the pressure first
float pressure = ReadPreasure();
//Convert the pressure to Hectopascals(hPa)
pressure /= 100;
//Calculate and return the altitude using the international barometric formula
return 44330.0f * (1.0f - (float)Math.Pow((pressure / seaLevel), 0.1903f));
}
}
}
ライブラリを使用したセンサーデーター読み取りアプリのサンプルは以下です：
using System;
using System.Threading;
using System.Diagnostics;
using Windows.UI.Xaml.Controls;
using Sensor.BME280;
namespace BME280_Test
{
public sealed partial class MainPage : Page
{
private BME280 bme280;
private Timer periodicTimer;
public MainPage()
{
this.InitializeComponent();
bme280 = new BME280();
initBme280();
}
private async void initBme280()
{
await bme280.Initialize();
periodicTimer = new Timer(this.TimerCallback, null, 0, 1000);
}
private void TimerCallback(object state)
{
var temp = bme280.ReadTemperature();
var press = bme280.ReadPreasure() / 100;
var humidity = bme280.ReadHumidity();
var alt = bme280.ReadAltitude(1013); // 1013hPa = pressure at 0m
Debug.WriteLine("Temp:{0:F2}℃ Humidity:{1:F2}% Press:{2:F2}hPa Alt:{3:F0}m", temp, humidity, press, alt);
var task = this.Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () =>
{
tempValue.Text = temp.ToString("F2") + "℃";
humValue.Text = humidity.ToString("F2") + "%";
pressValue.Text = press.ToString("F2") + "hPa";
});
}
}
}
電源オンの初回起動時に湿度情報が読み取れない問題があり（一旦アプリを終了して再度起動すると正常になる）、回避策としてレジスタの初期化を２回行っています（ライブラリソースの226行目）。Arudino用のライブラリコードを見ると設定を２回やるような処理は行っておらず、この点は謎です。
BME280のセンサーとしての精度ですが、手持ちのAM2302に比べて温度が高め、湿度が低めに出ます。湿度は5%以上の差分があり、センサー種別によって測定値に大きなばらつきがあります。温度はその他手持ちの温度計と比較するとAM2302の方が正確なため、湿度もAM2302の方が正確ではないかと思います。