File content as of revision 2:3898208e02da:

#include "Barometer.h"
#define DEBUG "BMP280"
#include "Logger.h"

// Calibration parameters stored on chip
static uint16_t dig_T1;
static int16_t dig_T2;
static int16_t dig_T3;
static uint16_t dig_P1;
static int16_t dig_P2;
static int16_t dig_P3;
static int16_t dig_P4;
static int16_t dig_P5;
static int16_t dig_P6;
static int16_t dig_P7;
static int16_t dig_P8;
static int16_t dig_P9;

void Barometer::bmp280_read_cal_reg(const uint8_t reg, char* val)
{
    *val = read_reg(reg);
    *(val + 1) = read_reg(reg + 1);
}

void Barometer::bmp280_read_calibration()
{
    bmp280_read_cal_reg(0x88, (char*)&dig_T1);
    bmp280_read_cal_reg(0x8A, (char*)&dig_T2);
    bmp280_read_cal_reg(0x8C, (char*)&dig_T3);
    bmp280_read_cal_reg(0x8E, (char*)&dig_P1);
    bmp280_read_cal_reg(0x90, (char*)&dig_P2);
    bmp280_read_cal_reg(0x92, (char*)&dig_P3);
    bmp280_read_cal_reg(0x94, (char*)&dig_P4);
    bmp280_read_cal_reg(0x96, (char*)&dig_P5);
    bmp280_read_cal_reg(0x98, (char*)&dig_P6);
    bmp280_read_cal_reg(0x9A, (char*)&dig_P7);
    bmp280_read_cal_reg(0x9C, (char*)&dig_P8);
    bmp280_read_cal_reg(0x9E, (char*)&dig_P9);
    LOG("Calibration parameters: T=[%u, %d, %d] P=[%u, %d, %d, %d, %d, %d, %d, %d, %d]",
         dig_T1, dig_T2, dig_T3,
         dig_P1, dig_P2, dig_P3, dig_P4, dig_P5, dig_P6, dig_P7, dig_P8, dig_P9);
}

enum Oversampling {
    kSkip = 0,
    kOversample_1x = 1,
    kOversample_2x = 2,
    kOversample_4x = 3,
    kOversample_8x = 4,
    kOversample_16x = 5,
};

// Time taken to read the pressure at a particular oversampling
// cf. page 18
static float waitTime_ms[] = {
    0, // skip
    6.4, // 1x
    8.7, // 2x
    13.3, // 4x
    22.5, // 8x
    43.2, // 16x
};

enum Filtering {
    kFilter_None = 0,
    kFilter_2x = 1,
    kFilter_4x = 2,
    kFilter_8x = 3,
    kFilter_16x = 4
};

void Barometer::setFilterCoefficient(const uint8_t iir)
{
    write_reg(0xF5, (iir & 0x07) << 1);
    INFO("Filter coefficient => %dx", 1 << iir);
}

void Barometer::takeMeasurement(const uint8_t tmpovr,
                                const uint8_t psrovr)
{
    // Start a forced measurement
    write_reg(0xF4,
              ((tmpovr & 0x07) << 5) |
              ((psrovr & 0x07) << 2) |
              0x01 /* force reading mode */);

    // wait until it's done
    wait_ms(waitTime_ms[psrovr]); // XXX: what does this mean for BLE?
}

// Returns pressure in Pa as double. Output value of “96386.2” equals 96386.2 Pa = 963.862 hPa
typedef int32_t BMP280_S32_t;
static BMP280_S32_t t_fine;
static double bmp280_val_to_Pa(BMP280_S32_t adc_P)
{
    double var1, var2, p;
    var1 = ((double)t_fine/2.0) - 64000.0;
    var2 = var1 * var1 * ((double)dig_P6) / 32768.0;
    var2 = var2 + var1 * ((double)dig_P5) * 2.0;
    var2 = (var2/4.0)+(((double)dig_P4) * 65536.0);
    var1 = (((double)dig_P3) * var1 * var1 / 524288.0 + ((double)dig_P2) * var1) / 524288.0;
    var1 = (1.0 + var1 / 32768.0)*((double)dig_P1);
    if (var1 == 0.0) {
        return 0; // avoid exception caused by division by zero
    }
    p = 1048576.0 - (double)adc_P;
    p = (p - (var2 / 4096.0)) * 6250.0 / var1;
    var1 = ((double)dig_P9) * p * p / 2147483648.0;
    var2 = p * ((double)dig_P8) / 32768.0;
    p = p + (var1 + var2 + ((double)dig_P7)) / 16.0;
    return p;
}

Barometer::Barometer(I2C &i2c) : I2CPeripheral(i2c, 0xEC /* address */)
{
    write_reg(0xE0, 0xB6); // reset
    wait_ms(2); // cf. datasheet page 8, t_startup
    const uint8_t chip_id = read_reg(0xD0);
    if (chip_id == 0x58) {
        bmp280_read_calibration();
        //setFilterCoefficient(kFilter_16x);
        INFO("Bosch Sensortec BMP280 ready");
    } else {
        WARN("Bosch Sensortec BMP280 not found (chip ID=0x%02x, expected=0x58)", chip_id);
    }
}

double Barometer::getPressure()
{
    takeMeasurement(kSkip, kOversample_16x);
    const uint8_t msb = read_reg(0xF7);
    const uint8_t lsb = read_reg(0xF8);
    const uint8_t xlsb = read_reg(0xF9);
    const uint32_t val = (msb << 12) | (lsb << 4) | xlsb;
    return bmp280_val_to_Pa(val) / 100.0;
}

double Barometer::getHeightFromPressure(const double p) {
    const double R = 287.05; //general gas constant
    const double g = 9.80665; // acceleration due to gravity
    const double T = 24.0; // temperature
    const double p0 = 1000.0; // hPa sea level
    const double h = (R / g) * T * log(p0 / p);
    return h;
}