Barometer

A barometer is a scientific instrument that is used to measure air pressure in a certain environment. The absolute barometric pressure sensor can measure pressure from 300 Pascal to 1100 hPa.

MetaWear RPro and Cpro, MMR, MMC, MTR, and MetaEnvironment boards come with a Bosch barometer.

The specific barometer model varies between the boards although both barometers are nearly identical save for a few settings. Bosch barometer functions are defined in the barometer_bosch.h header file where functions containing baro_bosch are barometer agnostic where as functions with baro_bmp280 and baro_bme280 are for those specific barometers.

Users can programatically determine which barometer is on their board with the mbl_mw_metawearboard_lookup_module function.

#include "metawear/sensor/barometer_bosch.h"

void barometer_who_am_i(MblMwMetaWearBoard* board) {
    switch (mbl_mw_metawearboard_lookup_module(board, MBL_MW_MODULE_BAROMETER)) {
    case MBL_MW_MODULE_TYPE_NA:
        printf("no barometer on this board\n");
        break;
    case MBL_MW_MODULE_BARO_TYPE_BMP280:
        printf("barometer type = BMP280\n");
        break;
    case MBL_MW_MODULE_BARO_TYPE_BME280:
        printf("barometer type = BME280\n");
        break;
    default:
        printf("unknown barometer type\n");
        break;
    }
}

Sensor Configuration

The Bosch barometers have 3 configurable parameters:

• Oversampling

• Infinite impulse filter (iir) coefficient

• Standby time

These operational parameters work in conjunction to control the noise, output resolution, and sampling rate. When you are done setting the configuration, call mbl_mw_baro_bosch_write_config to write the changes to the sensor.

void configure_barometer(MblMwMetaWearBoard* board) {
    // Set oversampling to ultra high resolution
    mbl_mw_baro_bosch_set_oversampling(board, MBL_MW_BARO_BOSCH_OVERSAMPLE_ULTRA_HIGH);

    // Set standby time to 62.5ms or closest valid valud
    mbl_mw_baro_bosch_set_standby_time(board, 62.5f);

    // Set iir filter coefficient
    mbl_mw_baro_bosch_set_iir_filter(board, MBL_MW_BARO_BOSCH_IIR_FILTER_AVG_4);

    // Write configuration to the sensor
    mbl_mw_baro_bosch_write_config(board);
}

Pressure Sampling

Pressure data is represented as a float and is in units of Pascals. To receive pressure data, simply subscribe or log the pressure data signal and then start the sensor.

#include "metawear/core/datasignal.h"

void setup_pressure_stream(MblMwMetaWearBoard* board) {
    auto data_handler = [](const MblMwData* data) -> void {
        // Cast value to float*
        printf("%.3fPa\n", *((float*) data->value));
    };
    auto pa_signal= mbl_mw_baro_bosch_get_pressure_data_signal(board);

    mbl_mw_datasignal_subscribe(pa_signal, data_handler);
    mbl_mw_baro_bosch_start(board);
}

Altitude Sampling

Altitude data is represented as a float and is in units of meters. To receive altitude data, simply subscribe or log the altitude data signal and then start the sensor.

#include "metawear/core/datasignal.h"

void setup_altitude_stream(MblMwMetaWearBoard* board) {
    auto data_handler = [](const MblMwData* data) -> void {
        // Cast value to float*
        printf("%.3fm\n", *((float*) data->value));
    };
    auto m_signal= mbl_mw_baro_bosch_get_altitude_data_signal(board);

    mbl_mw_datasignal_subscribe(m_signal, data_handler);
    mbl_mw_baro_bosch_start(board);
}