Data Signal¶
Data signals are an abstract representation of data producers. The API treats them as an event that contains data and represents them with the MblMwDataSignal struct.
They can be safely typecasted to an MblMwEvent as seen in the example in the previous section.
Data signals can come from the accelerometer:
// Get accelerometer data
auto acc = mbl_mw_acc_get_acceleration_data_signal(board);
Reading the battery level:
// Get battery level
auto batt = mbl_mw_settings_get_battery_state_data_signal(board);
Getting the switch state (is the button pushed or not):
// Get switch state
auto switch = mbl_mw_switch_get_state_data_signal(board);
There are many signals which are highlighted in the sections of our documentation:
// Get analog gpio value
auto analog_gpio = mbl_mw_gpio_get_analog_input_data_signal(board, pin, MBL_MW_GPIO_ANALOG_READ_MODE_ADC);
Data Handling¶
Signal data is encapsulated by the MblMwData struct.
The struct contains a:
Timestamp of when the data was created
Pointer to the data
Data type id indicating how to cast the pointer
An enumeration of data types is defined by the MblMwDataTypeId enum and structs wrapping non basic data types are defined in the types.h header file.
#include "metawear/core/data.h"
#include <chrono>
#include <iomanip>
using std::chrono::duration_cast;
using std::chrono::milliseconds;
using std::chrono::system_clock;
using std::chrono::time_point;
void data_printer(const MblMwData* data) {
// Print data as 2 digit hex values
uint8_t* data_bytes = (uint8_t*) data->value;
string bytes_str("[");
char buffer[5];
for (uint8_t i = 0; i < data->length; i++) {
if (i) {
bytes_str += ", ";
}
sprintf(buffer, "0x%02x", data_bytes[i]);
bytes_str += buffer;
}
bytes_str += "]";
// Format time as YYYYMMDD-HH:MM:SS.LLL
time_point<system_clock> then(milliseconds(data->epoch));
auto time_c = system_clock::to_time_t(then);
auto rem_ms= data->epoch % 1000;
cout << "{timestamp: " << put_time(localtime(&time_c), "%Y%m%d-%T") << "." << rem_ms << ", "
<< "type_id: " << data->type_id << ", "
<< "bytes: " << bytes_str.c_str() << "}"
<< endl;
}
In this example, we sample data from the accelerometer with MblMwDataTypeId MBL_MW_DT_ID_CARTESIAN_FLOAT, a Cartesion Float. This is casted as such in the callback:
#include "metawear/core/datasignal.h"
#include "metawear/core/types.h"
#include "metawear/sensor/accelerometer.h"
void enable_acc_sampling(MblMwMetaWearBoard* board) {
auto data_handler = [](const MblMwData* data) -> void {
// Cast value to MblMwCartesianFloat*
auto acceleration = (MblMwCartesianFloat*) data->value;
printf("(%.3fg, %.3fg, %.3fg)\n", acceleration->x, acceleration->y, acceleration->z);
};
auto acc_signal= mbl_mw_acc_get_acceleration_data_signal(board);
mbl_mw_datasignal_subscribe(acc_signal, data_handler);
mbl_mw_acc_enable_acceleration_sampling(board);
mbl_mw_acc_start(board);
}
Some types are as simple as floats, signed and unsigned integer.
The color detector sensor (available in an older version of MetaWear) has a more complicated type (that is rarely used):
#include "metawear/core/datasignal.h"
#include "metawear/core/types.h"
void colordetector_adc_stream(MblMwMetaWearBoard* board) {
auto adc_signal= mbl_mw_cd_tcs34725_get_adc_data_signal(board);
mbl_mw_datasignal_subscribe(adc_signal, [](const MblMwData* data) -> void {
// Cast value to MblMwTcs34725ColorAdc*
auto color_adc = (MblMwTcs34725ColorAdc*) data->value;
printf("{clear: %d, red: %d, green: %d, blue: %d}", color_adc->clear,
color_adc->red, color_adc->green, color_adc->blue);
});
mbl_mw_datasignal_read(adc_signal);
}
Streaming¶
Streaming data is sending live data from the sensors on the MetaWear board, through the Bluetooth link, to the device of your choice in real-time.
Aside from the latency of the Bluetooth link, data is received instantaneously.
Note that there are limits to the Bluetooth link as well as the sensors. Please see the Bluetooth SIG and the MetaWear datasheet to learn more.
To stream data live to your device, call mbl_mw_datasignal_subscribe with the desired data signal and a callback function for handling the received data. Terminating the live stream is done by calling mbl_mw_datasignal_unsubscribe.
#include "metawear/core/datasignal.h"
#include "metawear/sensor/switch.h"
void subscribe_switch(MblMwMetaWearBoard* board) {
static auto data_handler = [](const MblMwData* data) -> void {
if (*((uint32_t*)data->value)) {
cout << "Switch Pressed" << endl;
} else {
cout << "Switch Released" << endl;
}
};
auto switch_signal = mbl_mw_switch_get_state_data_signal(board);
mbl_mw_datasignal_subscribe(switch_signal, data_handler);
}
Logging¶
Alternatively, data can be logged and retrieved at a later time.
When the data is logged, it is stored in the board memory. The memory is finite and once it is full, old data may be overwritten by new data. Please consult the Tutorials and the datasheet of your board for more information.
The data must be retrieved at some point in time from the MetaWear board to the device of your choice using the logger APIs.
See the Logger section for more details.
Readable Signals¶
Some sensors will only send data when they receive a command to do so. These are typically either slower sensors or analog sensors where data doesn’t need to be read at 100Hz (such as the temperature sensor).
Data signals that represent this type of data source are called readable signals. You can check if a data signal is readable by calling mbl_mw_datasignal_is_readable.
The read command is issued by calling mbl_mw_datasignal_read or mbl_mw_datasignal_read_with_parameters. Most readable data signals will use the former function which does not require any additional parameters. The latter function is for reads that require additional parameters which are bundled into one struct.
Reading the humidity from the barometer is a good example of a single read:
signal = mbl_mw_humidity_bme280_get_percentage_data_signal(board);
mbl_mw_datasignal_read_with_parameters(signal);
Reading I2C is a good example of read_with_parameters:
parameters = MblMwI2cReadParameters(deviceAddress, registerAddress);
mbl_mw_datasignal_read_with_parameters(signal, parameters);
When using readable signals, you must decide up front if the data will be streamed or logged before interacting with it. That is, you should either have subscribed to or setup a logger for a readable signal before reading it.
Data Processing¶
Data signals can be fed through the on-board data processors to filter and/or transform the data in the firmware. By performing computations on the MetaWear side, you can reduce the amount of data that is sent over the radio and the amount of postprocessing that is done on your mobile device.
For example, a threshold processor can be used to determine if the ambient temperature has exceeded 40 degrees. A highpass filter can be used to determine if the board has moved or the comparison processor can be used to determine if and when the light in the room has been turned on.
Data processors can also be chained together to perform more complex tasks, such as using the rss, average, and threshold processors to determine if the board is in freefall based on the XYZ acceleration data.
See the Data Processor section for more details on the data processing system.
Here is an example where the x,y,z components of the accelerometer are combined using the rss processor to calculate the vector magnitude:
#include "metawear/core/dataprocessor_fwd.h"
#include "metawear/processor/rss.h"
#include "metawear/sensor/accelerometer.h"
void rss_accelerometer(MblMwMetaWearBoard* board) {
static auto data_handler = [](const MblMwData* data) -> void {
printf("acc rss= %.3fg\n", (float*) data->value);
};
static auto rss_ready = [](MblMwDataProcessor* processor) -> void {
// subscribe to the rss processor
mbl_mw_datasignal_subscribe((MblMwDataSignal*) processor, data_handler);
};
// Create an rss processor to transform the XYZ values into vector magnitude
// Do not need to compute rss on your device and less data is transmitted
auto acc_signal = mbl_mw_acc_get_acceleration_data_signal(board);
mbl_mw_dataprocessor_rss_create(acc_signal, rss_ready);
}
Component Signals¶
Some signals, such as the acceleration data signal, are composed of multiple values. While you can interact with them as a whole, sometimes it is more convenient to only use individual values.
To access the component values, call mbl_mw_datasignal_get_component with the signal and an index represnting which component to retrieve. If a signal is single valued, the function will return null.
In this example, only the z-axis is from the accelerometer is retrieved:
#include "metawear/sensor/accelerometer.h"
void component_demo(MblMwMetaWearBoard* board) {
auto acc_root = mbl_mw_acc_get_acceleration_data_signal(board);
// get z axis signal
auto acc_z = mbl_mw_datasignal_get_component(acc_root, MBL_MW_ACC_ACCEL_Z_AXIS_INDEX);
mbl_mw_datasignal_subscribe(acc_z, [](MblMwData* data) -> void {
//combined xyz data is MblMwCartesianFloat, individual axis is float
printf("z-axis: %.3f\n", *((float*) data->value));
});
}