Sensirion UPT Core

A library containing all the relevant types and definitions shared between components of the Unified Prototyping Toolkit (UPT) for ease of maintainability. There shouldn't be a reason to use it directly, but it is being used to create libraries compatible with the rest of the UPT.

Philosophy behind this library

Introduction:

Goal of this library is to provide a uniform design language across all UPT modules when handling sensor data, for convenient flow of information and preserving metadata.

Fundamentally, a data point in the UPT ecosystem comes from one of two sources:

• Wired: signal readings retrieved via a wired communications bus, eg. i2c communication
• Bluetooth Low Energy (BLE): signal readings transimtted via BLE advertisement from a Sensirion demo device

Unfortunately, further classification of a data point's origin is tricky:

• The information about the sensor authoring a measurement is not transmitted via BLE, it is thus in general not possible to assign a SensorType to each DataPoint received via BLE.
• Not all wired sensors support exact sensor classification. For example, while we know that a sensor responding to i2c commands on address 0x62 belongs in the SCD4X family of sensors, we have no way of knowing if it is a SCD40 or SCD41.

These two reasons motivate why we distinguish two sensor types, BLEGadgetType and SensorType, and the granularity of their definitions. To counter this granularity, fields for storing the authoring sensor's serial number or MAC address (depending on whether the data was received via i2c or BLE) are provided, to ensure a unique identifier is made available.

Measurement creation Pipeline:

To enable both total characterization and efficient storage of large quantities of measurements, it was decided to encapsulate the actual values in different layers. Let us go over each of the objects that make up a Measurement:

DataPoint:

At first, some measurement is either made via i2c, or received via interception of a BLE advertisement. This allows us to populate the DataPoint struct:

DataPoint(
    t_offset = *Milliseconds since startup*
    value = *Signal value*
);

We measure a relative time since startup because of its easy availability anywhere in the code, through the built-in Arduino method millis(). Conversion to Epoch (or another time format) usually requires an internet connection, making it less versatile. The underlying data type of the value field depends on the sensor and the signal being measured. For ease of interoperability, all values (independently of their original type) are stored as float in the DataPoint.

SignalType:

Any kind of signal that can be measured by a Sensirion sensor is available as a SignalType. Both a description of the physical quantity being measured and the SI units of the measurement can be queried through the provided methods unitOf() and quantityOf(). The SignalTypes of measurements by Sensirion's SCD4X series of CO₂ sensors are for example

SignalType::CO2_PARTS_PER_MILLION
SignalType::TEMPERATURE_DEGREES_CELSIUS
SignalType::RELATIVE_HUMIDITY_PERCENT

MetaData:

The metadata encodes information regarding a measurement's origin:

MetaData(
    devicePlatform = *WIRED or BLE*;
    deviceID = *MAC address or sensor serial No.*;
    DeviceType = *BLEGadgetType or SensorType*;
);

The first field, devicePlatform, enables us to interpret information contained in the other two fields. A method to obtain a descriptive string of the device type is available with deviceLabel(). The DeviceType is one of the available BLEGadgetTypes or SensorTypes available in the Sensirion ecosystem.

To pick up on the SCD4X example above, assuming its signals are retrieved using UPT Sensor Autodetection, the MetaData would look as such:

MetaData(
    devicePlatform = DevicePlatform::WIRED;
    deviceID = 151181767251340; // Some 48-bit int
    DeviceType = SensorType::SCD4X;
);

Please refer to the BasicUsage.ino script for a featured example on how to use the Measurement struct.

Bluetooth Protocol

Sensirion BLE Gadgets send their measurements encoded in their device advertisements, one way to enable data transmission without needing to establish a connection to the device. This encoding depends on the data being transmitted (to ensure the receiver knows how to interpret the data), and can be queried in a dictionary (implemented as a std::map) provided by this library.

SampleType and DataType

These two identifiers (SampleType is a uint8_t and DataType an enum) serve as the main ways to identify (1) the shape of the transmission message, and (2) the nature of the contents therein. The device sending the advertisement (such as an Arduino board running Sensirion's BLE DIY Gadget software) typically knows what signals it wants to send (and therefore selects an appropriate DataType), while the receiving device reads the SignalType field in the ManufacturerData part of the BLE Advertisement message and maps it to the corresponding DataType.

For example, a Sensirion MyCO₂ Gadget sends an advertisement identified by DataType::T_RH_CO2_ALT and SignalType = 8, and containing the temperature, relative humidity and CO₂ concentration measured by the device.

Please refer to the Sensirion BLE Communication Protocol for a list of available SignalTypes and for the structure of a Sensirion BLE Advertisement.

SampleConfig

Once the DataType of the message is determined, it can be used to query configuration information of the message, such as it size and signals it contains, using the provided sampleConfigSelector dictionary provided:

SampleConfig sampleConfig = sampleConfigSelector[myDataType];

SampleConfig contains the following fields, among others:

• SampleConfig.sampleSizeBytes: useful for determining the size of the Advertisement message
• SampleConfig.sampleSlots: a dictionary mapping the slots to SignalType.

To pick up on the MyCO₂ Gadget, the corresponding sampleSizeBytes reads 6 (the 3 signal values transmitted get 2 bytes each), while the sampleSlots informs us of the order of the signals, the aforementioned Temperature, Relative Humidity, and CO₂ concentration.

En/De-coding functions

The signals are transmitted as uint16_t. To ensure no information is lost, encoding and decoding functions are provided. Since these are not uniform (being both signal and DataType dependant), sampleSlots also provides pointers to the appropriate conversion functions.

Please refer to the BLE_example.ino script for a featured example on how to encode/decode sensor data in a BLE Advertisement message.

Library example scripts

Two example scripts are provided:

• BasicUsage: An example showing how to use the Measurement struct to handle sensor data in general;
• BLE_example: An example showing how to encode/decode senor data for transmission via BLE Advertisement. Note that the example does not actually use Bluetooth, so it can be run on a board that does not have an antenna.

On PlatformIO, the examples can be run by by executing the following command from the library's root directory (containing this README):

• For example BasicUsage: $ pio run -t upload or $ pio run -e basicUsage -t upload (environment BasicUsage is default)
• For example BLE_example: pio run -e ble_example -t upload

In both cases, the output can be monitored by using the PlatformIO device monitor:

pio device monitor

Arduino IDE users can navigate to File -> Examples -> Sensirion Gadget BLE Lib -> BasicUsage or BLE_example, and click the upload button. The example outputs can be examined using the serial monitor (make sure to use 115200 baud).

How to install

Requirements

This library requires standard library compatibility. Some boards such as Arduino AVR Uno do not ship with this functionnality. This library was developed and tested on Espressif ESP32 boards.

Arduino

1. Download Arduino IDE and setup the environment for ESP32 platform
   • Follow this guide
   • Detailed Instructions for advanced users: Arduino-ESP32
2. Start the Arduino IDE and open the Library Manager by selecting Sketch -> Include Library -> Manage Libraries.... Search for the Sensirion UPT Core library in the Filter your search... field and install it by clicking the install button.
3. Connect your ESP32 DevKit to your computer.
4. In the Arduino IDE, make sure you have the ESP32 Dev Module and the correct Port selected.
5. Select File -> Examples -> Sensirion Gadget BLE Lib -> BasicUsage.
6. Connect your board and click the upload button.
7. The Serial Monitor displays randomly generated mock Measurements.

For users interested in the BLE en/decoding features, example script BLE_example is available.

PlatformIO

1. To install the PlatformIO Core, follow the steps detailed here.
2. Open this project in PlatformIO.
3. After you've conected your board, Compile, Upload and view Serial output or example script BasicUsage using the terminal:

pio run -t upload && pio device monitor

4. The Serial Monitor displays randomly generated mock Measurements.

For users interested in the BLE en/decoding features, example script BLE_example is available. It can be run with

pio run -e ble_example -t upload && pio device monitor

The example script will now display mock BLE advertisements in the console.

To use this library in your project, add Sensirion UPT Core to your lib_deps in the platformio.ini file.

License

See LICENSE.