The Seeeduino LoRaWAN module is operated by the 32bit microcontroller ATSAMD21G18 (ARM® Cortex®-M0+) running at 48MHz. It has 256 KB flash memory (to store the program code) and 32 KB of RAM (to store variables, status information, and buffers). The operating voltage of the board is 3.3V (this is important when attaching sensors and other peripherals; they also must operate on 3.3V). The board offers 20 general purpose digital input/output pins (20 GPIOs), 6 analog input pins (with 12bit analog digital converters (ADC)), 1 analog output pin (with 10bit digital analog converter (DAC)), 2 serial ports (2 programmable Universal Asynchronous Receiver and Transmitters, UARTs). The board comes with an embedded lithium battery management chip and status indicator led, which allows to directly connect a 3.7V LiPo rechargeable battery that will be automatically recharged when the board is powered over its USB connector. The battery voltage level can be queried from analog input A4, the charging status (charging, full) from analog input A5. The Seeeduino LoRaWAN (without GPS module) is available in German shops for around 37 €.
The LoRa transmitter and receiver is encapsulated within an RHF76-052AM module from the Chinese company RisingHF. The RF module contains its own microcontroller, which implements the LoRaWAN protocol. The module is connected via the serial interface to the ATSAMD21G18 microcontroller and can be controlled by sending so-called 'AT' commands. The implemented LoRaWAN functionality is compatible with LoRaWAN Class A/C. The explanation of all supported commands as well as a number of examples on how to use the Seeeduino LoRaWAN are given on the Seeeduino LoRaWAN Wiki.
The board has 4 on-board Grove connectors. 'Grove' is a framework developed by the company Seeed Studio standardizing the connectors, operating voltages, and pin configurations for attaching peripherals like sensors, actuators, and displays to microcontrollers. Note that the Grove modules need to be able to operate (also) on 3.3V (instead of only with 5V), because the Seeeduino LoRaWAN board only provides 3.3V to the Grove connectors. Important hint: if you want to use the Grove ports, make sure to include the command "digitalWrite(38, HIGH)" in the setup() routine of your program. A low level on that pin deactivates the power supply of the four Grove ports.
The board has also the typical Arduino UNO connectors allowing to attach so-called Arduino shields (however, please note that the shields must be working with 3.3V; the normal operating voltage for the Arduino UNO microcontroller and its shields is 5V).
We attached a Bosch BME280 sensor module to the extension connectors of the microcontroller board using 5 wires. The employed BME280 sensor board is a cheap no-name product. VCC and GND are connected to 3.3V and GND of the microcontroller board respectively. SCL and SDA from the sensor board are connected to SCL and SDA of the microcontroller board. SDO from the sensor board is also connected to GND of the microcontroller; it selects 0x76 as the I2C device address (a high level, i.e. 3.3V, would set the device address to 0x77 - this is relevant, if two sensor modules should be operated on the same I2C bus). Note that there is also a Seeed Grove BME280 module available which alternatively can be used and connected to the first I2C Grove connector of the Seeeduino LoRaWAN board. The BME280 measures temperature in the range -40 - 85 ℃, with ±1.0°C accuracy; 0% - 100% relative humidity with ±3% accuracy; and atmospheric pressure in the range 300 - 1100 hPa (1 hPa= one hundred Pa) with ±1.0 hPa accuracy. It offers the two interface standards I2C and SPI (we are using I2C here and the default I2C address 0x76). The atmospheric pressure changes with altitude, hence, the BME280 can also be used to measure the approximate altitude of a place.
The sensor node has been programmed using the Arduino IDE. Please note, that in the Arduino framework a program is called a 'Sketch'.
In order to support the "Seeeduino LoRaWAN" board with the Arduino IDE, make sure to have installed the package "Seeed SAMD boards by Seeed Studio" in version 1.3.0 using the board manager in the Arduino IDE. This is also explained on a dedicated webpage from Seeed Studio. The sketch requires the software libraries "RTCZero", "Arduino_BME280", "Adafruit_Sensor", "Wire", and "LoRaWAN". The first three have to be installed using the library manager of the Arduino IDE, the fourth library is already installed with the Arduino IDE and the latter library comes with the "Seeeduino LoRaWAN" board installation.
After the sketch has successfully established a connection to The Things Network it reports the air temperature, relative humidity, air pressure, altitude, and the voltage of a (possibly) attached LiPo battery every 5 minutes. All five values are being encoded in two byte integer values each and then sent as a 10 bytes data packet to the respective TTN application using LoRaWAN port 33. Please note, that LoRaWAN messages can be addressed to ports 1-255 (port 0 is reserved); these ports are similar to port numbers 0-65535 when using the Internet TCP/IP protocol. Voltage, pressure, altitude, and humidity values are always greater or equal to 0, but the temperature value can also become negative. Negative values are represented as a two's complement; this must be considered in the Payload Decoding Function used in The Things Network (see below).
In between two sensor readings the microcontroller, the LoRaWAN module, and the sensor module are going into deep sleep mode to save battery power. During LoRaWAN data transmission the device draws up to 65mA current. When in sleep mode the entire node only draws around 0.06 mA power. Hence, with a 6600 mAh 3.7V LiPo battery and the current version of the sketch the system should be able to run for many years before recharging (not taking into account the self-discharging rate of the battery).
The source code is provided in the following section Arduino_Sketch_seeeduino_lorawan.ino
The services used for this sensor-node are:
- TheThingsNetwork service for LoRaWAN network service.
- TheThingsNetwork - OGC SensorWeb integration for uploading LoRaWAN sensor data into OGC infrastructure.
The LoRaWAN protocol makes use of a number of different identifiers, addresses, keys, etc. These are required to unambiguously identify devices, applications, as well as to encrypt and decrypt messages. The names and meanings are nicely explained on a dedicated TTN web page.
The sketch given above connects the sensor node with The Things Network (TTN) using the Over-the-Air-Activation (OTAA) mode. In this mode, we use the three keys AppEUI, DevEUI, AppKey. The DevEUI should normally be delivered with the sensor node by the manufacturer. However, it seems that there is no explicit DevEUI provided with the Seeeduino LoRaWAN module. Therefore, it has to be generated automatically together with the other two keys using the TTN console. Each sensor node must be manually registered in the TTN console before it can be started. This assumes that you already have a TTN user account (which needs to be created otherwise). In the TTN console create a new device with also the DevEUI being automatically generated. After the registration of the device the respective keys (AppEUI, DevEUI, AppKey) can be copied from the TTN console and must be pasted into the the proper places in the source code of the sketch above. Please make sure that you choose for each of the three keys are in the correct byte ordering (all are in MSB, i.e. in the same ordering as given in the TTN console). A detailed explanation of these steps is given on this page. Then the sketch can be compiled and uploaded to the Seeeduino LoRaWAN microcontroller. Note that the three constants (AppEUI, DevEUI, AppKey) must be changed in the source code for every new sensor node.
Using the OTAA mode has the advantage over the ABP (activation by personalization) mode that during connection the session keys are newly created which improves security. Another advantage is that the packet counter is automatically reset to 0 both in the node and in the TTN application.
Everytime a data packet is received by a TTN application a dedicated Javascript function is being called (Payload Decoder Function). This function can be used to decode the received byte string and to create proper Javascript objects or values that can directly be read by humans when looking at the incoming data packet. This is also useful to format the data in a specific way that can then be forwarded to an external application (e.g. a sensor data platform like MyDevices or Thingspeak ).
Such a forwarding can be configured in the TTN console in the "Integrations" tab. TTN_Payload_Decoder_Seeeduino_lorawan given here checks if a packet was received on LoRaWAN port 33 and then assumes that it consists of the 10 bytes encoded as described above. It creates the five Javascript objects 'temperature', 'humidity', 'pressure', 'altitude', and 'vbattery'. Each object has two fields: 'value' holds the value and 'uom' gives the unit of measure. The source code can simply be copied and pasted into the 'decoder' tab in the TTN console after having selected the application. Choose the option 'Custom' in the 'Payload Format' field. Note that when you also want to handle other sensor nodes sending packets on different LoRaWAN ports, then the Payload Decoder Function can be extended after the end of the if (port==33) {...} statement by adding else if (port==7) {...} else if (port==8) {...} etc.
The presented Payload Decoder Function works also with the TTN-OGC SWE Integration for the 52° North Sensor Observation Service (SOS). This software component can be downloaded from this repository. It connects a TTN application with a running transactional Sensor Observation Service 2.0.0 (SOS). Data packets received from TTN are imported into the SOS. The SOS persistently stores sensor data from an arbitrary number of sensor nodes and can be queried for the most recent as well as for historic sensor data readings. The 52° North SOS comes with its own REST API and a nice web client allowing to browse the stored sensor data in a convenient way.
We are running an instance of the 52° North SOS and the TTN-OGC SWE Integration. The web client for this LoRaWAN sensor node can be accessed on this webpage. Here is a screenshot showing the webclient:
Arduino_Sketch_seeeduino_lorawan/Arduino_Sketch_seeeduino_lorawan.ino
TTN_Payload_Decode.js
- Seeeduino LoRaWAN microcontroller
- Seeeduino LoRaWAN Wiki with instructions
- A short presentation on LoRaWAN basics and using the Seeeduino LoRaWAN board
On the RisingHF RHF76-052AM LoRaWAN module
On the Bosch BME280 sensor