Data Exchange Between Python on Linux and an Arduino Sketch

Overview

The container infrastructure provided by Arduino contains a pre-built Python image that you can use to run python applications on the Portenta X8. In this tutorial we're going to build a container based on a provided one. While all the peripherals can be accessed from the iMX8 processor running the Linux environment, it can be useful to let the onboard microcontroller take care of certain peripheral handling and just exchange the required data between the microcontroller and the Python application. In this tutorial you will learn how to do that. If you haven't done so, read through the foundations article to understand the fundamental concepts of the X8 and the provided infrastructure.

Goals

• Learn how the RPC mechanism on the X8 works
• Learn how to exchange sensor data between Linux and an Arduino sketch
• Learn how to modify a container and run it
• Learn how to use commands to debug the container and service infrastructure

Required Hardware and Software

• Portenta X8 board
• Portenta breakout board
• Any sensor (in this example we'll use an BME680 I²C module)

Python on the X8

Python is a modern and powerful scripting language that can be used for all sorts of use cases. In this tutorial we only read sensor data from an Arduino sketch without doing anything interesting with it, but you could extend the example and process the data further.

Communication Between Linux and Arduino Sketches

The python script will run on the Linux side and therefore on the iMX8 processor. The Arduino sketch on the other hand will run on the STM32H747 microcontroller. That allows for real-time processing on the Arduino side while running a fully fledged operating system on iMX8. However the two processors need a communication mechanism to exchange data with one another. The communication mechanism that is being used is referred to as RPC (Remote Procedure Call). To facilitate the communication the M7 core on the STM32H747 microcontroller is used which hands over any data / request to the M4 core. That means your Arduino sketch will solely run on the M4 core. Dual core processing on the Arduino side is currently not supported.

On the Linux side there is a service that takes care of sending data between the two worlds. It's called

m4-proxy

adb shell

sudo journalctl -fu m4-proxy

sudo systemctl restart m4-proxy

The Arduino Sketch

The Arduino sketch to read sensor data doesn't look much different from an ordinary sketch. The only thing that differs is that we expose the sensor data via RPC.

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1RPC.bind("temperature", []{ return bme.temperature; });
2RPC.bind("humidity", []{ return bme.humidity; });
3RPC.bind("pressure", []{ return bme.pressure / 100.0F; });
4RPC.bind("gas", []{ return bme.gas_resistance / 1000.0; });
5RPC.bind("altitude", []{ return bme.readAltitude(SEALEVELPRESSURE_HPA); });

Two additional header files need to be included:

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1#include <RPC.h>
2#include <SerialRPC.h>

The bind function makes the data available via the specified name e.g. "temperature". In our example an anonymous function is created that returns the corresponding sensor property whenever requested. Alternatively you could bind the name to an existing, named function instead. The data can then easily be requested using that name (e.g. "humidity") by querying the

m4-proxy

You can find the sketch in the software package here. You may need to change the sketch depending on what sensor you would like to read from. If you're using an I²C sensor, you can connect SCL to PWM6 and SDA to PWM8 on the Portenta breakout. That's because the labeled I²C pins on the Portenta Breakout are only available on the Linux side. If you're using an analog sensor you can connect it to any analog pin. Please refer to the pinout diagram on the Portenta Breakout documentation page.

Make sure you've installed the "Arduino Mbed OS Portenta Boards" core and upload the sketch to the X8 in the Arduino IDE or via Arduino CLI.

Debugging the Arduino Sketch

To check if the Arduino sketch is working correctly you may want to read the messages from the

Serial.println

py-serialrpc

adb push py-serialrpc /home/fio

Log into the X8 shell with

adb shell

serialrpc

sudo docker build . -t py-serialrpc

-t

cd..

sudo docker-compose up -d

-d

sudo docker-compose stop

Check if the container is running by executing

sudo docker ps

sudo docker-compose logs -f --tail 20

-d

The Python Application

The Python application requests the sensor data from the M4 over RPC and unpacks the message. Data can be requested by calling the function exposed over RPC on the M4 e.g.:

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1m4_proxy_address = 'm4-proxy'
2m4_proxy_port = 5001
3rpc_address = RpcAddress(m4_proxy_address, m4_proxy_port)
4rpc_client = RpcClient(rpc_address)
5temperature = rpc_client.call('temperature')

The files for the complete Python application can be found in the same package as the Arduino sketch (see above). Upload the

python-sensor-rpc

adb push python-sensor-rpc /home/fio

adb shell

python-sensor-rpc

sudo docker build . -t python-sensor-rpc

sudo docker-compose up

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1python-sensor-rpc_1  | ============================================
2python-sensor-rpc_1  | ==       Portenta X8 Sensor reading       ==
3python-sensor-rpc_1  | ============================================
4python-sensor-rpc_1  |
5python-sensor-rpc_1  | Temperature:  25.904266357421875
6python-sensor-rpc_1  | Humidity:  25.564695358276367
7python-sensor-rpc_1  | Pressure:  976.4400024414062
8python-sensor-rpc_1  | Gas:  136.496
9python-sensor-rpc_1  | Altitude:  311.0769348144531

Keep in mind that, whenever you change anything in the Python script on your computer you will have to sync it back to the X8 and re-build the container:

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1# On your computer
2adb push python-sensor-rpc /home/fio
3
4# On X8
5sudo docker-compose down
6sudo docker build . -t python-sensor-rpc
7sudo docker-compose up

Alternatively you could modify the files directly on the X8 using an editor such as VIM so you don't need to upload the files all the time. Re-building the container will be necessary in any case though. In case you wonder how to specify the python script that is executed when running a container, have a look at the

Dockerfile

ENTRYPOINT

ENTRYPOINT [ "python3", "m4_to_python.py"]

Conclusion

In this tutorial you learned how to use the docker infrastructure to build a container that runs a python application. You have also learned how to use the RPC mechanism to exchange data between the microcontroller and the iMX8 which runs the Linux operating system.

Next Steps

• You may now further process the data that you receive from the Arduino sketch and e.g. upload it to a cloud service or similar.
• Familiarize yourself with Docker commands to adjust the docker configuration to your needs.