Use your Arduino as a data acquisition card under Python
PyFirmata2 turns your Arduino into a data acquisition card controlled by Python.
Just upload the default firmata sketch into your Arduino and you are all set.
pyFirmata2 is an updated version of pyFirmata which adds precise sampling to the API so that it’s possible to filter signals and in general do signal processing. Instead of “sleep” commands which have unreliable timing the Arduino performs the sampling in its firmware and transmits the data then to pyFirmata2. The Python application simply registers a callback which is then called every time after new data has arrived.
This API has been used in my Digital Signal Processing (DSP) class to practise realtime filtering of analogue sensor data. Examples can be viewed on the YouTube channel of the class: https://www.youtube.com/user/DSPcourse
Install the Arduino IDE on your computer: https://www.arduino.cc/en/Main/Software
Start the IDE and upload the standard firmata sketch into your Arduino with:
File -> Examples -> Firmata -> Standard Firmata
The preferred way to install is with pip / pip3. Under Linux:
pip3 install pyfirmata2 [--user] [--upgrade]
and under Windows/Mac type:
pip install pyfirmata2 [--user] [--upgrade]
You can also install from source with:
git clone https://github.com/berndporr/pyFirmata2 cd pyFirmata2
Under Linux type:
python3 setup.py install
Under Windows / Mac:
python setup.py install
Create an instance of the Arduino class:
from pyfirmata import Arduino board = Arduino(Arduino.AUTODETECT)
which automatically detects the serial port of the Arduino.
If this fails you can also specify the serial port manually, for example:
board = Arduino('COM4')
Under Linux this is usually /dev/ttyACM0. Under Windows this is a COM port, for example COM4. On a MAC it’s /dev/ttys000, /dev/cu.usbmodem14101 or check for the latest addition: ls -l -t /dev/*.
Starting sampling at a given sampling interval
In order to sample analogue data you need to specify a sampling interval in ms. The smallest interval is 10ms:
board.samplingOn(samplinginterval in ms)
Calling samplingOn() without its argument sets the sampling interval to 19ms.
Enabling and reading from analoge pins
To process data at a given sampling interval register a callback handler and then enable it:
where myCallback(data) is then called every time after data has been received and is timed by the arduino itself.
You can also read additional analogue pins any time by issuing a read command:
This is useful for reading additional pins within a callback handler to process multiple pins simultaneously. Note that the data obtained by read() is read from an internal buffer which stores the most recent value received from the Arduino. This call is non-blocking. You also need to run enable_reporting() on that pin before you can use read().
Writing to a digital port
Digital ports can be written to at any time:
For any other functionality use the pin class below.
The pin class
The command get_pin requests the class of a pin by specifying a string, composed of ‘a’ or ‘d’ (depending on if you need an analog or digital pin), the pin number, and the mode (‘i’ for input, ‘o’ for output, ‘p’ for pwm). All seperated by :. Eg. a:0:i for analog 0 as input or d:3:p for digital pin 3 as pwm:
analog_0 = board.get_pin('a:0:i') analog_0.read() pin3 = board.get_pin('d:3:p') pin3.write(0.6)
Closing the board
To close the serial port to the Arduino use the exit command:
The directory https://github.com/berndporr/pyFirmata2/tree/master/examples contains two realtime Oscilloscopes with precise sampling rate, a digital port reader, the ubiquitous flashing LED program and a program which prints data using the callback handler.
Start your program from the (Anaconda-) console / terminal and never within Spyder. Here is an example for Windows:
(base) D:\> (base) D:\>cd pyFirmata2\examples (base) D:\pyFirmata2\examples>python realtime_two_channel_scope.py
The problem with Spyder is that it won’t let your Python program terminate properly which leaves the serial port in an undefined state. If you then re-run your program it won’t be able to talk to your Arduino. In the worst case you need to reboot your computer. Bottomline: use Spyder for editing, run the program from the console / terminal.
After an update still the old version is being used
If you use the –user option to install / update packages Python might keep older versions.
Solution: Do a pip uninstall pyfirmata2 multiple times until no version is left on your computer. Then install it again as described above.
The original pyFirmata was written by Tino de Bruijn. The realtime sampling / callback has been added by Bernd Porr.
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