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Use your Arduino as a data acquisition card under Python

Project description

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.

Installation

Upload firmata

Upload the standard firmata sketch into your Arduino with:

File -> Examples -> Firmata -> Standard Firmata

Install pyfirmata2

The preferred way to install is with pip / pip3. Under Linux:

pip3 install pyfirmata2

and under Windows/Mac type:

pip install pyfirmata2

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

Usage

Initialisation

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:

board.analog[0].register_callback(myCallback)
board.analog[0].enable_reporting()

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:

board.analog[1].read()

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:

board.digital[13].write(1)

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)

Example code

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.

Troubleshooting

No analogue data after stopping the program in Spyder

If you kill the Python program in Spyder the serial port will stay open and locked.

Solution: always terminate the program properly by closing all plot windows in Spyder or start the program from the (Anaconda-) console. This is the safest option because even a ctrl-c will close the serial port.

After an update still the old version is used

If you use the “-user” option python might keep the 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.

Credits

The original pyFirmata was written by Tino de Bruijn. The realtime sampling / callback has been added by Bernd Porr.

Project details


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