This page describes an experiment in using PyMata Express to communicate (using Firmata) between a Raspberry Pi and an Arduino microcontroller.

Installing PyMata Express on Linux/Raspberry Pi#

To install Pymata Express on Linux (including Raspberry Pi) and macOS computers, open a terminal window and type:

sudo pip3 install pymata-express

Installing Firmata Express on an Arduino#

The FirmataExpress page describes how to install it on an Arduino, using the Arduino IDE.

In a nutshell, once you've installed the Arduino IDE, using the Tools:Manage Libraries... command you install "FirmataExpress" and Erick Simoes' "Ultrasonic" library. Then open up the File:Examples:FirmataExpress:FirmataExpress sketch, set your IDE to the type of Arduino you have with Tools:Board, then use Sketch:Upload to upload the FirmataExpress sketch. Then your Raspberry Pi can communicate with it using Python sketches, an example of which is shown below.

Example Code#

Here is an example demonstrating using a callback to monitor the state changes of a digital input pin.

import asyncio
import time
import sys
from pymata_express import pymata_express

"""
Setup a pin for digital input and monitor its changes
using a callback.
"""

# Setup a pin for analog input and monitor its changes
DIGITAL_PIN = 12  # arduino pin number
IDLE_TIME = .001  # number of seconds for idle loop to sleep

# Callback data indices
# Callback data indices
CB_PIN_MODE = 0
CB_PIN = 1
CB_VALUE = 2
CB_TIME = 3


async def the_callback(data):
    """
    A callback function to report data changes.
    This will print the pin number, its reported value and
    the date and time when the change occurred

    :param data: [[pin, current reported value, pin_mode, timestamp]
    """
    date = time.strftime('%Y-%m-%d %H:%M:%S', time.localtime(data[[CB_TIME]))
    print(f'Pin: {data[[CB_PIN]} Value: {data[[CB_VALUE]} Time Stamp: {date}')


async def digital_in(my_board, pin):
    """
     This function establishes the pin as a
     digital input. Any changes on this pin will
     be reported through the call back function.

     :param my_board: a pymata_express instance
     :param pin: Arduino pin number
     """

    # set the pin mode
    await my_board.set_pin_mode_digital_input(pin, callback=the_callback)

    while True:
        try:
            await asyncio.sleep(IDLE_TIME)
        except KeyboardInterrupt:
            await board.shutdown()
            sys.exit(0)

# get the event loop
loop = asyncio.get_event_loop()

# instantiate pymata_express
board = pymata_express.PymataExpress()

try:
    # start the main function
    loop.run_until_complete(digital_in(board, 12))
except (KeyboardInterrupt, RuntimeError) as e:
    loop.run_until_complete(board.shutdown())
    sys.exit(0)

References#

• Introducing ItsyBitsy on Adafruit
• ItsyBitsy M4 Pinout