Exercise: Using An Arduino with a Raspberry Pi

Objective

Use an Arduino Uno as an accessory to a Raspberry Pi.

This exercise assumes you have the basic Raspberry Pi materials as specified in 1.c.iv.1, as well as the following materials:

  1. Arduino Uno with USB cable.

Overview

The Raspberry Pi GPIO interface is great for easy and fast interfacing to individual digital I/O lines and SPI and I2C busses. However, there are still circumstances under which using an Arduino makes sense. Even though the Raspberry Pi is much, much faster than an Arduino, Linux is not a real time system and so other processes can interfere with the precise timing of your program. And many devices have exising libraries for the Arduino, so the fastest way to test a part can be to use the existing Arduino code but transmit the data to and from the Raspberry Pi for display and further processing.

Some specific rationale for using the Arduino/Pi combination would be a project requiring features compelling both an Arduino and a Pi:

Requirements motivating a Raspberry Pi: networking, graphics, audio signal processing, machine learning, databases, data logging, machine vision, extensive numerical computations, use of Python or Pure Data, libraries available under Linux, transferring an existing Pure Data patch into embedded form.

Requirements motivating an Arduino: simple analog input, rapid start-up (e.g. no boot time), existing hardware device libraries, tight signal timing requirements, fast digital pattern generation such as for driving a stepper motor.

With enough work, many of these Arduino requirements can be satisfied with just a Pi and additional hardware such as an external ADC, but may require porting device libraries. The timing can be improved by adjusting Linux process priorities and disabling unneeded services.

But a short startup time is the hardest requirement to overcome with the Pi by itself. So for example, a sophisticated project might ideally have a failsafe mode in which the Arduino can still provide minimum behavior while the Raspberry Pi is booting up or in case of program failure.

Steps and observations

  1. Using a laptop, program the OneInOneOutASCII Arduino sketch onto an Arduino Uno. (This is also possible from the Pi, but faster from a laptop.) This is available in the physcomp-examples/support/Arduino folder.
  2. Attach the Arduino to any of the Raspberry Pi USB ports.
  3. Locate the Pure Data and Arduino exercise material on the Raspberry Pi. The course image for the Pi includes this at the following location: /opt/cmuphyscomp/physcomp-f15/exercises/Pure-Data/pd-arduino/
  4. As before, run the OneInOneOutAsciiTest.pd patch in pd. This can be done from the command line as pd OneInOneOutAsciiTest.pd. Note that no special permissions are required; Pure Data will communicate with the Arduino over USB and not access the Pi hardware.
  5. Click on the [/dev/ttyACM0] message near the top of the patch. This is the name given to the Arduino serial device under Linux. (If it doesn't work, click the [devices] message and examine the console output.)
  6. As before, examine the Time box for clock data from the Arduino, try the LED toggle, examine the A0 input box with analog data.

Comments

A useful tool for debugging the Arduino connection from the Raspberry Pi is the minicom text-mode terminal emulator which can be run from the command line. The minicom commands are prefixed with Control-A: type Control-A Z to see a menu, or Control-A Q to quit.