Objective

Communicate data between Pure Data and an Arduino using the USB serial bus.

The Arduino IDE and Pure Data solve different problems well. The Arduino has easy direct access to its hardware interface, and can create and respond to electrical signals with very low latency. Pure Data is well-suited to interactive real-time programming, but the latency and jitter is considerable higher than on a microcontroller, and on a laptop lacks access to hardware interfaces. This exercise will demonstrate one way of coordinating Pure Data and the Arduino to take advantage of the best features of each by linking them using a simple ASCII message protocol over the USB serial bus.

The serial communication does introduce significant latency between the Arduino and Pd, so this approach is strongest for systems in which Pd is used for tasks such as the GUI, visual display, sequencing, and overall event control. It would not be suitable for high-bandwidth tasks such as cycling individual digital outputs to create a stepper motor waveform. However, the extensibility of the protocol means that you can add those low-level hardware behaviors to the Arduino client program and invoke them via messages.

Setting up the exercise requires a couple of preparatory steps:

1. Compiling and loading the OneInOneOutASCII Arduino sketch on an Arduino Uno.
2. Determining the exact name of the serial port offered by the Arduino over USB.
3. Running the OneInOneOutAsciiTest PureData patch in pd-extended on your laptop. (The patch requires the moocow library and won't work in 'vanilla' Pd.)
4. Verifying or setting the serial port name in the patch.

The OneInOneOutASCII sketch is provided in the course distribution in the folder named support/Arduino/OneInOneOutASCII. If you have not cloned the course materials from github, you can download it directly from github. The source code of the sketch provides details of the messaging scheme

The Pd patch is available in the same materials folder as this file or directly from github.

Steps and observations

1. Upload the OneInOneOutASCII sketch onto an Arduino Uno.
2. Open the Serial Monitor, set the baud rate to 115200 and the line ending to Newline. You should see data messages start to stream by.
3. Enter 'poll 1000' in the input box and press enter. The message rate should slow.
4. Enter 'led 0' in the input box and press enter. The LED marked L should go off.
5. Enter 'led 1' in the input box and press enter. The LED marked L should go on again.
6. Note the exact serial port name from the Tools/Port menu in the Arduino IDE.
7. Exit the Arduino IDE and load the OneInOneOutAsciiTest.pd patch in pd-extended.
8. Near the top of the sketch are a set of message objects naming different serial ports. If one matches the port name on your machine, click it to open the port. If not, edit one of the boxes to match the port name and then click it.
9. Right below is a toggle labeled LED. Click it several times to send led control messages and observe the Arduino LED. On each press you should also see the RX LED flash briefly as the message is received on the Arduino.
10. Further down on the patch is a number box labeled 'Time'. Verify that it is continuously changing with the microsecond clock values from the Arduino.
11. Explore the other GUI areas on the patch.

Comments

This particular Arduino IDE sketch is going to serve as the basis for all the following exercises in which we connect hardware to the Arduino and control it from Pd.

For a challenge, examine the source code for both the sketch and the patch and work out how to add additional message types.