Now that you’ve assembled the MIDI interface for your Arduino, it’s time to try it out. That means you need a few basic sketches to test and debug the interface.

First, a philosophical word or two. Testing is one of the most neglected and least emphasized topics in engineering. Other than careful design, assembly and coding, nothing else is as important to the success of a project as testing. In practice, you should be thinking about testing while you design, code, layout and solder. Without getting too computer science-y, it all comes down to controllability and observability — the ability to apply a stimulus (test input) and the ability to measure and assess a response (expected vs. actual test output).

Take the case of our MIDI interface. There are two major subsystems: the MIDI IN circuit and the MIDI OUT circuit. We must drive the MIDI IN circuit with a keyboard (or some other device) that sends MIDI to the 5-pin MIDI IN connector. The Arduino reads the logic level bit-stream through its RX pin (pin D0). The messages in the bit-stream need to conform to the MIDI message standard. On the MIDI OUT side, the Arduino sends a logic level bit-stream into the MIDI OUT circuit via the TX pin (pin D1). Again, the bit-stream carries standard MIDI messages. We must observe the outgoing MIDI messages, perhaps by playing the MIDI stream into a MIDI-compatible keyboard or tone module.

In addition to controllability and observability, you also need a test plan that identifies the features to be tested and the order in which you intend to test those features. A carefully constructed plan can make your work more efficient and systematic. Again, taking the case of the MIDI interface, I intend to test the MIDI OUT side first because I can easily write a sketch to generate MIDI messages and that sketch does not require a working MIDI IN circuit. Once I know that the MIDI OUT is correct, I can use the MIDI OUT as part of MIDI IN testing. By testing MIDI OUT first, I can work around the limited controlability and observability of our system.

I hope this plan makes sense to you. Here’s a few additional points. We don’t need to test all MIDI standard messages and can restrict testing to simple 3-byte messages like NOTE ON and NOTE OFF. We need to have a reliable, known-good MIDI sender (like a keyboard or other controller) and MIDI receiver (like a tone module). I have two MIDI-compatible devices — a Roland SK-88 Pro Sound Canvas keyboard and a Boss DS-330 Dr. Synth module — that I like to use for testing. Both devices now have emeritus status, meaning that I won’t cry too much if I blow them up! I send the MIDI OUT from the Arduino to both devices and play the audio through self-powered speakers. I play the SK-88 keyboard and send MIDI NOTE ON and NOTE OFF messages to the MIDI IN circuit under test and read those messages on the Arduino.

With these preliminaries out of the way, let’s dive into the details.

MIDI OUT: Simple MIDI Sequencer

In order to test MIDI OUT, we need a sketch that sends MIDI NOTE ON and NOTE OFF messages via the TX pin into the MIDI OUT circuit. With a MIDI tone generator connected to the 5-pin MIDI OUT connector, we just need to listen to the audio from the tone generator to determine if the outgoing notes are correct.

I took the simple Arduino tone sequencer sketch and modified it to send MIDI NOTE ON and NOTE OFF messages. I replaced calls to the Arduino tone() function with code to send MIDI NOTE ON and NOTE OFF as appropriate. (Reusing code this way is known as “copy and modify.”) The new sketch is a simple MIDI sequencer that repeatedly plays back a musical sequence of notes through the MIDI OUT port. The sketch is a useful little program in itself — not just a specialized test.

Here are direct links to the source code for the sketch:

MidiNoteNum.h: Defines symbolic constants for the note names
ToneNote.h: Defines symbolic constands for note/rest durations
MidiSeq.ino: The simple MIDI sequencer Arduino sketch

The sketch does not use the Arduino MIDI library. It sends messages by making calls to the Arduino Serial library. I will use the Arduino MIDI library in future projects, especially when we need to parse complicated incoming messages.

The sketch sends messages on MIDI channel 1. So, be sure to have the destination tone generator configured to receive on channel 1 (or OMNI).

Here’s a possible gotcha that could drive you insane. The Arduino’s serial port is used for both uploading sketches and for MIDI communications. MIDI operates at a rather “unusual” data rate of 31,250 bits per second and most likely does not match the default rate established by the Arduino IDE. Even worse, the MIDI devices used for testing (i.e., keyboard controller) may be sending real time status messages that collide with commands from the IDE. Soooo, you may see messages about “sync” failure when uploading a sketch. If that is the case, then temporarily disconnect the MIDI IN circuit while uploading a sketch. Also, you will not be able to print to the serial port, etc. and see text serial messages in the IDE’s Serial Monitor window. Unfortunately, this limits the use of serial communication for tracing and debugging purposes when MIDI is connected and active.

MIDI IN: Echo MIDI IN to MIDI OUT

Once we know that the MIDI OUT port is working, it’s time to check the MIDI IN port. An easy way to test is to simply echo the incoming MIDI messages to the MIDI OUT port. If we have a MIDI keyboard connected to MIDI IN and a tone generator connected to MIDI OUT, then anything that we play on the keyboard should be heard through the tone generator. If the keyboard and tone generator are part of the same instrument (e.g., a synthesizer workstation, arranger or digital piano), be sure to turn LOCAL off. You want to hear the MIDI messages coming in through the instrument’s MIDI IN port.

Here is the code for the MIDI IN to MIDI OUT sketch.

//
// Send MIDI IN to MIDI OUT (THRU)
//

// Author:  P.J. Drongowski
// Date:    30 May 2016
// Version: 1.0
//
// Copyright (c) 2016 Paul J. Drongowski
// Permission granted to use, copy and distribute

// Uncomment the following line to use the Arduino 
// MIDI library instead of serial read() and Write().
// #define USE_MIDI_LIBRARY 1

#ifdef USE_MIDI_LIBRARY

#include <MIDI.h>
MIDI_CREATE_DEFAULT_INSTANCE() ;

void setup() {
  MIDI.begin(MIDI_CHANNEL_OMNI) ;
}

void loop() {
  if (MIDI.read())
  {
    MIDI.send(MIDI.getType(),
              MIDI.getData1(),
              MIDI.getData2(),
              MIDI.getChannel());
  }
}

#else

int midiByte = 0 ;

void setup() {
  // Initialize the serial port for MIDI
  Serial.begin(32500) ;
}

void loop() {
  if (Serial.available() > 0) {
    midiByte = Serial.read() ;
    Serial.write(midiByte) ;
  }
}
#endif

There are two ways to echo the incoming MIDI message stream. One way is to echo the bytes arriving on the serial input to the serial output. This is the most straightforward method. The other way is to use the Arduino MIDI library to read incoming MIDI messages and send the messages to the MIDI OUT port. The code (above) implements both methods and uses conditional compilation (#ifdef) to select and compile one of the methods. If you want to get your feet wet with the Arduino MIDI library, then define the symbol USE_MIDI_LIBRARY before the #ifdef.

Here are a few links for the Arduino MIDI library to help you get started.

Arduino MIDI Library site
Arduino MIDI Library Documentation
Deprecated site

The current version is 4.2. The Arduino IDE has a nice library management system: Sketch > Include Libraries > Manage Libraries.