You can play music with the Trellis M4 by turning it into a great input device for synthesizers! It can send standard MIDI (Musical Instrument Digital Interface) note data over USB MIDI to a software synth on your computer, laptop, (including web browser based synths!), or mobile device.

Using the built-in accelerometer (thanks to Analog Devices for the ADXL343!), we can also send control change (CC) data, such as mod wheel, and filter cutoff values, and even pitch bend settings just by tilting the Trellis M4! 

You can even use the Trellis M4 to control hardware synthesizers by using a conversion device that acts as a USB MIDI host and sends out traditional MIDI over a 5-pin DIN cable or more modern TRS 3.5mm MIDI connection. It's even possible to go really old school and convert to gate and CV (control voltage) for a modular synth or pre-MIDI device..

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Assembly and Familiarization

Take a look at the Trellis M4 main guide for lots more info and the assembly page in case you haven't yet built the enclosure.

MIDI is a venerable protocol (dating back to 1983) that is used to communicate between synthesizers, controllers, sequencers, sample players, computers, mobile devices, drum machines, and other electronic music making devices.

Electronic Music Gear

Electronic music gear is often made up of multiple discreet components that each have their specialized task to perform. They can communicate with each other through data messages, control voltages, gate and trigger signals, and/or audio signals in order to come together as a whole, united system capable of being performed, recorded, played back, and ultimately, producing audible music!

MIDI Messages

A simple and very common use case is to have a controller, such as a piano-style keyboard, send Note On and Note Off data to a music synthesizer (including software synths on your computer or mobile device).

Press a key and a message is sent telling the synth to play a specific musical note. Release the key and a message is sent to the synthesizer telling it to stop playing that note.

Getting a bit fancier than simple on/off messages, MIDI can also be used to send "continuous controller" CC messages, typically the result of turning a knob or pushing a slider on the keyboard controller. These can be used to sweep through the cuttoff frequency of a low pass filter, or modulate a tremolo, and many, many other parameters.

A similar scheme is also used to send pitch bend info -- often represented by a pitch bend wheel on the left side of a MIDI keyboard.

Trellis M4 MIDI

We can use the Trellis M4 to send any kind of MIDI message we like, right over USB. In this project, we've mapped the 32 buttons to play the "keys" on your synth, from a low C (MIDI Note 32) up to a high G (MIDI note 64). You can hold many buttons at once to create chords if you're using polyphonic synth software.

But that's not all! We can also play very expressive pitch bend and modulation with the Trellis M4. This is all thanks to the Analog Device ADXL343 accelerometer built right onto the board! Tilt left and right to pitch bend down and up, and tilt forward and backward to gradually adjust the filter or tremolo or whatever you like on MIDI CC 1.

In order to make it simple to get up and running with your Trellis M4 Expressive MIDI Controller with no programming required, we've created a drag-and-drop firmware you can use. You can skip to the page Custom Code in Arduino if you want to customize things!

If you're just getting started with your Trellis M4, there is lots more info here in the main Learn Guide!


First, download the firmware file linked in the button below and save it to your computer hard drive somewhere you'll be able to find it, such as your Downloads folder.

This program will replace CircuitPython, to get back to your CircuitPython projects, reinstall it by following

Install the Firmware

Plug your Trellis M4 into your computer with a good quality, data capable USB cable. Life is too short to go through the pain of accidentally using a power-only USB cable, so please round up any you own, cut them in half, travel to a distant land, bury them, and dance on their grave.

Your Trellis M4 will start up, and you'll see a purple indicator light on the back side.

Bootloader Mode

Now, we'll put the Trellis M4 into "bootloader" mode. In this mode it will appear as a USB drive on your computer and will be ready to receive a new .uf2 firmware file. Use a thin, pointed object such as a headphone plug to double-click the reset button on the back side of the board.

Once you have double-clicked the reset button, the indicator LED will turn green. You'll notice a new USB drive appear on your computer named TRELM4BOOT, this is the bootloader USB storage built right into the Trellis M4. It is now ready to receive the firmware file.

Drag and Drop

Now, drag the TM4MIDIC.UF2 file onto the TRELM4BOOT drive. The file will copy over in a few seconds and then the Trellis M4 will automatically restart itself (you'll see the TRELM4BOOT drive dissapear, don't worry, this is normal!). The status LED will return to purple, indicating regular operation mode.

Congratulations, you've updated the firmware and you're ready to play!

If you ever need to manually switch out of bootloader mode, simply press the reset button one time.

Test MIDI Output

You can test to see that your Trellis M4 is outputting MIDI signals properly by using a MIDI utility. These will report all activity coming from any MIDI controller plugged into your system. Here are some to try:

There are many, many, many, many (did we mention "many"?) synthesizers out there in the world that you can control using MIDI note and CC controls.

You may already have a favorite software synth, and chances are it'll work with the Trellis M4. In case you don't have one already picked out, here are some good ones to try.


Nearly any software synth (or DAW (digital audio workstation) you find will allow you to use USB MIDI to control the notes, pitch bend, and CC input. Here are some good ones to try:



Chrome Web Browser   




free open source         








Linux / Windows / mac os  

free open source         

Windows  / mac os







mac os    



Play Music

Once you've picked a synth (the Chrome browser ones are really quick to try out, hint, hint!) plug in your Trellis M4 and get playing! It's very likely the synth will recognize it immediately, but if not, check the preferences and choose the "Adafruit Trellis M4" MIDI device.

Here's an example using Helm.

Assign CC

The note on/off functions will work immediately, as should the pitch bend if it's supported in your software. Try holding some notes and then tip the controller left and right to bend down and up.

The forward and backward tilt will send CC 1 messages, which is usually interpreted as "mod wheel". This can be mapped by default to things like tremelo, but you may want to change it. In most software synths it's a matter of checking the help docs and reassigning it.

For example, here's how to re-assign a MIDI mapping for the x-axis tilt in Helm so that it will control Filter Cutoff Frequency:

And, here it is in action!

Enjoy playing with your new Trellis M4 Expressive MIDI Controller! If you want to further customize things, check out the page on coding it in Arduino.

Using with iOS Devices

Using the Trellis M4 with an iOS synthesizer, such as the excellent AudioKit Synth One, is easy, but you will need an adapter to connect the USB output of the Trellis M4 to the Lightning input of the iOS device.

The iOS Lightning to USB OTG cable allows you to connect the Trellis M4 right to your iPad, iPhone, or iPod Touch. The device will also provide power to your Trellis M4!

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Depending on your software, you may need to select and configure the Trellis M4 as the chosen MIDI device.


Synth One found it automatically, but you can use the MIDI Learn function to assign the x-axis (forward/backward) tilt to any knob.


Press the "MIDI Learn" button


Tap on the knob you want to control -- Here I've chosen the Filter cutoff frequency


Tilt the Trellis M4 forward and backward. The range doesn't matter, it just needs to register that that MIDI CC number (1 in this case) is the once changing


Press the "MIDI Learn" button to finish


Now, as you tilt the Trellis M4, the Filter cutoff frequency changes! Have fun exploring different parameters as you play.

If you want to customize the Trellis M4 Expressive MIDI Controller beyond the .uf2 firmware you dragged onto the board, we'll need to use Arduino IDE to code it.

First, get set up with Arduino IDE as detailed here.

Next, follow these instructions on getting the proper libraries installed. In this code we use the following libraries:

  • Adafruit_Keypad
  • Adafruit_NeoPixel
  • Adafruit_Sensor
  • Adafruit_ADXL343

Once you've updated the board definitions as shown, you'll be able to select Adafruit Trellis M4 (SAMD51) as your board for compiling and uploading.

Copy the code shown below and paste it into a new sketch in Arduino. Save the sketch as midi_keypad.ino and then upload it to your board.

Note: if you have any problem uploading to the board, double-click the reset button on the back of the board during the upload to set the Trellis M4 into bootloader mode. The on-board status LED (on the back) will be purple during normal running mode and switch to green when it is in bootloader mode.

// Trellis M4 MIDI Keypad CC
// sends 32 notes, pitch bend & a CC from accelerometer tilt over USB MIDI

#include <Adafruit_Sensor.h>
#include <Adafruit_ADXL343.h>
#include <Adafruit_NeoTrellisM4.h>

#define MIDI_CHANNEL     0  // default channel # is 0
// Set the value of first note, C is a good choice. Lowest C is 0.
// 36 is a good default. 48 is a high range. Set to 24 for a bass machine.
#define FIRST_MIDI_NOTE 36

Adafruit_ADXL343 accel = Adafruit_ADXL343(123, &Wire1);

int xCC = 1;  //choose a CC number to control with x axis tilting of the board. 1 is mod wheel, for example.

int last_xbend = 0;
int last_ybend = 0;

Adafruit_NeoTrellisM4 trellis = Adafruit_NeoTrellisM4();

void setup(){
  //while (!Serial);
  Serial.println("MIDI keypad & pitchbend!");

  // USB MIDI messages sent over the micro B USB port
  Serial.println("Enabling MIDI on USB");
  // UART MIDI messages sent over the 4-pin STEMMA connector (3.3V logic)
  Serial.println("Enabling MIDI on UART");
  if(!accel.begin()) {
    Serial.println("No accelerometer found");

void loop() {
  // put your main code here, to run repeatedly:

  while (trellis.available()){
    keypadEvent e =;
    int key = e.bit.KEY;
    Serial.print("Keypad key: ");    Serial.println(key);
    Serial.print("MIDI note: ");     Serial.println(FIRST_MIDI_NOTE+key);

    if (e.bit.EVENT == KEY_JUST_PRESSED) {
      Serial.println(" pressed\n");
      trellis.setPixelColor(key, 0xFFFFFF);
      trellis.noteOn(FIRST_MIDI_NOTE+key, 64);
    else if (e.bit.EVENT == KEY_JUST_RELEASED) {
      Serial.println(" released\n");
      trellis.setPixelColor(key, 0x0);
      trellis.noteOff(FIRST_MIDI_NOTE+key, 64);

  // Check for accelerometer
  sensors_event_t event;
  /* Display the results (acceleration is measured in m/s^2) */
  //Serial.print("X: "); Serial.print(event.acceleration.x); Serial.print("  ");
  //Serial.print("Y: "); Serial.print(event.acceleration.y); Serial.print("  ");
  //Serial.print("Z: "); Serial.print(event.acceleration.z); Serial.print("  ");Serial.println("m/s^2 ");
  int xbend = 0;
  int ybend = 0;

  if (abs(event.acceleration.y) < 2.0) {  // 2.0 m/s^2
    // don't make any bend unless they've really started moving it
    ybend = 8192; // 8192 means no bend
  } else {
    if (event.acceleration.y > 0) {
      ybend = ofMap(event.acceleration.y, 2.0, 10.0, 8192, 0, true);  // 2 ~ 10 m/s^2 is downward bend
    } else {
      ybend = ofMap(event.acceleration.y, -2.0, -10.0, 8192, 16383, true);  // -2 ~ -10 m/s^2 is upward bend
  if (ybend != last_ybend) {
    Serial.print("Y pitchbend: "); Serial.println(ybend);
    last_ybend = ybend;

  if (abs(event.acceleration.x) < 2.0) {  // 2.0 m/s^2
    // don't make any bend unless they've really started moving it
    xbend = 0;
  } else {
    if (event.acceleration.x > 0) {
      xbend = ofMap(event.acceleration.x, 2.0, 10.0, 0, 127, true);  // 2 ~ 10 m/s^2 is upward bend
    } else {
      xbend = ofMap(event.acceleration.x, -2.0, -10.0, 0, 127, true);  // -2 ~ -10 m/s^2 is downward bend
  if (xbend != last_xbend) {
    Serial.print("X mod: "); Serial.println(xbend);
    trellis.controlChange(xCC, xbend);  //xCC is set at top of sketch. e.g., CC 1 is Mod Wheel
    last_xbend = xbend;

  trellis.sendMIDI(); // send any pending MIDI messages


// floating point map
float ofMap(float value, float inputMin, float inputMax, float outputMin, float outputMax, bool clamp) {
    float outVal = ((value - inputMin) / (inputMax - inputMin) * (outputMax - outputMin) + outputMin);

    if (clamp) {
      if (outputMax < outputMin) {
        if (outVal < outputMax)  outVal = outputMax;
        else if (outVal > outputMin)  outVal = outputMin;
      } else {
        if (outVal > outputMax) outVal = outputMax;
        else if (outVal < outputMin)  outVal = outputMin;
    return outVal;


// Input a value 0 to 255 to get a color value.
// The colours are a transition r - g - b - back to r.
uint32_t Wheel(byte WheelPos) {
  WheelPos = 255 - WheelPos;
  if(WheelPos < 85) {
    return trellis.Color(255 - WheelPos * 3, 0, WheelPos * 3);
  if(WheelPos < 170) {
    WheelPos -= 85;
    return trellis.Color(0, WheelPos * 3, 255 - WheelPos * 3);
  WheelPos -= 170;
  return trellis.Color(WheelPos * 3, 255 - WheelPos * 3, 0);

The most likely things you'll want to change are the MIDI channel, the MIDI note range of the keypad buttons, and the CC channel numbers of the accelerometer tilt axes.

MIDI Channel

You can adjust the MIDI channel by changing the value of this line:

#define MIDI_CHANNEL 0 // default channel # is 0

To change this controller to send MIDI over channel 6, for example, you could make it read:

#define MIDI_CHANNEL 6

Note Range

The Trellis M4's 32 buttons are set up in this project as chromatic keys with the low note set to C3. This corresponds to MIDI notes 36 through 68. You can change this range by adjusting this line in the code:

#define FIRST_MIDI_NOTE 36

CC Channel

The x-axis (forward/backward) tilt of the Trellis M4's accelerometer is set by default to send out a range of 0-127 values over MIDI CC channel 1. This is typically received as "mod wheel" in most synthesizers. You can change this value, in case it isn't simple in your software synth to re-assign which knob is turned by CC 1. This line is the one to adjust:

int xCC = 1;

Once you're done making changes, upload the sketch to your Trellis M4 and test it out in your MIDI monitor utility and synth!

Test MIDI Output

You can test to see that your Trellis M4 is outputting MIDI signals properly by using a MIDI utility. These will report all activity coming from any MIDI controller plugged into your system. Here are some to try:

This guide was first published on Nov 11, 2018. It was last updated on Nov 11, 2018.