Let's make a simple MIDI note player in CircuitPython. This will give you a good idea of how to send MIDI data over the NeoTrellis M4's UART/serial port, which you can then adapt to make your own arpeggiators, sequencers, modal players, chord keyboards for polyphonic synths, and more.
CircuitPython is designed to simplify experimentation and education on low-cost microcontrollers. It makes it easier than ever to get prototyping by requiring no upfront desktop software downloads. Simply copy and edit files on the CIRCUITPY flash drive to iterate.
Are you new to using CircuitPython? No worries, there is a full getting started guide here.
Adafruit suggests using the Mu editor to edit your code and have an interactive REPL in CircuitPython. You can learn about Mu and its installation in this tutorial.
First, let's prep the board for that.
CircuitPython Prep
To get prep the Trellis M4 to run the sequencer code, follow these steps:
- Update the bootloader for NeoTrellis from the Trellis M4 guide
- Install the latest CircuitPython for NeoTrellis from the Trellis M4 guide
- Get the latest CircuitPython library pack, the version should match the major version of CircuitPython you are using, unzip it, and drag the libraries you need over into the /lib folder on CIRCUITPY. The latest library package includes support for Trellis M4.
https://github.com/adafruit/Adafruit_CircuitPython_Bundle/releases/
Now, copy the following code, paste it into Mu and then save it to your NeoTrellis M4's CIRCUITPY drive as code.py
A hat tip to Friend of Adafruit, Tod Kurt for creating the initial CircuitPython MIDI over UART example code.
# SPDX-FileCopyrightText: 2018 John Park for Adafruit Industries
#
# SPDX-License-Identifier: MIT
# Simple example of sending MIDI via UART to classic DIN-5 (not USB) synth
import adafruit_trellism4
from rainbowio import colorwheel
import board
import busio
midiuart = busio.UART(board.SDA, board.SCL, baudrate=31250, timeout=0.001)
print("MIDI UART EXAMPLE")
trellis = adafruit_trellism4.TrellisM4Express()
for x in range(trellis.pixels.width):
for y in range(trellis.pixels.height):
pixel_index = (((y * 8) + x) * 256 // 2)
trellis.pixels[x, y] = colorwheel(pixel_index & 255)
current_press = set()
while True:
pressed = set(trellis.pressed_keys)
for press in pressed - current_press:
x, y = press
print("Pressed:", press)
noteval = 36 + x + (y * 8)
midiuart.write(bytes([0x90, noteval, 100]))
for release in current_press - pressed:
x, y = release
print("Released:", release)
noteval = 36 + x + (y * 8)
midiuart.write(bytes([0x90, noteval, 0])) # note off
current_press = pressed
Hook up the NeoTrellis M4 to your synthesizer over the UART-to-MIDI adapter cabling and now when you press a button, you'll send a MIDI note on message, and when you release a button, it'll send a MIDI note off.
This can serve as your launching off point for more elaborate CircuitPython MIDI projects!
