Once you've finished setting up your QT Py RP2040 with CircuitPython, you can access the code and necessary libraries by downloading the Project Bundle.

To do this, click on the Download Project Bundle button in the window below. It will download as a zipped folder.

# SPDX-FileCopyrightText: 2022 Liz Clark for Adafruit Industries
# SPDX-License-Identifier: MIT

import board
import simpleio
import adafruit_mcp4725
import usb_midi
import adafruit_midi
from digitalio import DigitalInOut, Direction
from adafruit_midi.note_off import NoteOff
from adafruit_midi.note_on import NoteOn
from volts import volts

#  midi channel setup
midi_in_channel = 1
midi_out_channel = 1

#  USB midi setup
midi = adafruit_midi.MIDI(
    midi_in=usb_midi.ports[0], in_channel=0, midi_out=usb_midi.ports[1], out_channel=0
)

# gate output pin
gate = DigitalInOut(board.A1)
gate.direction = Direction.OUTPUT

#  i2c setup
i2c = board.I2C()
#  dac setup over i2c
dac = adafruit_mcp4725.MCP4725(i2c)

#  dac raw value (12 bit)
dac.raw_value = 4095

#  array for midi note numbers
midi_notes = []
#  array for 12 bit 1v/oct values
pitches = []

#  function to map 1v/oct voltages to 12 bit values
#  these values are added to the pitches[] array
def map_volts(n, volt, vref, bits):
    n = simpleio.map_range(volt, 0, vref, 0, bits)
    pitches.append(n)

#  brings values from volts.py into individual arrays
for v in volts:
    #  map_volts function to map 1v/oct values to 12 bit
    #  and append to pitches[]
    map_volts(v['label'], v['1vOct'], 5, 4095)
    #  append midi note numbers to midi_notes[] array
    midi_notes.append(v['midi'])

while True:
    #  read incoming midi messages
    msg = midi.receive()
    #  if a midi msg comes in...
    if msg is not None:
        #  if it's noteoff...
        if isinstance(msg, NoteOff):
            #  send 0 volts on dac
            dac.raw_value = 0
            #  turn off gate pin
            gate.value = False
        #  if it's noteon...
        if isinstance(msg, NoteOn):
            #  compare incoming note number to midi_notes[]
            z = midi_notes.index(msg.note)
            #  limit note range to defined notes in volts.py
            if msg.note < 36:
                msg.note = 36
            if msg.note > 96:
                msg.note = 96
            #  send corresponding 1v/oct value
            dac.raw_value = int(pitches[z])
            #  turn on gate pin
            gate.value = True

Upload the Code and Libraries to the QT Py RP2040

After downloading the Project Bundle, plug your QT Py RP2040 into the computer's USB port with a known good USB data+power cable. You should see a new flash drive appear in the computer's File Explorer or Finder (depending on your operating system) called CIRCUITPY. Unzip the folder and copy the following items to the QT Py RP2040's CIRCUITPY drive. 

  • lib folder
  • volts.py
  • code.py

Your QT Py RP2040 CIRCUITPY drive should look like this after copying the lib folder, volts.py file and the code.py file.

CIRCUITPY

volts.py File

The volts.py file is a helper file that contains a dictionary (volts[]) with the phenetic note name, the MIDI note number and the 1V/oct voltage. The range of notes are C2 to C7, or 0V to 5V. 

volts = [
    {'label':"C-2",'midi':36,'1vOct':0.000},
    {'label':"C♯2",'midi':37,'1vOct':0.083},
    {'label':"D-2",'midi':38,'1vOct':0.167},
    ...
    {'label':"B-6",'midi':95,'1vOct':4.917},
    {'label':"C-7",'midi':96,'1vOct':5.000},
]

How the CircuitPython Code Works

First, USB MIDI, the digital output for the gate signal, I2C and the DAC are setup. Note that board.I2C() is being used rather than STEMMA_I2C().

#  midi channel setup
midi_in_channel = 1
midi_out_channel = 1

#  USB midi setup
midi = adafruit_midi.MIDI(
    midi_in=usb_midi.ports[0], in_channel=0, midi_out=usb_midi.ports[1], out_channel=0
)

# gate output pin
gate = DigitalInOut(board.A1)
gate.direction = Direction.OUTPUT

#  i2c setup
i2c = board.I2C()
#  dac setup over i2c
dac = adafruit_mcp4725.MCP4725(i2c)

#  dac raw value (12 bit)
dac.raw_value = 4095

Mapping Voltages

Two arrays are created: midi_notes[] and pitches[]. midi_notes[] will hold the MIDI note numbers as defined in volts.py. pitches[] will hold the 12-bit DAC values that correspond with the 1V/oct values in volts.py.

#  array for midi note numbers
midi_notes = []
#  array for 12 bit 1v/oct values
pitches = []

The map_volts() function, uses simpleio's map_range() function to map the 1V/oct values from volts.py to 12-bit values for the MCP4725 DAC. These 12-bit values are then added to the pitches[] array.

#  function to map 1v/oct voltages to 12 bit values
#  these values are added to the pitches[] array
def map_volts(n, volt, vref, bits):
    n = simpleio.map_range(volt, 0, vref, 0, bits)
    pitches.append(n)

A for statement iterates through the dictionary in volts.py and uses map_volts() to map the 1V/oct values to 12-bit values. Then, the MIDI note numbers in volts.py are added to the midi_notes[] array.

The indexes of each array correspond with each other for the same note and will be used in the loop. For example, midi_notes[0] equals MIDI note 36 and pitches[0] equals 0, which are two different ways of saying note C2.

#  brings values from volts.py into individual arrays
for v in volts:
    #  map_volts function to map 1v/oct values to 12 bit
    #  and append to pitches[]
    map_volts(v['label'], v['1vOct'], 5, 4095)
    #  append midi note numbers to midi_notes[] array
    midi_notes.append(v['midi'])

The Loop

The loop begins by listening to incoming MIDI messages. In this instance of the code, the specific messages being listened for are NoteOn and NoteOff messages.

If a NoteOn message is received, the message's note number is checked against the midi_notes array and the matching index is defined as z. The DAC's value is set to pitches[z], sending the corresponding 1V/oct voltage as a 12-bit value. gate's value is set to True.

If a NoteOff message is received, the DAC's value is set to 0 and gate's value is set to False

while True:
    #  read incoming midi messages
    msg = midi.receive()
    #  if a midi msg comes in...
    if msg is not None:
        #  if it's noteoff...
        if isinstance(msg, NoteOff):
            #  send 0 volts on dac
            dac.raw_value = 0
            #  turn off gate pin
            gate.value = False
        #  if it's noteon...
        if isinstance(msg, NoteOn):
            #  compare incoming note number to midi_notes[]
            z = midi_notes.index(msg.note)
            #  limit note range to defined notes in volts.py
            if msg.note < 36:
                msg.note = 36
            if msg.note > 96:
                msg.note = 96
            #  send corresponding 1v/oct value
            dac.raw_value = int(pitches[z])
            #  turn on gate pin
            gate.value = True

This guide was first published on Aug 02, 2022. It was last updated on 2022-08-02 16:56:26 -0400.

This page (Code the MIDI to CV Skull) was last updated on Aug 06, 2022.

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