Raspberry Pi Pico as MIDI to CV converter

This entry is part 1 of 2 in the series Raspberry Pi Pico as MIDI to CV converter

I’ve been toying with the idea of building a MIDI to CV converter for at least a decade. Now I’ve finally had a go using MicroPython on the Raspberry Pi Pico.

Here’s how I managed to control my Moog Werkstatt-01 over MIDI using the Pico, a handful of components and a MCP4725 12-bit DAC:

The Hardware

BOM

  • Raspberry Pi Pico
  • HC11L optocoupler
  • 220Ω Resistor
  • 470Ω Resistor
  • small signal diode (eg 1N914)
  • MCP4725 module

Circuit Diagram

The Code

The code is written in Micropython. Its only external dependency is the SimpleMIDIDecoder class by @diyelectromusic. You can download the latest version on GitHub here: SimpleMIDIDecoder.py.

# MIDI to CV converter for Raspberry Pi Pico and MCP4725 DAC by @AxWax
#
# Demo: https://www.youtube.com/watch?v=aGfQHL1jU4I
#
# This is heavily based on and requires
# the SimpleMIDIDecoder library by @diyelectromusic, which can be found at
# https://diyelectromusic.wordpress.com/2021/06/13/raspberry-pi-pico-midi-channel-router/
#
#
# Wiring:
# serial midi input on GP1 (UART0 RX)
# gate output: GP17
#
# MCP4725 Pico
# GND GND
# VCC VBUS (5V)
# SDA GP6
# SCL GP7
# VOUT is the CV output
import machine
import time
import ustruct
import SimpleMIDIDecoder
# which MIDI note number corresponds to 0V CV
lowest_note = 40;
# create gate pin
gate = machine.Pin(17, machine.Pin.OUT)
gate.value(0)
#create an I2C bus
sda=machine.Pin(6)
scl=machine.Pin(7)
i2c = machine.I2C(1, scl=scl, sda=sda, freq=400000)
# calculate 1mV: steps / max V / 1000
mv = 4096 / 5.1 / 1000
# calculate mV per semitone
semitone = 83.33 * mv
# DAC function
def writeToDac(value):
buf=bytearray(2)
buf[0]=(value >> 8) & 0xFF
buf[1]=value & 0xFF
i2c.writeto(0x62,buf)
# Initialise the serial MIDI handling
uart = machine.UART(0,31250)
# MIDI callback routines
def doMidiNoteOn(ch, cmd, note, vel):
global semitone
writeToDac(int((notelowest_note)*semitone))
gate.value(1)
def doMidiNoteOff(ch, cmd, note, vel):
global semitone
gate.value(0)
# initialise MIDI decoder and set up callbacks
md = SimpleMIDIDecoder.SimpleMIDIDecoder()
md.cbNoteOn (doMidiNoteOn)
md.cbNoteOff (doMidiNoteOff)
# the loop
while True:
# Check for MIDI messages
if (uart.any()):
md.read(uart.read(1)[0])
view raw PicoMIDItoCV.py hosted with ❤ by GitHub

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