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((note-lowest_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])