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:
VIDEO
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
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# 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])
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