Use a Raspbery Pi Pico to add old school MIDI IN and MIDI OUT to MIDI devices with a USB B device port

This is a hardware and software project. The MIDI gadgets you will wire to this hardware will likely cost a lot more than than the hardware for this project. Please double-check my design and test your circuit and wiring before you connect it to anything expensive. I will take no responsibility for burning up your favorite keyboard or sound module.

This projects uses commercial hardware for testing. I do not officially endorse any hardware. I have not been paid by anyone to use this hardware. A carefully designed custom board would be a better solution. The hardware combinations described here would almost certainly fail electrical and RF compliance testing. Use at your own risk.

This project is designed to run on the RP2040 chip. It is tested on a Raspberry Pi Pico board, but any board that provides access to the USB port and UART 1 will probably be fine.

The project uses the RP2040 UART 1 port for driving the old school MIDI IN and MIDI OUT ports. You can use whatever circuit you would like for the current loop driver for MIDI OUT and the optical isolator for MIDI IN. This project is tested using the MIDI Featherwing board. If you use this board, please note that MIDI OUT is not isolated from your processor board UART 1 TX pin. Also note there are no ferrite beads on MIDI IN and MIDI OUT connector pins, so radio frequency interference (RFI) may be an issue with this circuit.

You will need to provide +5VDC to the VBus input. I use the VBUS pin from a picoprobe board.

If you use the Pico board, you will need to be able to connect your USB MIDI device to the Pico's micro USB connector. I use the least expensive micro USB to full size USB A OTG adapter I could find. I won't link to one here. Web search is your friend. Note that just using the adapter provides no current limit on the USB host VBus line, so please be careful what you connect to this.

If you choose to use the same hardware I did, wire the boards together as follows.

USB C Breakout board VBus pin -> Pico board VBUS Pin 40
USB C Breakout board GND pin  -> Pico board GND Pin 38
Pico board GND Pin 8 -> MIDI Featherwing board GND Pin 4
Pico board 3.3V Pin 36 -> MIDI Featherwing board 3.3V Pin 2
Pico board UART1 RX Pin 6 -> MIDI Featherwing board Pin 14
Pico board UART1 TX Pin 7 -> MIDI Featherwing board Pin 15

A photo of my development setup using a second Pico board as a picoprobe is below. The Pico board in the middle is running the MIDI adapter software. It is wired to the MIDI featherwing on the left and has the Micro USB to USB A adapter on it. The picoprobe is providing VBUS power and the serial port console. The picoprobe wiring is

picoprobe GND -> MIDI Pico GND
picoprobe GP2 -> MIDI Pico SWCLK
picoprobe GP3 -> MIDI Pico SWDIO
picoprobe GP4/UART1 TX -> MIDI Pico GP1/UART0 RX
picoprobe GP5/UART1 RX -> MIDI Pico GP0/UART0 TX
picoprobe VBUS -> Pico B VBUS

This project uses the the Raspberry Pi Pico SDK and it uses the tinyusb library for the USB stack. At the time of this writing my pull request for supporting USB MIDI host is still pending, so please use my forked tinyusb library until it is accepted.

This project also depends on the midi_uart_lib and ring_buffer_lib projects. They are included as git submodules.

I am running Ubuntu Linux 20.04LTS on an old PC. I have Visual Studio Code (VS Code) installed and went through the tutorial in Chapter 7 or Getting started with Raspberry Pi Pico to make sure it was working first. I use a picoprobe for debugging, so I have openocd running in a terminal window. I use minicom for the serial port terminal (make sure your linux account is in the dialup group).

The Pico SDK uses the main repository for tinyusb as a git submodule. Until the USB Host driver for MIDI is incorporated in the main repository for tinyusb, you will need to use my forked version. This is how I do it.

1. If you have not already done so, follow the instructions for installing the Raspberry Pi Pico SDK in Chapter 2 of the Getting started with Raspberry Pi Pico document.
2. Set the working directory to the tinyusb library

cd [some directory where you installed the pico SDK]/lib/tinyusb

3. Create an "upstream" remote.

git remote add upstream https://github.com/hathach/tinyusb.git

4. Change the "origin" remote to point at my fork

git remote set-url origin https://github.com/rppicomidi/tinyusb.git

5. Get the code from my fork into your local repository

git fetch origin

6. Get the midihost branch code branch

git checkout -b midihost origin/midihost

Clone the midiusb2host project to a directory at the same level as the pico-sdk directory.

cd [one directory above the pico-sdk directory]
git clone --recurse-submodules https://github.com/rppicomidi/midi2usbhost.git

Enter this series of commands (assumes you installed the pico-sdk and the midid2usbhost project in the $HOME/foo directory)

export PICO_SDK_PATH=$HOME/foo/pico-sdk/
cd $HOME/foo/midi2usbhost
mkdir build
cd build
cmake ..
make

The build should complete with no errors. The build output is in the build directory you created in the steps above.

Enter this series of commands

cd midiusb2host
mkdir build
cd build
touch compile_commands.json

Run these once in a terminal before you launch VS Code. The first sets up the environment and the second launches openOCD for use with the picoprobe

export PICO_SDK_PATH=$HOME/projects/pico/pico-sdk/
gnome-terminal -- openocd -f interface/picoprobe.cfg -f target/rp2040.cfg
gnome-terminal -- minicom -D /dev/ttyACM0 -b 115200

Finally, launch VS Code

code

The first time you run the project, in VS Code, File->Open Folder... and select the midiusb2host folder. Click OK.

You will be prompted to set up the Kit. Choose GCC for arm-none-eabi [your version]

In VS Code, select Run->Start Debugging from the file menu. The first time, you will be prompted to select the launch target. Select midi2usbhost.

VS Code debugger will load the code to your target Pico board and halt at main(). Press the triangular run icon to start it running. If all goes well, you should see the LED on your Pico board blinking off and on once per second and your should see the following in your minicom terminal window:

Pico MIDI Host to MIDI UART Adapter
Configured MIDI UART 1 for 31250 baud