Use a Raspberry Pi Pico to interconnect a MIDI host, a serial MIDI device and up to 4 MIDI devices via a USB hub. If you have a Pico W or compatible board, this project supports connection from a Bluetooth MIDI client (iPad, phone, PC...)
This project uses the RP2040 processor's built-in USB port as a USB device port for connection to a USB MIDI host like a PC or Mac. It uses the RP2040's PIOs plus 2 GPIO pins to create a USB host port, and it uses the RP2040 processor's built-in UART1 port for serial port MIDI. Bluetooth MIDI uses the Pico W's built-in WiFi/Bluetooth module.
You configure how the MIDI streams connect by using serial terminal command line interface (CLI) connected to either the RP2040's UART0 port or the RP2040's built-in USB port's serial (CDC ACM) interface.
The software uses some of the Pico board's program flash for a file system to store configurations in presets. If you save your settings to a preset, then the midi2piousbhub software will automatically reload the last saved preset on startup and when you plug a Connected MIDI Device to the hub. You can back up any or all of your presets to a USB Flash drive connected to the USB hub. Presets are stored in JSON format.
My first test circuit used a Raspberry Pi Pico board, a USB A breakout board, and a hand wired MIDI I/O port. The circuit is the same as the hardware described in the usb_midi_host README "Software-based USB Host Port: Pico_PIO_USB Library" section except I wired a MIDI IN and MIDI OUT port to pins GP4 and GP5 like the midi2usbhost project shows.
However, it should also run on the Adafruit Feather RP2040 with USB Type A Host with pins D4 and D5 wired to an Adafruit MIDI FeatherWing board (take FeatherWing 3.3V power and ground from the RP2040 board), or any similar hardware configuration.
I am running Ubuntu Linux 24.04LTS 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). I found that when debugging using the picoprobe whilst also using the Pico board as a USB device connected to the same PC, I had to connect the picoprobe first, then connect the target Pico board via a USB hub. Your experience may be different.
You do not need to use the picoprobe or the UART 0 output at all. The USB device connector on the RP2040 target board serves both as a MIDI port and serial port console.
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. In particular, make sure PICO_SDK_PATH is set to the directory where you installed the pico-sdk.
At the time of this writing, the Pico C/C++ SDK version is 1.5.1. It has some
issues with Bluetooth Support that are fixed in the develop branch of the pico-sdk.
To address these issues, please check out the develop branch of the pico-sdk and
get the latest version.
cd ${PICO_SDK_PATH}
git checkout -b develop origin/develop
git pull
git submodule update -- lib/btstack
git submodule update -- lib/cyw43-driver
The USB MIDI host driver is currently not part of the TinyUSB stack. It is an
application host driver found in this project's lib/usb_midi_host directory.
The Pico SDK uses the main repository for TinyUSB as a git submodule. The version
of TinyUSB that ships with the Pico SDK 1.5.1 does not support application host
drivers. That feature was added 15-Aug-2023. You will likely need the latest version
of the TinyUSB library for this code to work correctly. The following describes
how to make sure your Pico SDK version's TinyUSB library supports application host
drivers.
- Set the working directory to the tinyusb library and make sure you are on the main branch.
cd ${PICO_SDK_PATH}/lib/tinyusb
git checkout master
- Check the date on the last commit to the TinyUSB library master branch.
git log -1
- If the
Date:is >= 15-Aug-2023, your TinyUSB library should be fine. If not, get the latest
git pull
The latest TinyUSB does not come with PIO USB driver source code. To install that source, you need to have Python 3 installed on your computer. From a command line, type
cd ${PICO_SDK_PATH}/lib/tinyusb
python tools/get_deps.py rp2040
For more information, see the TinyUSB documentation
Clone the midi2piousbhub project to a directory at the same level as the pico-sdk directory.
cd ${PICO_SDK_PATH}/..
git clone --recurse-submodules https://github.com/rppicomidi/midi2piousbhub.git
Enter this series of commands (assumes you installed the pico-sdk and the midid2usbhub project in the ${PICO_MIDI_PROJECTS} directory)
If your system is based on a Pico W board, enter this command first
export PICO_BOARD=pico_w
For all boards, enter this commands.
export PICO_SDK_PATH=${PICO_MIDI_PROJECTS}/pico-sdk/
cd ${PICO_MIDI_PROJECTS}/midi2piousbhub
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.
If your project works for some USB MIDI devices and not others, one
thing to check is the size of buffer to hold USB descriptors and other
data used for USB enumeration. Look in the file tusb_config.h for
#define CFG_TUH_ENUMERATION_BUFSIZE 512
Very complex MIDI devices or USB Audio+MIDI devices like DSP guitar pedals or MIDI workstation keyboards may have large USB configuration descriptors. This project assumes 512 bytes is enough, but it may not be for your device.
To check if the descriptor size is the issue, use your development computer to dump the USB descriptor for your device and then add up the wTotalLength field values for each configuration in the descriptor.
For Linux and MacOS Homebrew, the command is lsusb -d [vid]:[pid] -v For Windows, it is simplest to install a program like Thesycon USB Descriptor Dumper.
For example, this is the important information from lsusb -d 0944:0117 -v
from a Korg nanoKONTROL2:
bNumConfigurations 1
Configuration Descriptor:
bLength 9
bDescriptorType 2
wTotalLength 0x0053
bNumInterfaces 1
bConfigurationValue 1
iConfiguration 0
bmAttributes 0x80
(Bus Powered)
MaxPower 100mA
This is the important information from the Thesycon USB Descriptor Dumper for a Valeton NUX MG-400
0x01 bNumConfigurations
Device Qualifier Descriptor is not available. Error code: 0x0000001F
-------------------------
Configuration Descriptor:
-------------------------
0x09 bLength
0x02 bDescriptorType
0x0158 wTotalLength (344 bytes)
0x04 bNumInterfaces
0x01 bConfigurationValue
0x00 iConfiguration
0xC0 bmAttributes (Self-powered Device)
0x00 bMaxPower (0 mA)
You can see that if CFG_TUH_ENUMERATION_BUFSIZE were 256 instead of 512,
the Korg nanoKONTROL2 would have no trouble enumerating but the Valeton
NUX MG-400 would fail because TinyUSB couldn't load the whole configuration
descriptor to memory.
- Connected MIDI Device: a MIDI device connected to a USB hub port or to a serial port MIDI DIN connector.
- USB ID: A pair of numbers the Connected MIDI Device reports to the hub when it connects. They are supposed to be unique to a particular product.
- Routing Matrix: The software that sends MIDI data to and from Connected MIDI Devices
- Terminal: a MIDI data input to or output from the Routing Matrix.
- FROM terminal: an input to the Routing Matrix. It will be a MIDI OUT signal from a Connected MIDI Device.
- TO terminal: an output from the Routing Matrix. It will be a MIDI IN signal to a Connected MIDI Device.
- Port: usually a group of 1 MIDI IN data stream and one MIDI OUT data stream associated with a Connected MIDI Device. A Port of a Connected MIDI Device may omit MIDI IN or MIDI OUT, but not both. Ports are numbered 1-16
- Direction of a terminal: either FROM the Connected MIDI Device's MIDI OUT or TO the Connected MIDI Device's MIDI IN.
- Nickname: a more human name than specifying a device port's FROM and TO data streams using a USB ID, a Port number and a Direction. Nicknames have a maximum of 12 characters. The default nickname for a port in a given direction is the USB ID followed by either a "F" for a FROM data stream or "T" for a TO data stream, followed by the port number (1-16). For example, "Drumpads" above was renamed from "0000-0000-F1"
- Product Name: a name that identifies the the attached MIDI device. The Connected MIDI Device sends it to the hub on connection; it is a more friendly name than USB ID, and is the easiest way to assocate the Connected MIDI Device with all the other info.
Show a list of all available commands and brief help text.
List all Connected MIDI Devices currently connected to the USB hub followed by the built-in devices. For example:
USB ID Port Direction Nickname Product Name
0499-1622 1 FROM lead-out reface CS
0499-1622 1 TO lead reface CS
1C75-02CA 1 FROM keys Arturia Keylab Essential 88
1C75-02CA 1 TO keys-in Arturia Keylab Essential 88
1C75-02CA 2 FROM faders Arturia Keylab Essential 88
1C75-02CA 2 TO faders-in Arturia Keylab Essential 88
0000-0000 1 FROM Drumpads MIDI IN A
0000-0000 1 TO TR-707 MIDI OUT A
0000-0001 1 FROM DAW-OUT PC MIDI
0000-0001 1 TO DAW-IN PC MIDI
0000-0002 1 FROM iPad-OUT BT MIDI
0000-0002 1 TO iPad-IN BT MIDI
Rename the nickname for a product's port. All nicknames must be unique. If you need to hook up more than one device with the same USB ID, then you must do so one at a time and change the nickname for each port before attaching the next one to the hub.
Send data from the MIDI Out port of the MIDI device with nickname <From Nickname> to the MIDI IN port of the device with nickname <To Nickname>. If more than one device connects to the TO terminal of a particular device, then the streams are merged.
Break a connection previously made using the connect command.
Disconnect all routings.
Show a connection matrix of all MIDI devices connected to the hub. A blank box means "not
connected" and an x in the box means "connected." For example, the following shows
MIDI OUT of the "keys" device connected to the MIDI IN of the "lead" device.
TO-> | | | |
| | | |
| | | |
| | | f |
| | | a |
| l | k | d |
| e | e | e |
FROM | | a | y | r |
v | d | s | s |
| - | - | - |
| i | i | i |
| n | n | n |
------------+---+---+---+
lead | | | |
------------+---+---+---+
keys | x | | |
------------+---+---+---+
faders | | | |
------------+---+---+---+
Save the current setup to the given <preset name>. If there is already a preset with that name, then it will be overwritten.
Load the current setup from the given <preset name>. If the preset was not previously saved using the save command, then print an error message to the console.
Copy the specified preset to USB flash drive to a file on the drive named /rppicomidi-midi2usbhub/<preset name>. If no preset name is given, then all presets are copied to the
flash drive.
Copy the specified preset from the USB flash drive directory /rppicomidi-midi2usbhub/<preset name> to the file system on Pico board's program flash.
Reformat the LittleFs file system in the Pico's program memory. It delete all presets.
Print information about the LittleFs file system
List all files in the LittleFs file system. If you specify a path, then list the contents
of the path directory. For now, the only directory path is /.
Deletes the file with name <filename> in the LitteFs file system
Change current directory of the current USB flash drive to path. If path is not specified,
equivalent to f-cd / (i.e., set to the drive root directory).
Change current drive number for the USB flash drive. Will only need to do this if you have more than one flash drive plugged in. When you plug in a drive, the code automatically sets the drive number to the latest drive.
List contents of the current directory on the current USB flash drive if path is not
specified. Otherwise, list the contents of the specified path.
Print the current directory path of the current USB flash drive.
Change real-time clock date. The date and time is used for external flash drive file timestamps.
Change the real-time clock time of day. The date and time is used for external flash drive file timestamps.
Print the current date and time as read from the on-chip real-time clock. The time has a resolution of 2 seconds as because that is what is required for flash drive file timestamps. The initial date and time will be the last time you built the msc-rp2040rtc library.
Disconnect an active Bluetooth MIDI connection or report "Already disconnected."