USB-MIDI ⇄ DIN-MIDI Bridge (Arduino UNO + USB Host Shield + MIDI Shield)

A simple bidirectional bridge between a class-compliant USB-MIDI device (e.g., Akai MPK mini MK3) and a 5-pin DIN MIDI interface.
Built with PlatformIO, using:

• USB Host Shield Library 2.0 (felis/USB-Host-Shield-20)
• Arduino MIDI Library (FortySevenEffects/MIDI Library)

USB messages are forwarded to DIN OUT; DIN IN messages are forwarded to the attached USB-MIDI device.

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Hardware

• MCU: Arduino UNO (ATmega328P)
• USB Host Shield: MAX3421E-based (standard Arduino shield form factor)
• MIDI Shield: DIN IN/OUT/THRU + RUN/PGM switch

Stack order

UNO (bottom)
└─ USB Host Shield (MAX3421E)
   └─ MIDI Shield (DIN IN/OUT/THRU, RUN/PGM)

Pin usage

• USB Host Shield: SPI via ICSP header, SS = D10
• MIDI Shield:
  • DIN IN → UNO D0 (RX)
  • DIN OUT → UNO D1 (TX)
  • RUN/PGM switch present on shield (see notes below)
• Power: 5 V & GND from the UNO
  (Ensure the Host Shield’s VBUS (5 V) jumper is set correctly so it can power a bus-powered USB device.)

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Features

• Bridges common channel voice messages:
  • Note On/Off, Control Change, Program Change, Pitch Bend, Aftertouch (poly & channel)
• Two-way: USB → DIN and DIN → USB
• Minimal and fast—no additional buffering threads

Not included (yet): SysEx & system real-time/common forwarding (easy to add later).

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Directory Layout

usb-midi-bridge/
├─ platformio.ini
└─ src/
   └─ main.cpp

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Software Requirements

• PlatformIO (VS Code extension or CLI)
• Libraries (fetched automatically via lib_deps):
  • felis/USB-Host-Shield-20
  • FortySevenEffects/MIDI Library @ ^5.0.2

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Quick Start

1. Clone this project into your PlatformIO workspace.
2. Insert libraries: PlatformIO will auto-install from platformio.ini.
3. Upload:
   • On the MIDI Shield, set RUN/PGM to PGM while uploading (prevents Serial port conflicts).
   • After upload completes, flip it back to RUN.
4. Connect:
   • Plug your USB-MIDI controller (e.g., MPK mini MK3) into the Host Shield’s USB-A.
   • Connect DIN OUT to your synth’s MIDI IN, and/or feed DIN IN from an external source.

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platformio.ini

[env:uno]
platform = atmelavr
board = uno
framework = arduino

lib_deps =
  felis/USB-Host-Shield-20
  FortySevenEffects/MIDI Library @ ^5.0.2

; Note: The UNO’s hardware Serial (D0/D1) is used for DIN MIDI at 31250 baud.
; To avoid serial conflicts during upload, set the MIDI Shield’s RUN/PGM switch
; to PGM while uploading; switch it back to RUN after the upload completes.

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src/main.cpp (summary)

• Uses USBH_MIDI to receive/send USB-MIDI packets.
• Uses Arduino MIDI Library on Serial (D0/D1) at 31250 baud.
• Forwards:
  • USB → DIN: Reads up to 3-byte channel messages and writes them to Serial.
  • DIN → USB: MIDI callbacks translate events into USB packets.

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How it Works

• USB → DIN: MidiHost.RecvData(m) fetches incoming USB-MIDI bytes; the sketch writes them to UART at 31250 baud (DIN).
• DIN → USB: The MIDI library invokes handlers (e.g., handleNoteOn), which re-encode status/data bytes and call MidiHost.SendData(...).

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Testing

1. Power up the stack; plug in the USB-MIDI keyboard.
2. Press keys—your synth connected to DIN OUT should play.
3. (Optional) Send DIN IN messages from another source; they should appear on the USB device.

If nothing happens:

• Confirm the USB device is class-compliant MIDI (no vendor drivers required).
• Provide sufficient power (bus-powered controllers can draw ~100–500 mA).
• Re-seat shields; check the Host Shield’s LEDs (enumeration).
• Double-check the RUN/PGM switch state (RUN during operation).

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Troubleshooting

• UnknownPackageError / library not found
  Use the exact slugs:

  • felis/USB-Host-Shield-20
  • FortySevenEffects/MIDI Library Or temporarily reference GitHub:

  lib_deps =
    https://github.com/felis/USB_Host_Shield_2.0
    FortySevenEffects/MIDI Library

• Upload fails / Serial busy
  Set MIDI Shield RUN/PGM → PGM while uploading.

• No sound / device not detected

  • Verify the Host Shield enumerates the device (power & cable).
  • Confirm DIN cabling direction (OUT → IN).

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Extending

• SysEx bridge: Use USBH_MIDI::SendSysEx(...) and handle larger payloads from RecvData(...).
• System real-time: Forward timing/clock/Start/Stop messages.
• Debug UART: Add AltSoftSerial (on different pins) at 115200 for logging since Serial is reserved for 31250 baud MIDI.
• RUN/PGM input: Wire the switch to a digital pin to auto-mute DIN during uploads.

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License

This project is licensed under GPL-2.0.
See the included LICENSE file for the full text.

Why GPL-2.0?

This project links against USB Host Shield Library 2.0, which is GPL-licensed. Therefore, the combined work must be distributed under a GPL-compatible license. The Arduino MIDI Library is MIT-licensed and is compatible with GPL.

Third-Party Licenses

• USB Host Shield Library 2.0 — GPL-2.0
• Arduino MIDI Library (FortySevenEffects) — MIT

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Credits

• USB Host Shield Library 2.0 by Oleg Mazurov & contributors
• MIDI Library by FortySevenEffects & contributors