Midi bff

qtpy_midibff

QTPy MIDI BFF — a small MIDI interface board for QTPy / Xiao class boards (RP2040 or compatible). The board breaks out the host MCU's hardware UART to standard MIDI IN and MIDI OUT jacks (TRS or 5-pin DIN).

Hardware

The PCB is meant for easy assembly using only through-hole components. It is essentialy a QTPy "doubler" with a standard MIDI In and MIDI Out circuit, with a 4-pin I2C header for a standard OLED display (usually 128x32 SSD1306).

Assembling

Coming soon.

Arduino sketches

Requires arduino-pico board core.

Set Tools > USB Stack: Adafruit TinyUSB before uploading any sketch.

Libraries needed (Arduino Library Manager):

• Adafruit TinyUSB
• MIDI Library
• Adafruit SSD1306 + Adafruit GFX Library (OLED sketches only)
• Adafruit SPIFlash + SdFat - Adafruit Fork + ArduinoJson (MSC sketch only)

midi_interface — USB-MIDI ↔ UART-MIDI bridge

arduino/midi_interface/

Forwards all MIDI between USB and UART in both directions. Includes scaffolding to filter or transform messages — uncomment the examples or add your own:

• Drop realtime messages (clock, active sensing, etc.)
• Transpose notes on a channel
• Remap channels

arduino-cli compile \
  --fqbn rp2040:rp2040:adafruit_qtpy:usbstack=tinyusb \
  arduino/midi_interface

SysEx messages up to 127 bytes are forwarded. See the source for notes on larger SysEx.

midi_interface_oled — bridge with OLED display

arduino/midi_interface_oled/

Same bridge as midi_interface, with a 128x32 SSD1306 I2C OLED that displays each incoming message. Realtime messages (clock, active sensing) are forwarded but not shown, to avoid flicker.

arduino-cli compile \
  --fqbn rp2040:rp2040:adafruit_qtpy:usbstack=tinyusb \
  arduino/midi_interface_oled

midi_interface_msc — bridge with USB flash drive and config file

arduino/midi_interface_msc/

Extends midi_interface_oled with a USB Mass Storage device backed by the RP2040's internal flash (the filesystem partition). The drive contains a config.json file that controls runtime behaviour — no recompile needed to change settings.

{
  "name": "MIDI BFF",
  "manufacturer": "todbot",
  "filter_realtime": false,
  "transpose": 0
}

Edit config.json on the drive, eject it, then reset the board to apply the new settings. The filter_realtime and transpose fields are applied in both directions.

Requires a board Flash Size setting that includes a 1MB filesystem partition. Compile commands for each supported board:

# QTPy RP2040 (8MB flash)
arduino-cli compile \
  --fqbn rp2040:rp2040:adafruit_qtpy:usbstack=tinyusb,flash=8388608_1048576 \
  arduino/midi_interface_msc

# Xiao RP2040 (2MB flash)
arduino-cli compile \
  --fqbn rp2040:rp2040:seeed_xiao_rp2040:usbstack=tinyusb,flash=2097152_1048576 \
  arduino/midi_interface_msc

midi_interface_simple — simplified bridge for learners

arduino/midi_interface_simple/

Same USB-MIDI ↔ UART-MIDI bridge with the same filter/transform scaffolding, but with SysEx handling and advanced notes removed. A good starting point if you just want to read and modify the forwarding logic without the extra complexity.

arduino-cli compile \
  --fqbn rp2040:rp2040:adafruit_qtpy:usbstack=tinyusb \
  arduino/midi_interface_simple

CircuitPython sketches

Requires CircuitPython 9+ firmware on the board.

Libraries needed:

• tmidi — copy tmidi.py to /lib/ on your device or install with circup tmidi
• adafruit_ssd1306 — install with circup install adafruit_ssd1306 (OLED sketches only)

midi_interface — USB-MIDI ↔ UART-MIDI bridge

circuitpython/midi_interface/

Forwards all MIDI between USB and UART in both directions. Same filter/transform scaffolding as the Arduino version — uncomment the examples or add your own:

• Drop realtime messages (clock, active sensing, etc.)
• Transpose notes on a channel
• Remap channels

SysEx messages up to 127 bytes are forwarded. Increase the sysex_buf_* bytearray sizes in the source to raise the limit.

Copy code.py to the root of your device, or copy the whole midi_interface/ folder and rename code.py to suit.

midi_interface_oled — bridge with OLED display

circuitpython/midi_interface_oled/

Same bridge as midi_interface, with a 128x32 SSD1306 I2C OLED that displays each incoming message. Realtime messages are forwarded but not shown, to avoid flicker. The sketch continues as a plain MIDI bridge if no display is connected.

Copy code.py to the root of your device, or copy the whole midi_interface_oled/ folder and rename code.py to suit.

Hardware

Schematic and PCB in schematics/midi_bff/ (KiCad).